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eHam Forums => Boat Anchors => Topic started by: K9AXN on March 13, 2015, 05:57:00 PM



Title: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 13, 2015, 05:57:00 PM
This is a common complaint regarding the Hallicrafters SX-115

“The Selectivity switch positions don't seem to be effective.”

Simple fix for the problem:

To fix the problem, simply replace the Z5U Ceramic disc capacitors on the selectivity switch along with C73 and C86 with Polypropylene film capacitors and realign the 50Kc I.F.  If you would like to understand why, read on.

If there is any question in your mind that the Z5U capacitors need replacing, do two things.

1.  Measure the capacity of the capacitors.  The .01uf caps are easy to reach.  Measure them with the radio cold.  They will be very close to .0082uf not .01uf --- (-5%/Decade).  That is a result of aging.  Don't think they age?  Remove one and place it in an oven at 280 degrees for about two hours.  Then let it sit for 48 hours and measure the capacity --- it will be .01uf like new.  Now it begins to age again.  The heating does not hurt it.  The baking process was used by Mfrs to adjust them to tolerance.

  
2.  Measure the caps while the radio is cold .0082uf, then warm the radio to operating temperature.  Operating temperature on the SX-115 is approximately 20 degrees C above room temperature.  Now measure the capacity.  It will be approximately .0073uf --- a long way from .01uf.  


The following link is a video that was shot while heating a .01uf capacitor on the selectivity switch from 22 degrees C to 55 degrees C.  We recorded the capacity cold 22C .0083uf then at the operating temperature 42 degree C point .0071uf.  Then heated it further to 55 degrees C.

www.k9axn.com/SX-115/SX-115%20ceramic%20temperature.avi

The values of those capacitors are Critical, they control the frequency offset for the five selectivity settings as well as coupling.  Hallicrafters made a component selection error.  

I believe the SX-115 was designed before the SX-117 by a year or two and the SX-117 is part of the story.  The SX-115 was the flagship and the SX-117 was supposedly the poor Man's copy.  

Hallicrafters made a component selection error.  

They apparently decided that the capacitors used in the BAND PASS FILTER circuits of the SX-115 could be high end 10% ceramic class 2 capacitors and somehow ceramic class 3, Z5U capacitors were used.    

Every engineering or design document explicitly warned the use of Class 3 ceramics in tuned circuits, timers, and analog applications were patently poor design practices.  How they missed is a mystery because the SX-115 is the only radio designed with the 50Kc I.F. system that used the ceramic capacitors.  The SX-100 Mark2 and SX-117 used expensive General instruments film capacitors.  The SX-88, SX-76, SX101, SX-96, and early SX-100 used paper or film capacitors that did not have the instability with voltage or temperature that the ceramic Z5U has.  

Hal used .01uf and .0047uf 10% ceramic Z5U capacitors to fit out the BAND WIDTH switch.  I believe they meant to use Class 2 caps that would probably work.  

Note: The SX-115 is the only 50Kc I.F. radio ever built by Hal to use the ceramic Z5U caps to implement the Band width logic.  This was a mistake as you will see below.    

The ceramic Z5U is one of the most nonlinear, volatile ceramic capacitors ever available.  The schematic calls for a 10% ceramic disc capacitor.  It does not specify the class 3 Z5U.  What does the 10% mean?  It says at room temperature, the capacity will be within 10% of the stated value; that is, DISREGARDING APPLIED VOLTAGE which has a profound influence on capacity.  All bets are off when voltage is applied.  With AC the capacity begins to vary at less than 1 volt.  This would cause linearity problems although not obviously perceptible.  The vast change in capacity with temperature and aging however, will cause the center point in each of the band width center positions to shift along with the band width.  

The .5Kc selectivity position, the anchor position is set by alignment to center at 50.750Kc 750 cycles above the BFO; resulting in a band pass at 50.500 to 51.000Kc.  The 1Kc, 2Kc, 3Kc, and 5Kc center points are shifted out by the capacitors used on the selectivity switch; If the capacity changes, so too does the frequency offset center position.    

Data sheet information for the Z5U.  The Z and 5 are the low and high charted limits of temperature which are +10 to +85 degrees C.  The U states that the capacity will vary from +22% to –56% over the temperature range.  That's the charted range.  Anything beyond will vary radically more.

  

The Polypropylene film capacitors vary <=2% over a wider temp range and the capacity does not vary with voltage whereas the capacity of the Z5U will vary up to 60% over the voltage range, an additional 20% due to aging, and who knows what due to temperature change.  

HOW THE SWITCH LOGIC WORKS:

The .5Kc position:
The 390pf capacitors control filters center point at 50.750Kc and the 2.2pf cap connecting the two transformers provides coupling.  The .5Kc position shorts around all of the other capacitors. This is the unaltered center of the band pass and the point that you aligned first. It is at 50.75Kc and is 500 cycles wide. This covers 50.500Kc to 51.000Kc at the 6db point with the carrier at 50.000 Kc. The 6db audio band pass is from 500 cycles to 1000 cycles.

The 1Kc position:
Introduces all of the capacitors on the switch in parallel inserted in series with the 390pf fixed caps and removes the short around.  This reduces the effective capacity, moving the band pass center upward to 51.000Kc  The removal of the short and insertion of the capacitors not only anchors the new band center at 51.000 Kc but provides coupling of the secondary load to the primary widening the band pass to 1Kc centered at 51.000Kc.  

The 1Kc position is the most efficient of the selectivity positions.  The combination of the values of the capacitors selected to anchor the new band center at 51.000Kc and the coupling effect of the capacitors combined with the load impedance result in the most efficient position.    


The 2Kc Position:
Removes a .01uf capacitor moving the band center point to 51.500Kc.  The added coupling of load from the secondary expands the band pass but not properly.  A 180 ohm resistor is added to further correct the band pass to 2Kc.  This creates a band pass from 50.500Kc to 52.500Kc at 6db rendering an audio band pass of 500 to 2500 cycles.

The 3Kc position:  
Removes the .0047uf capacitor moving the band center to 52.000Kc.  The added coupling of load from the secondary expands the band pass but again not properly.  A 220 ohm resistor is added to further correct the band pass to 3Kc.  This creates a band pass from 50.500Kc to 53.500Kc at 6db rendering an audio band pass of 500 to 3500 cycles.  


The 5Kc position:
Removes the last .01uf capacitor and reinserts the .0047 capacitor moving the band center to 53.000Kc.  The added coupling of load from the secondary further expands the band pass but again not properly.  A 390 ohm resistor is added to further correct the band pass to 5Kc.  This creates a band pass from 50.500Kc to 55.500Kc at 6db rendering an audio band pass of 500 to 5000 cycles.


The above paragraph says it all as to why the Z5U is a design error.  The variation in capacity with temperature, voltage, and age disqualifies it.  The band width center positions will not remain where they should be using the ceramic capacitors.


I hope this helps and hope you change those caps out for Polypropylene.  That is a fine instrument --- keep it that way.

  

MORE INFORMATION FOLLOWS:

If you should choose to replace those capacitors, Please measure them before you remove them.  The front .01uf is disconnected when the switch is in the 2Kc position and the second when in the 5Kc position.  Measure when cold and when it reaches operating temperature; after an hour.  The .01uf should measure .0082 cold and .00074 warm.  They don't affect the 500 cycle position because it is a short around, however the other positions will overlap each other to varying degrees.

Here's an interesting experiment.  After you remove the ceramic caps, put them in your oven at 280 degrees for 2 hours then take them out and let them cool and settle for 48 hours.  Now for the magic; Measure the capacity.  They will be .01uf like new and the aging process begins again; at 5%/decade.  

The baking process is how the MFGRS used to alter the capacity to the target value.  You would always find a 10% capacitor at =< 2%.        


The aging process combined with nonlinear –10,000 ppm Temp comp personality of the Z5U Ceramic will result in the .01uf cap becoming a .0074uf Cap and will do nothing but get worse over time.

Conclusion:

Replace the two .01uf and one .0047uf ceramic disc caps on each selectivity switch as well as C73 and C86 .02uf ceramic disc caps.  

C86 is a sleeper.  It is used to bypass the screen BUT is also a critical functioning member of the tuned circuit coupling the components to both sides of the second 50Kc I.F. circuit.  It must be replaced with Polypropylene.   Both C73 and C75 are exposed to +215 volts which further reduces their capacity after the accumulated reduction caused by aging and temperature.  

 

NOTE:  The four other paper looking capacitors in the SX-115 are not paper but high quality Mylar film.  If you have already replaced them with Ceramic, remove them and replace with them with Polypropylene.  C100 and C111 are timers for the fast AGC which controls the first RF amplifier.  Using ceramic caps relegates the fast AGC to a second rate performer.  All of those Mylar caps were deliberate choices used in critical circuits requiring linear time constants.  The Z5U is patently nonlinear in every sense and should not be used to replace the film caps in this radio.

 

Kindest regards Jim K9AXN

  

COPYRIGHT  James Liles


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: N8CMQ on March 13, 2015, 06:56:56 PM
Thanks!

 Very nice information, and I appreciate your work!

I am currently rebuilding an SX-101 mk III, and I am doing more
than the cap change. It needed more than 'orange drops'...

One thing I have found working on my 101 is, the tube sockets are
conductive at DC. I have been replacing all of the plastic sockets
with ceramic sockets due to the conductivity between pins.

The local oscillator socket was the worst case, and I suspect it was
the reason the local oscillator tube failed. I checked the paper caps
when I removed them, and they were leaky, but the socket was still
worse.

My first reaction was to try cleaning the socket, but that didn't work
one bit. I suspect the socket was overheated and carbon formed
between the pins. Definitely a lot of work replacing the sockets,
and I hope the final results on the air are how I remember a 101
really worked!


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 13, 2015, 07:45:43 PM
Hi Jeff,

Did you save the caps that were on the selectivity switch?  If you did were they truely leaky and if so please do a post mortem to see if they were paper of film.  I'm guessing but they were probably film and still good as day one.

Thanks and let me know Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: N8CMQ on March 13, 2015, 10:27:31 PM
Hi Jim,

The six caps off the selectivity switch are molded paper.

I am not sure what the name is, but they are black with color stripes.
Normally, black beauties had the name, and the value printed on the cap.
These caps look the same size, but no printing, just stripes.

These are the modern version of the wax paper caps they used to use.

The new caps are film tho, as are the resistors!


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 14, 2015, 02:03:57 AM
Are you sure that it was a Hallicrafters 'engineer' that changed to ceramic caps and not some bright spark in production or purchasing with little understanding? My experience generally was that someone in production 'engineering' or purchasing who didn't really understand what they were up to would 'reduce cost' by some change that didn't really work. The classic was a marine tx with lots of phenolic bars and so on around the variometer and brass screws: production 'reduced cost' by changing to cadmium plated steel and the result was transmitters catching fire on tests because of the losses!


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 14, 2015, 07:57:06 AM
Thanks Jeff,

We're trying tp nail down the transition from paper to Mylar in the Hallicrafters product line.  Would like to hear from someone who has an S76 or SX-96 that can look at the caps on the selectivity switch.  Would like to know what version of cap they had.

Kindest regards Jim K9AXN   


Hi Jim,

The six caps off the selectivity switch are molded paper.

I am not sure what the name is, but they are black with color stripes.
Normally, black beauties had the name, and the value printed on the cap.
These caps look the same size, but no printing, just stripes.

These are the modern version of the wax paper caps they used to use.

The new caps are film tho, as are the resistors!

[/quote]


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 14, 2015, 08:12:59 AM
Good morning Pete,

You may be right.  The Hal engineers were some of the most forward thinking creative guys around at the time.  They did more with less than most.  Documentation was not their strong point. 

----------------------------------------------------------------------------------------------------------

Are you sure that it was a Hallicrafters 'engineer' that changed to ceramic caps and not some bright spark in production or purchasing with little understanding? My experience generally was that someone in production 'engineering' or purchasing who didn't really understand what they were up to would 'reduce cost' by some change that didn't really work. The classic was a marine tx with lots of phenolic bars and so on around the variometer and brass screws: production 'reduced cost' by changing to cadmium plated steel and the result was transmitters catching fire on tests because of the losses!
[/quote]


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: WA1RNE on March 14, 2015, 08:31:28 AM
Quote
Are you sure that it was a Hallicrafters 'engineer' that changed to ceramic caps and not some bright spark in production or purchasing with little understanding?

 The way components are specified in most manufacturing organizations is this: (simplified)

 > Once engineering has a working design that meets specifications (including any qualification testing depending on the type), engineering creates the parts list/bill of materials and with manufacturing, purchasing and quality sign offs, it's approved for production.

If manufacturing and/or purchasing (supply chain) wants to make a component change, that change would be submitted as an Engineering Change Request (or a similar process name) and go before a review board with all the above mentioned representatives.

Purchasing/Supply Chain would not have unilateral approval authority to make a component change - it's way out of their area of expertise, even 50 years ago, so I highly doubt they did this on their own. This does sound like an oversight on engineerings part - it's been known to happen.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on March 14, 2015, 09:16:06 AM
I have never seen ceramic caps used for LC selectivity selections in Hallicrafters receivers.  Molded plastic caps, yes.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 14, 2015, 10:43:47 AM
Purchasing/Supply Chain would not have unilateral approval authority to make a component change - it's way out of their area of expertise, even 50 years ago, so I highly doubt they did this on their own.

I have known it happen a number of times over my 49 years in the industry. Before my time, but I heard about it from the engineer concerned before he retired, that there was a case of a military UHF airborne transceiver, made under licence from Collins. The purchasing guys bought a cheaper dynamotor (which was also the fan for the 4X150 PA tube) that tended to get hot, but didn't have the temperature rating for the varnish. The varnish evaporated and set up an explosive mixture in the box. In one high performance fighter, it only needed one spark and there was a loud bang behind the pilot's head, leading in several cases to them not standing on the order of their going, but used Martin-Baker's ejection seat. This happened a number of times before one brave soul determined that the aircraft still flew OK but had no radio and carefully brought it home, where they found the problem.....

Linear ICs are even more of a problem - second sources often aren't, even simple things like 747 dual op-amps. Being in the semiconductor industry for 32 years probably makes one even more cynical.....


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 14, 2015, 07:09:36 PM
Good evening,

your right about the caps in the Hallicrafters selectivity circuits.  THE ONLY receiver that Hal ever built that was fitted out with ceramic capacitors in the selectivity circuits was the SX-115.  It was a mistake.  I have never seen anything but ceramic in the SX-115 selectivity circuits.  If you have one take a look.  Won't take a moment, you don't even have to take it out of the case.  Just change em out to Polypropylene it will be obvious.  Case closed.

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on March 15, 2015, 05:27:24 AM
I'm surprised they did that--as you noted they (Z5U dilectric)  are terrible caps for temperature stability.  They make good thermistors! Even body heat shifts the value. Most of my Hallicrafters are SX-101 or earlier.

pete


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 20, 2015, 05:29:25 AM
The article is incorrect as to the function of the capacitors being discussed, and it has a tone of disrespect for Hallicrafter engineers. Worse, it explains the circuit incorrectly. If we are going to point out a design error, we need to make sure we actually understand the circuit.

If we take time to understand how the capacitors actually work, we see the capacitor choice was not all that bad based on available components. The important thing is that we understand how the circuit actually works.

The capacitor's function is to change the coupling of the resonant circuits, not the tuning. The frequency shift is an unintended minor consequence to altering the coupling.

The IF is a coupled pair, like most IF transformers, with the exception the coupling is not via magnetic fields. Coupling via magnetic fields would result in fixed selectivity unless the mutual coupling could be altered.  

http://mysite.du.edu/~etuttle/electron/elect34.htm

(I hope that link works. If it doesn't, old Handbooks should have something.)

If you look at each IF inductor pair, you see two small mica capacitors. In the SX115, the capacitors are 390 pF. These are the capacitors that resonate the inductors.

With the inductors isolated in cans, and no mutual coupling, the IF would have no path through the resonant circuits. To solve this, the "ground" end of the 390pF capacitors are connected to a common point and floated.  There is a 2.2 pF that determines absolute minimum coupling, and sets the maximum selectivity by assuring a certain minimum coupling coefficient K.

The basic formula is BW = k*f where BW is bandwidth, k is coupling coefficient, and f is IF frequency. This system functions by altering k (coupling), not f (frequency).

The large capacitors "ground" the common point of the two 390 pF caps. The small voltage developed across the large capacitors controls the coupling between each IF can resonant pair, which are resonated by the 390 pF caps.

The simple effect on "value" on the 390 pF is that of a large fixed cap in series with a small fixed cap. We have 390x2 in series with a .01uF, or 10,000 pF. Using standard formulas, we see the effective capacitance is 724 pF, or 362 pF per tuned coil. If the capacitors drift low to  8000 pF, the change is 710 pF or 355pF per tuned section.  So a change from 10000 pF to 8000 pF only results in a tuning capacitance change of 7 pF, or about 2% drift. The actual circuit change would be much less than this, because the wiring and components (including tubes) "load" the IF inductors with "fixed" capacitance, so the delta is reduced.

2% drift in capacitance is 1.4% drift in frequency. There are things that offset this effect. One thing is the bandwidth increases as the coupling capacitor becomes smaller.   Since bandwidth increases as the shunt capacitance becomes smaller, the effect is primarily a gradual reduction in selectivity.  

When those capacitors were new, 50 years or more ago, they were a good choice. They do not have to be high stability parts. When in the narrowest position, there is a short from the center of the 390's to ground and all coupling is via the 2.2 pF capacitor. This is the alignment point.

As the "short" is increased to a few ohms of capacitive reactance, by the .01, .01, and .0047 in parallel, we have 24,700 pF to ground or about 125 ohms reactance. Even if that drifts, the resonance still overlaps and it is a small selectivity change.

The real killer in the system is leakage resistance.

Years ago, around 1970, I modified an SX101 by adding 15 resonant circuits on a small chassis with a new RCA CA series  integrated amplifier to restore gain. Like Hallicrafters, I used silver mica resonating capacitors and standard disc ceramics for the shunt coupling caps. My values were a lot different, but the basic circuit was the same. I didn't worry at all about using a C0G or NP0 capacitor, except in the resonating cap.  

What Hallicrafters did was perfectly fine. They were good engineers. They just didn't have capacitors available for reasonable cost and size that would last 50 years. Those capacitor do not need to be exceptionally temperature stable, but they do need to be high leakage resistance (fairly high Q).

By the way, there are many things that factor into the operation, this was just a quick overview. There are resistors that intentionally add loss, for example, to keep gain the same as coupling coefficient k changes. I just wanted to set the record straight as to the function of the large capacitors or SHORT that Hallicrafters switches in. They could have used inductors instead of capacitors, because they do not "tune" the IF coils. The switched capacitors set mutual coupling, the 390 pF sets the tuning, and the 2.2 pF sets the maximum selectivity by setting the absolute minimum coupling when the BW switch shorts the .01's and .0047.

Those guys in the design lab knew a lot more than we think. They just had to work with 50 year old parts.  :)
  
73 Tom


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on March 20, 2015, 06:06:43 AM
But, the bumble caps used in the earlier SX-101 series was a magnitude better than Z5U dielectric caps.  They had better choices available.  Z5U is fine for bypassing, but I won't want to see it used in a critical coupling application.

Sometimes manufacturers cut a few corners to save a few pennies. It isn't always about using the best available parts for an application.

Pete


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 20, 2015, 06:11:52 AM
Tom,

BW = k*f where BW is bandwidth, k is coupling coefficient, and f is IF frequency


Not quite, because Q comes into it.

The response  at any offset f0 from the centre frequency f for a pair of tuned circuits is

E0/E = [(p+1)/{([{2Qf0/f}2 - (p-1)]2 + 4p)} 0.5

where p = k2Q2, E0 is the output at the offset and E is the output at centre frequency

From whence for a coupled pair

f0/f = +/-(2Q)-1({p-1}+/-[(p+1)2(E0/E)2-4p}0.5)0.5


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 20, 2015, 08:53:19 AM
Good morning Tom,

You don't know anything about me.  Take a moment and look at www.k9axn.com then you can decide what my affiliation is with Hallicfrafters engineers.  I was one of the people they worked with to test some of the SR-400, SR-400A, and SR-2000 engineering changes etc.  My opinion regarding the Hallicrafters designers is top of the line and the most creative of the times.  There were non better and that includes Collins.  I make mistakes and so to do other people in the design field.

There is not one statement that I made that is not 100% accurate.

First I will point you to the selectivity charts.  How do you suppose the center of the bandpasses moved away from the carrier and what do you suppose caused the move??  If you insert a capacitor in series with another, will the capacity increase or decrease?  Now get your calculator out and redo the math.  First I will ask you the combined capacity for each selectivity setting.  OK what is it?  Now calculate the resonant point.  Voila, the resonant point has moved the band center away from the carrier to accommodate the widened pass band created by coupling and the added resistors will reduce the Q to reduce the over coupling to critical.  The distance the center moved is as I stated.  Please reread the numbers.

Have you aligned an SX-115 or SX101?  Having an SX-101  --- yes you have.  First thing done on the 50.75 I.F. is to align the system at 50.75 using the .5Kc position.  That uses none of the caps or resistors on the selectivity switch.  However, if you reexamine the schematic there is a .02uf Z5U -20+80 capacitor in series with the 390's when in the .5Kc position.  Do you understand the -20+80 meaning?  That circuit is never shorted together in the second 50Kc tuned circuit.

Revisit the circuit design.  The two 390uf caps share the capacity in the selectivity circuit.  They're essentially in parallel but series with the selectivity caps and the resistors in series with the secondary winding.  I would encourage you to recalculate your supposition.  I to make frequent miscalculations.  
  
The capacitors that were used in those circuits are virtually condemned in every engineering research paper ever written.  If you should find one that contradicts anything that I have stated please bring it to my attention.  What I stated was correct!!!  The Hallicrafters engineers were cognizant of the frailties of the Ceramic Z5U capacitor and I believe one of the most careful design groups selecting components.  If you read the statement, I said the SX-115 is the ONLY radio using the 50Kc I.F. made by Hallicrafters to use the Z5U in the Selectivity circuits.  I never said it was negligence, more likely an oversight.

Judging from your tone, I wonder if you responded to this as a proxy for the belligerent one?  I sincerely hope not.

If not my apologies.

Have a super day and yes I consider you a friend in that you feel strongly about the great mark that the Hallicrafters people made on the era of radio.

Kindest regards Jim K9AXN  
 


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 20, 2015, 09:04:36 AM
But, the bumble caps used in the earlier SX-101 series was a magnitude better than Z5U dielectric caps.  They had better choices available.  Z5U is fine for bypassing, but I won't want to see it used in a critical coupling application.

Sometimes manufacturers cut a few corners to save a few pennies. It isn't always about using the best available parts for an application.

Pete

It isn't critical, and more important it doesn't even work the way the outline at the start of this thread describes.

The particular capacitors do not have to be exceptionally temperature stable.

My description was just a really rough overview, but if we are going to get a "feeling" for how the system works we have to understand the basic function of the components.

The capacitors determine mutual coupling. They are switched so they are open (maximum k and maximum bandwidth), or progressively decrease in shunt reactance until finally a "short" appears. At the point where it is a short, the 2.2 pF sets the tuned circuit coupling.

The actually system is much more complex than this, but this is the simplest operational description that explains the system.

If a capacitor drifts what we might assume is significant in value, the effect on the system is in fact relatively small. They are not tuning capacitors, they do not "shift the frequency" except as a small unintended effect. That shift is primarily in the upper passband frequency limit. There are three that are connected in various combinations. The 5,3,2,1, and .5 kHz sequence is:

5kHz  = 4700 pF
3 kHz  = 10,000 pF
2kHz = 14,700 pF
1 kHz = 24,700 pF
.5 kHz = short  

A normal drift from using non-special parts obviously had minimal effect. The frequency is primarily determined by the 390 pF caps. As a matter of fact, if we went from 10,000 pF to 24,700 pF for the bandwidth determining capacitor the center frequency of a 500 Hz wide 50.849 kHz IF would move from 51.229 to 51.756 kHz.  It moves ~500 Hz center for a change from 1kHz to 3 kHz bandwidth and the passband still overlaps other stages.

There is no amount of drift in the bandwidth capacitor that pulls the IF out of passband.

The bandwidth determining capacitor is not critical for normal drift of a standard capacitor, and that is why they used what they used. If the capacitor goes bad and goes severely off, like leaky or shorted, that is another issue.

73 Tom





Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 20, 2015, 09:29:15 AM
Good morning Tom,

<snip>

Kindest regards Jim K9AXN  
 

Jim,

This is nothing personal. The components Hallicrafters picked were 100% suitable for the application. It was not an "engineering mistake". I have no problem improving old radios, but we have to be reasonably accurate in understanding how the circuit works, or we can't accurately decide why the parts they used were used.

In this case, the numbers I gave were correct. I gave an example of a single stage assuming 100k ohms source and load impedance and the approximate values of the Hallicrafters parts:

Quote
5kHz  = 4700 pF
3 kHz  = 10,000 pF
2kHz = 14,700 pF
1 kHz = 24,700 pF
.5 kHz = short  

A normal drift from using non-special parts obviously has minimal effect. The frequency is primarily determined by the 390 pF caps. As a matter of fact, if we went from 10,000 pF to 24,700 pF for the bandwidth determining capacitor the center frequency of a 500 Hz wide 50.849 kHz IF would move from 51.229 to 51.756 kHz.  It moves ~500 Hz center for a change from 1kHz to 3 kHz bandwidth and the passband still overlaps other stages.

This is the reason they used the type of components they used. I'm certain we would all like the 50-100 year old radios we have to use parts from the last 20 years, but they were built in the 1950's and earlier. There is no reason to use a precision capacitor or a temperature stable capacitor for the bandwidth capacitors, because the primary frequency determination is in the stability of the 390 pF caps and the inductors.  

The circuity has a 390 pF "resonating" capacitor from each inductor in series with a selectable .0047,   0.01,  .0147, and .0247 capacitance. The dominant reactance at 50 kHz is in the inductor and the 390 pF (plus circuit strays). The large capacitors, which have relatively low reactance compared to the 390 pF capacitors, primarily set bandwidth by changing coupling.

If that circuit were designed today, there would be little reason to use an NP0 TC capacitor. Why then are we calling it a mistake when temperature stable caps in the 10,000 pF range were terribly expensive in 1950? What I see is a 2015 analysis mistake, not a 1950 mistake.  :)

They are 50 year old parts. If they have gone bad, change them. If you want to be overly picky, use C0G's or something. It won't work significantly different than a standard Z5U, however it might make us feel.  The change from 2kHz bandwidth to 1kHz BW is 10,000 pF. I doubt a .01uF capacitor is going to drift anything remotely close to .005 uF.

73 Tom


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 20, 2015, 10:03:12 AM
But what about the change in k with tolerance, temperature and age?? Of course, the other 'slight' complication is that one has to assume that the coil Q is the same as 50 odd years ago for many calculations to make sense. For bottom capacity coupling - where one uses large value capacitors - then

k =  (C1C2)0.5/Cc

where C1, C2 are the tuning capacitors and Cc is the coupling capacitor.

So a 20% variation in coupling capacitance means a 20% variation in k, and if one wanted critical coupling where kQ = 1, you could be kQ = 1.2 with the -3dB point on one coupled pair at where the -6dB point should be, or you could have kQ = 0.8 where the -6dB point would be about -8dB.

So from a tolerance viewpoint, stable close tolerance capacitors would be a best bet, and even in the late 1950s, high Q and reasonably stable, close tolerance polystyrene film caps were available. In the sorts of values that would be wanted, they were made with multiple wires to the foils to keep Q up, and the temperature stability and repeatability was enough for stable filters in FDMA telephone systems when used with suitable ferrites - which is where the fallacy has come from in some circles that polystyrene capacitors and ferrite cored coils always compensate each other.

There are a lot of designs out there where coupling is by means of top capacity coupling: that, unless Q is very low, ends up with small capacitors with a necessarily high tolerance and thus an ill defined value of k. But that is very popular coupling method amongst amateurs.....

Of course, where inductors are variable, then using capacitive coupling helps keep kQ constant over the tuning range, and vice versa when the tuning is by variable capacity.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 20, 2015, 10:11:18 AM
Good morning again Tom,

I didn't take it personal and am happy to engage in technical discussion; and respect you for responding.  

The facts and numbers are not negotiable.  Your calculations are simply incorrect.  What I stated about the function of those capacitors and resistors is absolutely correct.  The band centers are also correct.  Please review the Hallicrafters published band pass locations.  Those charts are correct and the measurements that I made match the charts.  If you should have sweep capabilities just sweep it and it will be obvious.  Please take a moment and rethink your supposition.  

I will ask two questions:

The parts that they picked were an absurd choice for that application.  What research paper did you find that suggests the use of Class 3 ceramic capacitors in any tuned circuit --- ever, even 50 years ago?

Why did the Hallicrafters folks use paper then film caps in every other 50Kc system?  Even paper is legions better that Class 3 ceramic, that is if they didn't become dangerous with age.  The SX-117 used Film caps. It was built during the same period for those who could not afford the flagship SX-115.  I can unequivocally state that the SX-115 in every other respect is a superb radio.

Case closed for me.  If you want your selectivity circuits to work properly, replace those caps with Polypropylene.  This is not a negotiable notion; simply good design practice and yes the difference is distinguishable.

Have a super day --- Kindest regards Jim

  


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 20, 2015, 11:46:13 AM
But what about the change in k with tolerance, temperature and age?? Of course, the other 'slight' complication is that one has to assume that the coil Q is the same as 50 odd years ago for many calculations to make sense. For bottom capacity coupling - where one uses large value capacitors - then

k =  (C1C2)0.5/Cc

where C1, C2 are the tuning capacitors and Cc is the coupling capacitor.

So a 20% variation in coupling capacitance means a 20% variation in k, and if one wanted critical coupling where kQ = 1, you could be kQ = 1.2 with the -3dB point on one coupled pair at where the -6dB point should be, or you could have kQ = 0.8 where the -6dB point would be about -8dB.


Peter,

Certainly a better capacitor would be more stable for stability of coupling and selectivity, but these receivers have a relatively poor shape factor anyway. I didn't get into a problem with coupling capacitor stability even with ~100 Hz bandwidths and shape factors of 3 or 4, which was about the limit of what I could do with 15 or 20 inductors.

The actual filter I used was a current carrier filter from a utility company. I moved it to the IF frequency by changing capacitors across the inductors, and increased the bandwidth by increasing coupling. The tuning capacitors, of course, had to be stable silver micas. The coupling capacitors were perfectly fine using standard disc capacitors at temperatures encountered in the box.

The important points here are:

1.) the large value capacitors do not move the IF frequency significantly in comparison to IF bandwidth

2.) the 390 pF capacitors, along with the inductors, are the primary frequency determining parts and are critical for stability 

3.) the 2.2 pF and the quality of the "short" at the switch set the 500Hz bandwidth. All the capacitors do is increases the coupling factor from that minimum point

4.) the effect of decreasing Cbw is to narrow bandwidth. The lower frequency -6dB point of each stage stays almost constant, while the center frequency shifts upward. The upper frequency -6dB shifts upwards the most. 

5.) because of circuit loading and limited inductor Q, none of which is known, we can be sure the shift is less than a perfect circuit. A perfect circuit doesn't appear to be a problem with good normal non-special capacitors.

It doesn't seem like Hallicrafters made any engineering mistake. They used the parts they had available 50-60 years ago.

It's unfortunate a few 50 year old parts are frequently bad today, but that doesn't mean someone made a mistake. It also doesn't mean we have to overkill the circuit. I'd probably use an X7R or something tighter than a Z5U, but most Z5U's are far better than spec and would likely not be noticeable. A C0G or NP0 would be not be any improvement.

These things have pretty wide skirts, even when perfect. 

We should not confuse 50 year old bad parts with bad engineering.

73 Tom


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 20, 2015, 12:18:36 PM
Tom,

I can't agree. To use capacitors which give a wide tolerance on kQ when that defines selectivity is not good practice, although I suppose for an amateur receiver, it's not so important. For marine receivers subject to Type Approval and definite requirements on selectivity (back in the days when ships had radio officers and used Morse!), it would not have done at all, and it was usual to use silver mica for coupling and tuning capacitors, as well checking every IF coil for inductance, tuning range and Q.  Similarly, professional receivers for non-maritime applications used close tolerance capacitors in switched selectivity IF stages.

My father's HQ170 had ceramic coupling caps, and the IF responses never quite matched up with their curves! Eddystone in the UK did a number of receivers where the coupling was changed by moving the relative position of the IF coils, similar to the method used in the BC453. That at least has the advantage that the coupled circuit far off resonance still has a monotonic response, while other coupling methods degenerate into either a low pass or a high pass response, albeit with high attenuation.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 20, 2015, 12:59:06 PM
The selectivity at best is already so wide, the caps matter very little.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 20, 2015, 05:27:56 PM
Hi Tom,

You say the selectivity is lousy.  What kind of capacitors did you use in your radio?

You also say those large capacitors have nothing to do with the resonant point.

Please get your calculator out and do a simple set of calculations that should help to understand the issue.

On the SX-115 the second 50Kc tuned circuit you will find C90, C91, and C92.  You must look at C86 also.  You say why because it's only a bypass capacitor.  It is a direct part of that resonant circuit.  Careful, there are many many (Bypass capacitors that participate in resonant circuits and also bypass RF to ground).

Calculate the resonant frequency of the switch in the .5Kc position.  and remember to use C86 the .02uf bypass/coupling capacitor.  The coil will be 25.7Mh.

Next calculate the resonant frequency when in the 1Kc position.  And remember the .02uf capacitor.

Next the 5Kc position.

You say those capacitors don't shift the resonant position?  Please just do the math, there is no argument here, let it speak for itself.

Now on to the Z5U class 3 capacitors behavior.

First look at the following snippet.   http://www.k9axn.com/SX-115/SX-115%20ceramic%20temperature.avi

It represents one of those Z5U's in the SX-115.  Note the starting capacity at room temperature 22 degrees C and the capacity as we heat it to 42 degrees C.  There is a pause when we reach 42 degrees.  That's the temperature behavior of the Z5U.  Think I'm doing magic and mirrors, do it yourself and note at room temperature it's at .0083 not .010 from aging.  compound that with temp shift and now we're at .0073.  Lets frost the cake and add 215 volts to the .02.  It will loose even more capacity.  the ,02 is .0165uf.  subtract at least 10% now the .02 is .015uf.

The Q at 100Kc for the Z5U is 254.  The Polypropylene Q 2900 at 100Kc and the DF for the Polypropylene 1/10 of the ceramic. 

If it makes sense to you to use ceramic Z5U caps in that circuit, don't lament the selectivity is deficient.

Do the math Tom and lets see if we can come to some conclusion.

Kindest regards Jim K9AXN
 


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: JS6TMW on March 21, 2015, 06:47:52 AM
"I'm surprised they did that--as you noted they (Z5U dilectric)  are terrible caps for temperature stability.  They make good thermistors! Even body heat shifts the value. Most of my Hallicrafters are SX-101 or earlier."

Correct - I used the effect in a miniature telemetry transmitter for ultracentrifuge rotor temperature measurement, back in 1965. Worked like a charm.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 21, 2015, 02:34:59 PM
Good afternoon Tom,

This is a truly a friendly question.  I'm and old timer that has probably forgotten more than I ever knew but you said something that bounced all around my grey matter looking for a rational solution.

You mentioned that the filter design used in the 50Kc I.F. systems, the nearest skirt is anchored 500 cycles from the carrier and the outside skirt is moved out to accommodate the wider pass band.  Would that not drag the center of the pass band along with it?  Or is it that the center is moved and the skirts are expanded?  Or are they both the same?  I got the feeling that you felt that there was some distinction between both.

Have a super evening Tom

Kindest regards Jim K9AXN

  


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 21, 2015, 07:57:00 PM
Tom is doing such a good job I wouldnt even have responded but Jim cant seem to help himself by taking cheap shots with no provocation, a trait Ive followed of his for many years. And yes Ive called him out several times elsewhere on various subjects so to him Im belligerent since he cant tolerate being questioned and will never back down or admit an error. Mr Smiley Perfect.

I doubt if Jim ever had a "friendly question" when in a discussion that he is losing; more like a guy with a preachers smile and a knife ready for your back. He has been trying to sell this idea for awhile and has been shopping it around. This time it hit the ground....hard.

As far as the SX-115; the claim the filter caps are Z5U's is completely bogus, at least in mine, as they are 10% "close tolerance" as called out in the manufacturers catalogs of the time. Z5U wasnt even a common term then since the EIA standards hadnt been set yet and they were simply called Hi-K by most and undefined in many cases.

My SX-115 selectivity curves are very close to the manual and the receiver is used often with a HT-32B and NCL-2000 for vintage CW, SSB and some AM and is a pleasure to use and absolutely not selectivity hampered. Halli seemed to come out with a real good radio every 10 years or so starting with the SX-28. 

The SX-117 came out a year after the SX-115, which was a very hard sell, and the claim they used film caps is more pure BS if the parts list is to be believed as they call out paper tubular and that is certainly what they looked like and leak tested in mine as I replaced with generic yellow film caps I matched to within 1% of each other. Oh and yes, surprise surprise I have both models and can easily refute a lot of nonsense here. The SX-117 is quite a bit inferior in many areas, especially selectivity, but certainly far better overall performance than most of the bottom dwelling stuff Hallicrafters were turning out in the 60's and on the way to oblivion with most of the other leaders of the past.

Both manuals even state that the selectivity steps are a matter of Q, coupling and resistance and it is obvious by examining the circuit that resonance is NOT a factor since it is an RC circuit and not LC. Halli used the same method as far back as the SX-96 and National in the NC-300, both around 1955 introductions. Halli simply copied a good idea or maybe there was some cross breeding going on(-;

Another thing Jim doesnt understand is the voltage used to determine capacitance in manufacturing is very low and likely at no more than 100 Hz at those capacitance values. Of course there will be some shift with real vacuum tube voltages which is why the Z5U +80 % variable is quoted and much higher than needed cap voltages were available to mitigate the shift if needed. The Z5U is also stated that it is not to be used in frequency determining circuits or where high tolerance is required. But that is all moot since a Hi-K cap was not used in the filter.
In addition, if you read his various pronouncements as if they were actually factual you would find that the voltage/capacitance shift charts used as examples are for 16V SMT devices, not 500-1000V disc ceramics.

I completely agree that the REAL Halli engineers knew what they were doing with the SX-115 and did it reasonably well but I still prefer my NC-300 on AM after a few mods.

Have at it Tom, Im sure most on here will enjoy it. Jim is seething with anger since he didnt expect a high level challenge to his competence...the more platitudes and sweet words he uses the more dangerous he becomes; watch your back.

I consider this whole exercise over...

Carl


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 22, 2015, 11:25:02 AM
Tom, I genuinely was trying to break the ice and apologize if it sounded off tone.

Carl, I respectfully disagree with your findings.  I won't reply to every issue today but will answer every issue individually; have several grandchildren to entertain.   

First:

The SX-115 build did in fact use Ceramic Z5U 10% capacitors for the selectivity switch.

Note the photo's in the following url of the parts list and capacitors removed from an unmolested SX-115.  They unambiguously define exactly what kind of capacitors they are as well as what was defined in the build list.

http://k9axn.com/_mgxroot/page_10879.html

Carl, if the caps in your radio are not as shown in the photo, they may have been replaced already.

For all other SX-115 owners, I would suggest you check your radios for the Ceramic caps and replace them.

Note, I described the use of the capacitors as a design error because they were used in all SX-115's as defined in the parts list.  I Probably should have defined the use of those caps as a manufacturing selection error even though they were listed in the parts list.  Anyone that knows me also knows that I consider the Hallicrafters design team the premier group of the era.

Carl, the next issue is the capacitors used in the SX-117.  I believe there are a few after the SX-117 issue and will finish with the calculations to support the statements that I wrote concerning how those caps were used.

Thanks for the reply Carl and have a great day.  I genuinely wish we could have a more friendly relationship.  I guess that's one of lifes lessons.   

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on March 22, 2015, 12:38:19 PM
In the SX-101A, as far as I remember, the selectivity curve shifted center frequency as BW was changed. That assymetrical response was used too advantage for the sideband reversal system, and for CW.   I can't see where using anything other than stable capacitors would improve the situation, or yield satisfactory results.

Pete


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 22, 2015, 01:46:25 PM

The SX-117 came out a year after the SX-115, which was a very hard sell, and the claim they used film caps is more pure BS if the parts list is to be believed as they call out paper tubular and that is certainly what they looked like and leak tested in mine as I replaced with generic yellow film caps I matched to within 1% of each other. Oh and yes, surprise surprise I have both models and can easily refute a lot of nonsense here. The SX-117 is quite a bit inferior in many areas, especially selectivity, but certainly far better overall performance than most of the bottom dwelling stuff Hallicrafters were turning out in the 60's and on the way to oblivion with most of the other leaders of the past.

Carl, I again respectfully disagree with your above assertion.

The capacitors used in the SX-117 and even late SX-100's were called paper but were actually high quality General Instruments film capacitors that are as perfect today as when they were installed. 

All of the Hal radios including the SR-150, SR-400, SR-400A, SR-2000 and others called capacitors PAPER to qualify them as components that participate in circuits that require stable, predictable, and linear components.  Both the paper and film caps fit that description.  The ceramic Z5u does not --- as published in any research paper or formal design document.

Please observe the url    http://k9axn.com/_mgxroot/page_10880.html

You will find the unmolested SX-117 photos of the selectivity switch and parts list revealing.

Those capacitors that you described as leaky paper were probably as good as new when you replaced them.

They are precisely as I described; High quality General Instruments FILM capacitors.  Dissect one for yourself.  Test them for Q, DF, leakage, aging, hysteresis, change with applied voltage, inductance, self resonance, whatever you have instruments to test for.  They are 100% qualified to participate in that resonant filter circuit; THE Z5U are absolutely not.  Every vendors engineering team was informed of the limitations of the Class 3 ceramics when they were first marketed over 50 years ago. 

Have a great day!

Kindest regards Jim K9AXN
 


 


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 22, 2015, 03:51:35 PM
The SX-115 build did in fact use Ceramic Z5U 10% capacitors for the selectivity switch.

Quote
Note the photo's in the following url of the parts list and capacitors removed from an unmolested SX-115.  They unambiguously define exactly what kind of capacitors they are as well as what was defined in the build list.

http://k9axn.com/_mgxroot/page_10879.html

Carl, if the caps in your radio are not as shown in the photo, they may have been replaced already.

For all other SX-115 owners, I would suggest you check your radios for the Ceramic caps and replace them.

Note, I described the use of the capacitors as a design error because they were used in all SX-115's as defined in the parts list.  I Probably should have defined the use of those caps as a manufacturing selection error even though they were listed in the parts list.  Anyone that knows me also knows that I consider the Hallicrafters design team the premier group of the era.

Nice photos but Erie nor anyone else was using Z5U anything at least thru the 1961-64 production years of the SX-115 according to my various Erie industrial catalogs. Id suggest that yours are from a very late production in a later year to finish up remaining SX-115 parts and not waste anything.

The capacitors used in the SX-117 and even late SX-100's were called paper but were actually high quality General Instruments film capacitors that are as perfect today as when they were installed. 

All of the Hal radios including the SR-150, SR-400, SR-400A, SR-2000 and others called capacitors PAPER to qualify them as components that participate in circuits that require stable, predictable, and linear components.  Both the paper and film caps fit that description.  The ceramic Z5u does not --- as published in any research paper or formal design document.

Please observe the url    http://k9axn.com/_mgxroot/page_10880.html

You will find the unmolested SX-117 photos of the selectivity switch and parts list revealing.

Those capacitors that you described as leaky paper were probably as good as new when you replaced them.

They are precisely as I described; High quality General Instruments FILM capacitors.  Dissect one for yourself.  Test them for Q, DF, leakage, aging, hysteresis, change with applied voltage, inductance, self resonance, whatever you have instruments to test for.  They are 100% qualified to participate in that resonant filter circuit; THE Z5U are absolutely not.  Every vendors engineering team was informed of the limitations of the Class 3 ceramics when they were first marketed over 50 years ago


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 22, 2015, 04:21:35 PM
Carl,

What is the difference between testing for Q and DF?


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 22, 2015, 05:45:55 PM
Ignore my last post as I hit send prematurely and then went out for dinner!



Quote
The SX-115 build did in fact use Ceramic Z5U 10% capacitors for the selectivity switch.

Note the photo's in the following url of the parts list and capacitors removed from an unmolested SX-115.  They unambiguously define exactly what kind of capacitors they are as well as what was defined in the build list.

http://k9axn.com/_mgxroot/page_10879.html

Carl, if the caps in your radio are not as shown in the photo, they may have been replaced already.

For all other SX-115 owners, I would suggest you check your radios for the Ceramic caps and replace them.

Note, I described the use of the capacitors as a design error because they were used in all SX-115's as defined in the parts list.  I Probably should have defined the use of those caps as a manufacturing selection error even though they were listed in the parts list.  Anyone that knows me also knows that I consider the Hallicrafters design team the premier group of the era.

Nice photos but Erie nor anyone else was using Z5U anything at least thru the 1961-64 production years of the SX-115 according to my various Erie industrial catalogs. Id suggest that yours are from a very late production in a later year to finish up remaining SX-115 parts and not waste anything.

Quote
The capacitors used in the SX-117 and even late SX-100's were called paper but were actually high quality General Instruments film capacitors that are as perfect today as when they were installed.  

All of the Hal radios including the SR-150, SR-400, SR-400A, SR-2000 and others called capacitors PAPER to qualify them as components that participate in circuits that require stable, predictable, and linear components.  Both the paper and film caps fit that description.  The ceramic Z5u does not --- as published in any research paper or formal design document.

Please observe the url    http://k9axn.com/_mgxroot/page_10880.html

You will find the unmolested SX-117 photos of the selectivity switch and parts list revealing.

Those capacitors that you described as leaky paper were probably as good as new when you replaced them.

They are precisely as I described; High quality General Instruments FILM capacitors.  Dissect one for yourself.  Test them for Q, DF, leakage, aging, hysteresis, change with applied voltage, inductance, self resonance, whatever you have instruments to test for.  They are 100% qualified to participate in that resonant filter circuit; THE Z5U are absolutely not.  Every vendors engineering team was informed of the limitations of the Class 3 ceramics when they were first marketed over 50 years ago

Again nice photos but that series of caps did not exist, that I can find, during the 1962-64 years of SX-117 production. The fact is that the first year or two appear to have used Pyramid paper caps which became part of GI. The 118 series is indeed likely mylar and came out at or after the end of historical quoted production or even later as Ive suggested for the SX-115. I can find no record of it in Pyramid or GI documentation thru 1967.

And as has already been stated it is not a resonant circuit and using your dogmatic terminology it is not debatable.

Claiming that paper means mylar in the parts list is a stretch since it simply shows it wasnt updated during production runs which was not uncommon in the industry. As a National Service Manager I had to fight to get an up to date manual with schematic and parts list for the HRO-60 which was likely the longest produced model radio on record.

Unless you have first hand documented knowledge of what those part # pertain to and running production order dates we will never have the full picture no matter how much grandstanding is done.

Carl


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 22, 2015, 05:49:31 PM
Quote
What is the difference between testing for Q and DF?

Ask Jim.....it is his comment


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 22, 2015, 06:15:56 PM
Let's call the troups back and regroup.
This is my K9AXN's statement NOT Carls.  Read his statement that I attached at the end of this note.

---------------------------------------------------------------------------------------------------------------------------------

Carl, I again respectfully disagree with your above assertion.

The capacitors used in the SX-117 and even late SX-100's were called paper but were actually high quality General Instruments film capacitors that are as perfect today as when they were installed. 

All of the Hal radios including the SR-150, SR-400, SR-400A, SR-2000 and others called capacitors PAPER to qualify them as components that participate in circuits that require stable, predictable, and linear components.  Both the paper and film caps fit that description.  The ceramic Z5u does not --- as published in any research paper or formal design document.

Please observe the url    http://k9axn.com/_mgxroot/page_10880.html

You will find the unmolested SX-117 photos of the selectivity switch and parts list revealing.

Those capacitors that you described as leaky paper were probably as good as new when you replaced them.

They are precisely as I described; High quality General Instruments FILM capacitors.  Dissect one for yourself.  Test them for Q, DF, leakage, aging, hysteresis, change with applied voltage, inductance, self resonance, whatever you have instruments to test for.  They are 100% qualified to participate in that resonant filter circuit; THE Z5U are absolutely not.  Every vendors engineering team was informed of the limitations of the Class 3 ceramics when they were first marketed over 50 years ago. 

Have a great day!

Kindest regards Jim K9AXN


------------------------------------------------------------------------------------------------------------------------
CARLS statement:

Tom is doing such a good job I wouldnt even have responded but Jim cant seem to help himself by taking cheap shots with no provocation, a trait Ive followed of his for many years. And yes Ive called him out several times elsewhere on various subjects so to him Im belligerent since he cant tolerate being questioned and will never back down or admit an error. Mr Smiley Perfect.

I doubt if Jim ever had a "friendly question" when in a discussion that he is losing; more like a guy with a preachers smile and a knife ready for your back. He has been trying to sell this idea for awhile and has been shopping it around. This time it hit the ground....hard.

As far as the SX-115; the claim the filter caps are Z5U's is completely bogus, at least in mine, as they are 10% "close tolerance" as called out in the manufacturers catalogs of the time. Z5U wasnt even a common term then since the EIA standards hadnt been set yet and they were simply called Hi-K by most and undefined in many cases.

My SX-115 selectivity curves are very close to the manual and the receiver is used often with a HT-32B and NCL-2000 for vintage CW, SSB and some AM and is a pleasure to use and absolutely not selectivity hampered. Halli seemed to come out with a real good radio every 10 years or so starting with the SX-28. 

The SX-117 came out a year after the SX-115, which was a very hard sell, and the claim they used film caps is more pure BS if the parts list is to be believed as they call out paper tubular and that is certainly what they looked like and leak tested in mine as I replaced with generic yellow film caps I matched to within 1% of each other. Oh and yes, surprise surprise I have both models and can easily refute a lot of nonsense here. The SX-117 is quite a bit inferior in many areas, especially selectivity, but certainly far better overall performance than most of the bottom dwelling stuff Hallicrafters were turning out in the 60's and on the way to oblivion with most of the other leaders of the past.

Both manuals even state that the selectivity steps are a matter of Q, coupling and resistance and it is obvious by examining the circuit that resonance is NOT a factor since it is an RC circuit and not LC. Halli used the same method as far back as the SX-96 and National in the NC-300, both around 1955 introductions. Halli simply copied a good idea or maybe there was some cross breeding going on(-;

Another thing Jim doesnt understand is the voltage used to determine capacitance in manufacturing is very low and likely at no more than 100 Hz at those capacitance values. Of course there will be some shift with real vacuum tube voltages which is why the Z5U +80 % variable is quoted and much higher than needed cap voltages were available to mitigate the shift if needed. The Z5U is also stated that it is not to be used in frequency determining circuits or where high tolerance is required. But that is all moot since a Hi-K cap was not used in the filter.
In addition, if you read his various pronouncements as if they were actually factual you would find that the voltage/capacitance shift charts used as examples are for 16V SMT devices, not 500-1000V disc ceramics.

I completely agree that the REAL Halli engineers knew what they were doing with the SX-115 and did it reasonably well but I still prefer my NC-300 on AM after a few mods.

Have at it Tom, Im sure most on here will enjoy it. Jim is seething with anger since he didnt expect a high level challenge to his competence...the more platitudes and sweet words he uses the more dangerous he becomes; watch your back.

I consider this whole exercise over...

Carl
----------------------------------------------------------------------------------------------------------------------------------------
CARLS second statement:

The SX-117 came out a year after the SX-115, which was a very hard sell, and the claim they used film caps is more pure BS if the parts list is to be believed as they call out paper tubular and that is certainly what they looked like and leak tested in mine as I replaced with generic yellow film caps I matched to within 1% of each other. Oh and yes, surprise surprise I have both models and can easily refute a lot of nonsense here. The SX-117 is quite a bit inferior in many areas, especially selectivity, but certainly far better overall performance than most of the bottom dwelling stuff Hallicrafters were turning out in the 60's and on the way to oblivion with most of the other leaders of the past.
 


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 22, 2015, 06:22:45 PM
Response for G3RZP

Q and DF --- essentially just opposites.  The shack may have an instrument that measures one or the other.

Guess I don't know how to use Quote --- what a mess I made of this thing.

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 23, 2015, 01:16:38 AM
Q and DF - not opposites but reciprocals. So you only need to measure one of them.....


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 23, 2015, 07:36:32 AM
Hi Pete,

Thanks for the semantic correction.

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 23, 2015, 10:18:06 AM
Good morning Carl,

Here is a Quote from your statement:

Another thing Jim doesn't understand is the voltage used to determine capacitance in manufacturing is very low and likely at no more than 100 Hz at those capacitance values. Of course there will be some shift with real vacuum tube voltages which is why the Z5U +80 % variable is quoted and much higher than needed cap voltages were available to mitigate the shift if needed. The Z5U is also stated that it is not to be used in frequency determining circuits or where high tolerance is required. But that is all moot since a Hi-K cap was not used in the filter.
In addition, if you read his various pronouncements as if they were actually factual you would find that the voltage/capacitance shift charts used as examples are for 16V SMT devices, not 500-1000V disc ceramics

-------------------------------------------------------------------------------------------------------------------------------
My response:

1.  Z5U and High K are by any definition the same --- the Z5U is a high K capacitor.  What was called High K before the
     EIA published standards documents were class 3 ceramic --- the Z5U.  Medium K was essentially Class 2 i.e. the X7R.
     High K capacitors were used in the SX-115 selectivity circuits.

2.  There is no +80 tolerance code for these caps however there is a +80 -20 tolerance code.  You forgot the -20.  In
     either case the +80 -20 has nothing to do with the change in capacity with temperature.  The Z5U is +22% -56%.
     The +80 -20 is the tolerance you can expect when you install the cap at room temperature.
     Your theory that the +80% offsets the negative change in capacity with temperature forgot that you might get a -20%.

3.  You state the capacitors that I speak of regarding the variation in capacity with applied voltage are 16V SMT and
     if I would test capacitors rated at 500v to 1000v, the variation would be minimal. 
     The following url demonstrates Z5U .022uf 1000v ceramic disc capacitor being tested for the voltage effect.  The
     capacitor is rated at 1000v and the meter is set for 500v.  The maximum voltage available is 600v.

     Note:  It starts at 0v .0246uf, 5v .0256, 30v .0259, 75v .0228, 85v .0177, 120v .0154, 250v .0064, 300v .0057,
     500v .0026uf.  It has lost 90% of it's capacity at half it's rated voltage.  If I had used that capacitor even for bypass at
     300v it would have been a .0057uf when I expected a.022uf cap.  Would you use it?  The Ceramic MLCC caps are
     universally just as bad even the 500v caps.  The point is, with high K caps Z5U etc, you don't know what you have
     unless you test them.  With Polypropylene you know.

   http://www.k9axn.com/capacitor/variable%20voltage/junkcap.avi

Have a good day.  I will respond to one issue at a time with proof of concept so each can be closed without confusion.

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 23, 2015, 10:35:05 AM
Forgot to send the capacity change with voltage video of the Polypropylene capacitor.  Note it's absolutely stable.

 http://www.k9axn.com/capacitor/propylene001.avi

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W9NVN on March 24, 2015, 04:01:16 AM
Jim K9AXN, I must agree with Tom in that the receivers were ok in their time,however the fact is that the type of selectivity they employed is by nature broad as a barn door!! When put next to a Collins 75A-4 or 75S-3, the great SX-115 falls short in the selectivity area no matter what type of cap you use..Now let me ask you, have you taken two SX-115s and installed your cap mod in one and did a A B test with one unmodified and measured the results and if so what were the results?? Any improvement as I see it would be small at best since no matter what cap you employ are limited by the circuit design which dates all the way back to the 1950s!!As I recall,they came out with this in 1961,so the design is 1950s era.. The shape factor of the SX-115 is extremely poor by today's standards and no match for my Icom 751A, -6db 2.4 khz and 1.75 shape factor!!! The SX115 had a number of design flaws such as the poor choice of the 6BA7 mixer which I found to be too noisy..I have had 5 of the receivers in my time and still have one I use for a monitor. Also, I originally purchased one of the SX-115s from one of the original design engineers and as I was making the purchase at his QTH,I noticed he had another SX-115 in his work area,and when I inquired about that one, he said he modified the 6BA7 mixers to 12AT7s as I recall and I noticed the rx had a much lower noise floor then the one I purchased.So much for the poor 6BA7,it was so good he changed to the 12AT7!! Also note they did not use the 6BA7 after the SX-115 which is why the SX-117 has a much lower noise floor.. Any way the selectivity circuit in the SX-115 is very lossy,for example when you are working cw in the 1kc position and the signal is 10/9 and you switch in the .5kc filter, the signal drops down to S 2 OR 3,way too much loss,again that was 1950 design..Did your cap change do anything to correct the loss???? Now I wish to correct you on the Hallicrafters engineers being the best in the business and even better than Collins!? Let me point out the HT-32,32A,32B had about 5 stages of audio before it hit the 6146 and all of this audio and no,none, not even a hint of ALC!!?? Same with the HT-37..You could never crank the audio past 2 or 2 and half before flatopping due to the lack of ALC or compression,these lacked talk power,no punch at all.. Ever listen to a 32S1 driving a 30S1,now that is talk power..The engineer who designed the rf section in the HT-32 was W9OYU,I think he was a cw man..How about the HT33B,no ALC, 800 watts pep out max,and a whopping 117 volt ac primary with a 22 amp circuit breaker,who had a dedicated 117 volt ac line in their shack @22amps??? The list goes on..Let tell you what former chief engineer Bob Orwin told me one time I was at his QTH in Berwyn IL," Hallicrafters was a schlock outfit"he said that not me..The rigs made before the 60s were much better IMHO,I feel after the 115 they kind of went down hill after that..I also had the SR-400A and HA-20 which Mr.Orwin went through and corrected many of the errors, it was a pretty good rig with dual receive and noise blanker.My friend Ken Miller had the SR-400 you said was the desirable one,maybe I should of got that one..Any way to sum up why didn't you catch this alleged error in 1962 when these rigs were in vouge rather than 2015? Mostly I would be interested in the A-B if you preformed that..I will say the SX-115 did have some of the best strong signal handling characteristics of any receiver around,and a very good notch filter..However,on this one I must side with with Tom W8JI as he is rite on track.I must conclude even if you change the cap,the -60 db point is too wide and the shape factor is less then stellar!!! Plus the loss when you kick in the .5kc filter,a weak cw signal you will loose in the noise..Please accept my comments in the correct spirit,just my opinion and all comments are in due respect..Just not sure of the value of this once again in 2015,I mean if it gave you a shape factor of something like 4.2 that might be of benefit,any way lets see what you found..Respectfully and 73, Nick W9NVN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 24, 2015, 06:57:00 PM
Good evening Nick,

First I would like to compliment you for the courtesy you show in your response, even though you have serious and opposing opinions.  That's how the QTH Hallicrafters list works.  

I will ask you to work with me to bring this thing to conclusion.  But first, I need to understand how you interpret the selectivity chart focusing on the .5Kc and 1Kc switch settings.  Use the 2Kc setting to determine the shape factor.

Carl, I received 23 unsolicited responses regarding the capacitors in the SX-115 and all have the Z5U that you see in the photo on my site.  Even more responded with the SX-117.  Every SX-117 had the General instruments film Caps.  Also, the caps that you see in the photo of the Z5U's are of the caps that I removed from an SX-115 that has been waiting in my inventory for 15 years to restore and yes, I have done a comparison regarding selectivity.
See http://k9axn.com/_mgxroot/page_10881.html

This is an issue that I won't address again.


Nick, why wait till 2015?  Because the complaints have become numerous enough to draw attention.  Why?  Because those caps loose 5% of their capacity every year compounded with the radical change in capacity with temperature have damaged the selectivity design.  I will fully explain the reasons later.  Have to see how this goes.

-----------------------------------------------------------------------------------------------------------------------------------
On the fun side:

You had an SR-400A and Bob Orwin worked on it.  Bob was a remarable man.  Why did you sell it?

The reason that I feel the Hallicrafters guys were extraordinary is they did so much with so little whereas the Collins people probably had a credit card.  No other design shop successfully built a 2000 watt desk top transceiver.  

Have a super evening Nick and get back to me with your interpretation of the shape of the selectivity chart.

Kindest regards Jim K9AXN    


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 24, 2015, 07:51:32 PM
I believe we are over-focusing on the coupling-setting capacitors because:

1.) The 390pF caps and the inductors primarily determine IF frequency.

2.) The reactance of the .01's are pretty low. If they drift a little, or have a little leakage, they don't change the overall system much.  After all, Hallicrafters takes the capacitance from a dead short (500Hz position) to 4700 pF (5kHz position) with a minimal move in peak frequency.

3.) As the frequency progressively shifts, bandwidth increases at an even greater rate. Who cares if the center moves when the BW has gotten wider at a much higher rate than the move?

4.) Each coupled pair stage has relatively poor skirts. This is caused by several things, the least worry being the mutual coupling capacitor system. The inductors have a Q in the 150-200 range, the ground end of the inductors have a circuitous path through bypass capacitors, and they load the transformers with resistors to compensate gain and prevent double hump curves.

5.) I have a time understanding all this focus on the type of 10,000pF capacitors used to shunt the high side common of the 390 pF capacitors in the SX115. The IF can RF paths have resistors and .02 bypasses hanging on them in series with the rather lengthy path. Specifically the path is IF top > 390 > ~.01uF selectivity control > ground > .02 bypass > IF coil bottom.  Hallicrafters at least partly cleaned up that loop in the SX101 by bumping the bypass up to a .047 and isolating the RF through the B+ line with a 3300 ohm.

Not that the path length through the bypass capacitors means much, because they are still just 50 kHz L/C circuits.
 

If the capacitors are bad, I would certainly change them. If someone "feels" changing the caps from good normal disc caps somehow makes a silk purse out of the sow's ear of using 50kHz cans for selectivity, I really cannot get into that. They feel what they feel.

These were very good receivers **in their day**. I grew up on receivers like the SX101 in the 70's, I had no money I had to make do. I went the whole L/C filter route, augmenting the 50kHz IF with a huge L/C filter with well over a dozen cans. It was still, unfortunately, just an L/C filter.

The best thing I ever did with my SX101, besides building a 1st IF noise blanker for LORAN, was ditching the 50kHz L/C system and building 1650 kHz lattice filters.

I think it is a little misplaced to spend so much time criticizing a 50 year old design because of a capacitor that is probably one of the least critical and important parts in the IF system. The resonance path is IF coil top > 390 pF > ~.01uF > ground > .02uF bypass > IF can bottom. Some of the paths have resistors inserted. Good luck on making a substantial difference by changing the type of one component in that lengthy loop and Q<200 inductor. 

73 Tom



Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 25, 2015, 01:47:13 AM
The tolerance of the coupling cap is fairly important for a reproducible response, but only if the Q is controlled - which means tested on each component. That's an expensive thing to do, and probably an unlikely thing for Hallicrafters to do for an amateur receiver - the SX73 would have been a different matter.

But considering the age of the receiver, one has to question whether the coil Qs are still the same as at manufacture? It is by no means unknown for moisture to have got in over the years, leading to a degree of corrosion that is extremely difficult to see, and if the coils are wax impregnated, very likely to have happened. Even polystyrene varnish is not totally proof against this as the plasticiser eventually tends to evaporate. If the Q has changed, then the circuit kQ will have changed and thus the response....

After all, we still don't have that much experience of electronic components to guarantee a life for all of them to 50 plus years while maintaining all their original characteristics, even though many do very well - I have some SN7404N in my remote antenna switch that are date coded 1969. Probably cost one or two dollars each then - when dollars were worth a lot more than now! I suspect many of the resistors in the radio will have drifted way past their 20% original tolerance....

Anyone interested in the theoretical effects of the various methods changing coupling in coupled tuned circuits on centre frequency can find a thorough analysis in Sturley's book 'Radio Receiver Design, Part 1'.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 25, 2015, 07:34:21 AM
The tolerance of the coupling cap is fairly important for a reproducible response, but only if the Q is controlled - which means tested on each component. That's an expensive thing to do, and probably an unlikely thing for Hallicrafters to do for an amateur receiver - the SX73 would have been a different matter.

That's right. I think the problem in this thread, which seems to take way too much time for such a simple system, is assignment of more importance to the type of capacitor than is required for what is really a pretty sloppy basic circuit.

The filtering is actually 1940's technology. 

If you look at the resonance path in an SX115, the path includes the inductor and a 390 pF, both with several thousand ohms reactance, in series with **multiple** much lower impedance components. That circuitous path varies with the particular inductor's position in the radio, it is not as "clean" in the SX115 as in later receivers, and it is nowhere as clean as the path in a ARC5 receiver for nav beacons (the Q-5er with 85kHz cans).

As I said, the path for resonance is through resistors and bypass capacitors that have significant reactance and resistance compared to the .01's being hammered on, the exact path depending on the particular stage.

Even the NC300 and 303 National return the inductor resonance path through .047uF bypasses (like the SX101), but National bumped the reactance of the coils and resonating capacitors several times higher than the Hallicrafters, so the .047's mean less.

None of these receivers enjoy any particular selectivity improvement by using high quality parts than factory. They are inductor Q limited. Replacing defective parts is another story.

Selectivity is changed with a multifaceted approach of loading the inductors with resistors, as well as changing mutual coupling via the shunt capacitors being discussed.

This was state of the art for Ham receivers using LC selectivity, although the National Radio design was actually better. 
Quote
But considering the age of the receiver, one has to question whether the coil Qs are still the same as at manufacture? It is by no means unknown for moisture to have got in over the years, leading to a degree of corrosion that is extremely difficult to see, and if the coils are wax impregnated, very likely to have happened. Even polystyrene varnish is not totally proof against this as the plasticiser eventually tends to evaporate. If the Q has changed, then the circuit kQ will have changed and thus the response....

I have around 10 Hallicrafters receivers, and the coils all pretty much measure the same out of circuit. It takes a lot of wire to resonate with 800 pF (150pF in the National), and iron technology wasn't as refined in the 1950's when these cans were likely developed.

The skirt response is so wide with a few L/C stages and the reactance of the shunt bandwidth caps are so low compared to the 390 pF they are in series with, it is all a moot point. It like discussing oxygen free copper and audio sound with tube type audio people. Just the other day someone was telling me how much better his guitar amp sounded with a 5AU4 rectifier than a 5U4, because the lower emission acts like a volume limiter.

73 Tom


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 25, 2015, 09:32:43 AM
Tom,

I will ask you to humor me.  At least allow me the opportunity to make my point.  I will prove each point given the opportunity.

I asked Nick to review the selectivity chart and tell us what it meant i.e. the 60db shape factor.  Armed with that information, lets compare it to what those radios exhibit today.  I consistently hear 6/1, anything to 1 except what the chart claims.  You and Nick asked me why I waited until 2015.  Valid question --- because the problem progressively became worse over the years until the concensus was, It's broad as a barn door.

Do you think even for a moment that the Hal guys would have gotten away with the vast discrepancy between performance and published data???  Wouldn't there have been a critique by someone challenging the chart when it hit the market???  Need a bit of common sense here.

The fundamental issue here is the pattent misuse of components.  

-----------------------------------------------------------------------------------------------------------------------------------------
Nick, this may answer part of your issue with the significant drop in the s meter when changing from the 1Kc to .5Kc position.  Little history.  They changed the 2.2pf coupling cap to 3.3pf in the SX-117 and about a year later changed it back to 2.2pf because it didn't make the .5Kc parameter.  Don't know what happened after that.

Changing C86 to Polypropylene helped with the .5Kc issue but the delta was still uncomfortable so changed the 2.2pf coupling cap to a ceramic plate NPO cap.  Now very little change from 1Kc position.  The original 2.2pf cap had a Q of 250 and the ceramic NPO Q of 3000.  After cap changes to this radio all positions closely follow the selectivity chart.
NOTE: I have yet to replace the second 2.2pf coupling cap in the extended chassis --- might be the final fix for the .5Kc position.

Tom, in the SX-115, C86 is in series with the 390pf cap and coil.  It participates in the circulating current, however It does not participate in the common leg so there is no coupling except the 2.2pf cap between the 390pf caps.  All of the other caps are shorted around.  C86 is a .02uf class 3 Z5U type capacitor and the tolerance is -20 +80.  That is the EIA Z tolerance spec. Has nothing to do with temperature but what comes out of the box.

That capacitor today measures .016uf not .02.  Is that a problem?  No not perceptible.  ITS Q is 250 at 100Kc as compared to a Q of 2900 at 100Kc for a Polypropylene cap.  The Polypropylene has 10 times the Q!  That capacitor participates in a tuned circuit.  Is there an effect?  Well before replacing it the .5Kc position had a 3.5Kc at-47db and after 1.5Kc.  Manufactures and research protocol specifically states no Class 3 ceramic Z5U type caps are to participate in tuned circuits.  That cap is Class 3.  
---------------------------------------------------------------------------------------------------------------------------------------
Your Quote Tom:

 3.) As the frequency progressively shifts, bandwidth increases at an even greater rate. Who cares if the center moves when the BW has gotten wider at a much higher rate than the move?
-----------------------------------------------------------------------------------------------------------------------------------------

My response:  Think about what you just stated.

---------------------------------------------------------------------------------------------------------------------------------------

I asked you or anyone to calculate the center frequency of each position and have not received an answer.  It's incredible simple math and the answerer's agree closely with my original numbers regarding the shift of the center frequency.  The shift in frequency anchoring the band pass combined with increased band width place the 6db point on the lower skirt 500 cycles from the carrier.  This was presented in the original post.  

You say that the magnitude of the capacity of the coupling caps precludes any distinguishable difference.

With the capacity of the three capacitors combined being .0247uf reduced to .018uf by age and temperature, the new 1Kc band pass becomes much closer to 2Kc than 1Kc.  Where did the 1Kc position go and how about the other positions?

Thanks for listening Tom and please be patient.

Kindest regards Jim K9AXN    
      


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 25, 2015, 09:54:55 AM

>I have around 10 Hallicrafters receivers, and the coils all pretty much measure the same out of circuit.<

About 50 years ago, a fellow apprentice at Marconi's got himself an AR88, which at that time would have been no more than 25 years old at the most. He could never get the IF response as per book, as when the IF transformer Q was measured, it so low that it was obviously the cause when you did a few calculations. So he un-wound every transformer, noting the number of turns and the wire type that he took off and the spacing between windings.  He then chatted with one of the ladies in the coil winding department, passed over a couple of bottles of gin (they were actually home brewed and distilled quite illegally by a little old lady friend of his mother who lived up the street from him  - she had an illegal still in her cellar which dated from Victorian times and produced VERY good gin!) and the transformers were rewound.

The AR88 aligned beautifully after that.....response exactly as per book. Shortly after a whole load of AR88 spares appeared on the surplus market, including IF transformers. But they may have been no better....


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 25, 2015, 10:03:36 AM
A little humor helps Pete.  Thanks.

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 25, 2015, 10:18:38 AM
Jim,

That (true) story goes part of the way to explain why I am old and cynical about electronics in general and radio in particular. One thing I have learned is that for a repeatable design in production with coupled tuned circuits, coupling capacitors need to be fairly tight in tolerance: top coupling with capacitors of under about 5pF is very dependent on capacitor value, and is really generally only acceptable where a loose tolerance and low Q can be used.

But despite that, top capacity coupling is used in a lot of manufactured gear and in many cases, leads to marginal performance.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 25, 2015, 11:26:28 AM
This is pretty difficult to read, but I will try:


Tom,

I will ask you to humor me.  At least allow me the opportunity to make my point.  I will prove each point given the opportunity.

I asked Nick to review the selectivity chart and tell us what it meant i.e. the 60db shape factor.  Armed with that information, lets compare it to what those radios exhibit today.  I consistently hear 4/1, 6/1, anything to 1 except what the chart claims.  You and Nick asked me why I waited until 2015.  Valid question --- because the problem progressively became worse over the years until the concensus was, It's broad as a barn door.

Do you think even for a moment that the Hal guys would have gotten away with the vast discrepancy between performance and published data???  Wouldn't there have been a critique by someone challenging the chart when it hit the market???  Need a bit of common sense here.

Yes, I certainly think they "would get away with" a discrepancy for multiple reasons. 

Quote
The fundamental issue here is the pattent misuse of components. 

Nick disagrees with some of my statements but he challenged them in a courteous tone.  For that I respect him and the dialog is productive.  I may be wrong in some things but at least give me the courtesy to prove it up.

This is technical. When technical becomes personal or social, it loses meaning and takes too long. 

Quote
They changed the 2.2pf coupling cap to 3.3pf and about a year later changed it back to 2.2pf because it didn't make the .5Kc parameter.  Don't know what happened after that and can't remember what radio.  Will have to dig through the engineering notes.

The 2.2 and 3.3 pF change the 500Hz BW coupling. They have rapidly decreasing effect on wider bandwidths.

Quote
Changing C86 to Polypropylene helps a bunch with the .5Kc issue but the delta is still uncomfortable.  On my radio all positions closely follow the chart after the change. 

C86 is one of the bypasses I spoke of.  The entire circuit is a bit sloppy, in that the path for the resonance and coupling is inductor top>390>>selectivity caps or short>ground>B+ bypasses>inductor bottom.

The ideal capacitor for C86 would be one with zero impedance at 50 kHz, unless the engineer originally planned for some finite value.



Quote
Tom, in the SX-115, C86 is in series with the 390pf cap and coil.  It participates in the circulating current, however It does not participate in the common leg so there is no coupling except the 2.2pf cap between the 390pf caps.  All of the other caps are shorted around.  C86 is a .02uf class 3 Z5U type capacitor and the tolerance is -20 +80.  That is the EIA Z tolerance spec. Has nothing to do with temperature but what comes out of the box.

That is absolutely not correct. That is the maximum delta for the capacitance over the temperature range.

Z is the low temperature
5 is the upper temperature
U is the maximum allowed capacitance change over that range

It is maximum allowed drift over a thermal range change.

X7R would be preferred, but for the small thermal changes in a radio room, coupled with the fact most capacitors are significantly better that minimum, and coupled with the fact the capacitor has a small effect on the system.....a Z5U would likely be fine.

 
Quote
That capacitor today measures .016uf not .02.  Is that a problem?  No not perceptible.  ITS Q is 250 at 100Kc as compared to a Q of 2900 at 100Kc for a Polypropylene cap. 


That doesn't matter much when the rest of the system has a net Q<150. It also doesn't matter much when the capacitor reactance is low compared to the tuned circuit reactance.

Just as a random example, suppose there are two capacitors in series. One is 7500 ohms with a Q of 300, and one is 500 ohms with a Q of 100. The ESR's of the two components are 25 ohms and 5 ohms. So out of the 30 ohms loss, the larger capacitor with a Q of 100 is only responsible for 17% of the loss.

This is why I think it is misplaced to focus only on one part, and think the least critical part has a huge effect.

So the TC issue was misunderstood, and so is the net effect of component Q.


Quote
The Polypropylene has 10 times the Q!  That capacitor participates in a tuned circuit.  Is there an effect?  Well before replacing it the .5Kc position had a 3.5Kc at-47db and after 1.5Kc.  Manufactures and research protocol specifically states no Class 3 ceramic Z5U type caps are to participate in tuned circuits.  That cap is Class 3.   

The 390 pF are the tuning capacitors. The bypasses are bypasses. The coupling caps are shunt components working in series with the junction of two 390 pF caps in series. The BW adjustment caps are either a short, .0247uF, .0147 uF, .01uF, or .0047 uF. The BW adjustment caps are in series with either resistors, or with bypass caps and resistors, depending on the position in the radio. All of that is in series with inductor ESR, which is substantial because of inductor loss. 

Quote
My response:  The increased coupling results in a wider bandwidth, but at a LESSER rate than the frequency shift and not enough to bracket the new center resonant frequency.  Why would they add load resistors if the band width increased at a more rapid rate than the frequency shift?  The increased coupling and new resonant frequency are equal only in the 1Kc position, not the 2Kc, 3Kc of 5Kc.  Those positions need the additional load resistors to equalize the width.

I asked you or anyone to calculate the center frequency of each position and have not received an answer.  It's incredible simple math and the answerer's agree closely with my original numbers regarding the shift of the center frequency.  The shift in frequency anchoring the band pass combined with increased band width place the 6db point on the lower skirt 500 cycles from the carrier.  This was presented in the original post. 

I cannot afford to work for you by doing assignments. I am way behind on paying work.

I do have something archived that basically explains how the system works:

http://www.w8ji.com/selectivity_hallicrafters.htm

Quote

 You say that the magnitude of the capacity of the coupling caps precludes any distinguishable difference.

With the capacity of the three capacitors combined being .0247uf reduced to .018uf by age and temperature, the new 1Kc band pass becomes much closer to 2Kc than 1Kc.  Where did the 1Kc position go and how about the other positions?

That is correct. All drift really does is change the selectivity, but the capacitors are one tiny part of a very complex circuit. There are more than a half dozen things altering selectivity when that switch is moved, including resistors that are changed. Changing one or two parts, especially the least critical parts, isn't likely to have a dramatic effect unless the parts are just bad compared to when the radio was new.

With a $100K budget we could probably redesign the SX115 circuit to be as good as any L/C filter using modern components could be, but it would still be an L/C filter. It would never approach a 4 pole crystal filter, unless we had a row of IF cans extending almost into the next room. This is because each resonator in a crystal filter is Q>100,000, compared to Q<200 in a L/C filter.

I think the burden falls on the person who has time to really analyze the circuit, and not just pull "this part should only be used in this place" statements out of the air.

Even the length of time this has taken is just a superficial overview, but I hope it has clarified the meaning of capacitor specs and why the effect of a component depends on the contribution of that component to a complex system.

I don't believe the engineers made any serious mistakes, especially one correctable by a very elementary, casual, and very incomplete look at the system. They worked with what they had to work with in 1950, or whenever the actual design evolved. I think there are actually far more mistakes and oversights, and lack of operational understanding,  in the analysis critical of the 1950's engineers.

It is just an L/C circuit filter. It will never be a crystal filter.

73 Tom


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: AC2EU on March 25, 2015, 02:16:50 PM
The SX-115 selectivity horse is dead guys...   ::)
Can we give it a nice burial?  :'(


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 25, 2015, 03:31:45 PM
The SX-115 selectivity horse is dead guys...   ::)
Can we give it a nice burial?  :'(


I think there is always something to be learned when people disagree, or when things are discussed. If everyone agrees, no one learns a thing.





Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 25, 2015, 04:23:17 PM
Good afternoon Tom,  I agree with you on your comment about learning.  Just think, we could agree on everything, get bored and talk about the flower garden instead.  All in jest Jim --- truely.  Have a great day.  

Take a look at the following url http://wiki.xtronics.com/index.php/Capacitor_Codes.

Note table 2, the tolerance codes and table 3 the dielectric codes for the Z5U.

The U in the Z5U is the capacity change over the temperature range which is +20 -56.

The +80 -20 stamped on the cap is the Z in the tolerance codes and has nothing to do with the temperature range.  If you can find a +80 -20 in the 3 digit codes i.e. please forward it.  Is the document in the url correct or incorrect?  You can't simply manufacture a coding system.  No Z5U or ceramic cap will be +80% unless it catches fire and shorts.

----------------------------------------------------------------------------------------------------------------------------------------

C86 is a dual function capacitor Bypass and coupling --- the circulating current passes through it in the primary circuit.  If that is not so, please explain.

----------------------------------------------------------------------------------------------------------------------------------------

Here are the calculated resonant frequencies for each of the selectivity positions using .025250mh.

.5Kc ---- 50.743  (50.000Kc carrier)  pass band (50.500 to 50.1000) for .5Kc  
1Kc  ---- 51.142  (50.000Kc carrier)  pass band (50.642 to 51.642) for 1Kc
2Kc  ---- 51.411  (50.000Kc carrier)  pass band (50.411 to 52.411) for 2Kc
3Kc  ---- 51.697  (50.000Kc carrier)  pass band (50.197 to 53.197) for 3Kc
5Kc  ---- 52.806  (50.000Kc carrier)  pass band (50.306 to 55.306) for 5Kc

Because the values of those capacitors changed the center frequencys would now be.

.5Kc ------------ same
1Kc -------------  (50.000Kc carrier)  pass band (50.243 to 51.243)
2Kc -------------  (50.000Kc carrier)  pass band (49.743 to 51.753)
3Kc -------------  (50.000Kc carrier)  pass band (49.243 to 52.430)
5Kc -------------  (50.000Kc carrier)  pass band (48.250 to 53.250)

The pass band would have straddled the carrier and if you were copying SSB, you would have to shift the BFO everytime you changed the selectivity setting.  Don't think their right?  get your calculator out.

If your considering the coupling effect changing the frequency it's not going to fly.  The two sides of that filter are essentially in phase coupling resistance back with very little J.  Also, with the calculations clearly resulting in the resonant points being where you would expect them for proper operation, it's going to be a hard sell otherwise.

I'll say it again those capacitors are used for coupling and to shift the center frequency to accommodate the expanded band pass so the 6db point on the lower skirt remains approximately 500 cycles from the carrier.  Their values are chosen to shift the frequency to the appropriate position.  

They provide adequate band width expansion only in the 1Kc position.  In the 2Kc, 3Kc, and 5Kc position the series resistors provide the added load --- reduced Q necessary to expand the pass bands to the appropriate width.

That's how it works.  

Everything is there.  My radio works as the selectivity chart defines it.  It's not as broad as a barn door it has a shape factor of approximately 4.1 to 1.  If that satisfies you replace the caps.  Otherwise attach two hinges and install it on your neighbors barn.  That's a great radio in all respects.  The Hal guys simply overlooked something.

Tom, I wish you well and hope you try the change.  You have nothing to loose and much to gain.

Good luck and Kindest regards Jim K9AXN          

 


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 25, 2015, 05:13:22 PM
Nick,

Replace the caps with Polypropylene not Polyethylene.  Much beter caps.  The selectivity factor is about 4.1 to 1.

You are going to like that radio.

Good luck --- Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on March 26, 2015, 11:14:44 AM
There was an article in Electric Radio Magazine a few years back who's author felt that changing the Sprague bumble caps in the SX-101 series IF selectivity circuits would ruin the selectivity curves since he felt that the engineers had spent an enormous amount of time including the internal stray inductances for those caps in endless calculations... his theory was that any modern replacement cap would be detrimental to the original filter design.  He obviously had too much time on his hands, but the publisher had material to fill a page, and that seems to be the rage in publishing these days.

Not to pick nits, but those ceramics are known to be unstable, they can become microphonic and may be hydroscopic.  If I'm going to waste my time restoring a 600 dollar radio, I'm not going to lose a lot of sleep of whether a 15 cent part might be better than a 13 cent part.  Just use the best quality material that is available that is suited for the application. Hallicrafters engineers designed equipment to be affordable.  I can see where Z5U might be preferred for RF bypassing, but that dilectric would be on the bottom of my list for use in a resonant circuit. That doesn't mean I don't think it work, but there are better choices.

I'm not arguing with Tom--he may have valid points that the original .01 ceramic caps would meet specs. The radios obviously worked when they left the factory, but it is now 70 years later.   I know that the Sprague bumble bees in my SX-101 receivers had some leakage so while doing a complete restoration they were replaced, and at a very modest cost. All in all, I was pleasantly surprised at the selectivity positions on the SX-101 receivers. Not the best shape factors, but for simple LC filters did a passable job for 1950 era technology.

Pete



Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 26, 2015, 02:17:52 PM
Nick,

Replace the caps with Polypropylene not Polyethylene.  Much beter caps.  The selectivity factor is about 2.1 to 1.

You are going to like that radio.

Good luck --- Kindest regards Jim K9AXN

I'd have to actually see that measurement to believe it, because it is far afield of anything I have measured.
 
Shape factor is typically at -3 and -60 Db points, although people are free to move that around as long as they define it. Hallicrafters defined the advertised BW at -6 dB, rather than the more traditional -3 dB, which tends to make the numbers look a little better.

The SX101 series, which is an improvement over the SX115 through better IF B+ path decoupling, is about 3-4 kHz wide at -60 dB in the 500Hz position. It is about 16-18 kHz wide in 5 kHz selectivity at -60 dB.  So if we use Hallicrafters looser -6dB bandwidth definition, the shape factor is about 7 in the 500Hz mode and 3.5 in the widest setting.

The skirt limitation is primarily in the Q of inductors and the number of inductors. Shape factors in the Hallicrafter receivers are absolutely typical of fairly good L/C filter IF systems. Even the NC300 and 303, which are a bit better designs, are not near 2:1 shape factor.

This is why, in the 1970's, I abandoned the L/C filters and moved to 1650 kHz four and eight pole filters in my SX101's.

Something else I'd like to ask you about is this statement from one of your papers:

Quote
Have you ever wondered why the 50.5Kc was changed to 50.75 in the SX-100 MARK2?
There were two reasons.  The MARK1 had a problem with the BFO feeding backward to
the AVC diode.  When the BFO was enabled, it generated AVC voltage and the S-Meter
indicated signal when there was none.  The path was from the BFO to the detector
diode V7, to T6, to T5, then to the AVC detector diode in V7.  The BFO frequency,
50Kc, was within the band pass of T6 and T7.  It passed backward through the
transformers.  Yes, T5 and T6 will pass a signal in both directions equally.  Adjusting the
transformer band pass from 50.50Kc to 50.75, 250 cycles further away from the BFO
reduced the effect but was not a complete fix.  That was yet to come in the SX-117.   

I'm having serious trouble following the logic in that text. Here is why:

1.) Two stages of IF tuned circuits have a -6 dB bandwidth of at least 1.75 kHz when in the maximum selectivity position. The move of 250 Hz would pretty much be meaningless inside a BW>1.75kHz window.

2.) More important, the BFO **has to move in direct sync with the IF center** for the same CW note pitch, otherwise the pitch changes.

Can you explain what I am missing, and how moving the IF center frequency 250 Hz reduces BFO back-feed to the AGC detector? Both 1 and 2 above exist as facts, and either one along precludes reducing BFO into the AGC with any small frequency shift.

73 Tom



Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 26, 2015, 07:10:15 PM


The quote is correct regarding the change from 50.50Kc to 50.75Kc.  What do you not understand???  Do you have another explanation?  I'm not interested in your opinion.  Electronic or physics facts are not subject to the democratic process.  We don't get to change the rules of physics by voting or with simple opinions.  

Your statement is this:

Quote
Have you ever wondered why the 50.5Kc was changed to 50.75 in the SX-100 MARK2?
There were two reasons.  The MARK1 had a problem with the BFO feeding backward to the AVC diode.  When the BFO was enabled, it generated AVC voltage and the S-Meter
indicated signal when there was none.  The path was from the BFO to the detector
diode V7, to T6, to T5, then to the AVC detector diode in V7.  The BFO frequency,
50Kc, was within the band pass of T6 and T7.  It passed backward through the
transformers.  Yes, T5 and T6 will pass a signal in both directions equally.  Adjusting the transformer band pass from 50.50Kc to 50.75, 250 cycles further away from the BFO
reduced the effect but was not a complete fix.  That was yet to come in the SX-117.   

My question is this:

You claim they moved the IF 250Hz to reduce back-feed through the two IF cans to the AVC detector. That makes absolutely no electrical sense at all for two reasons:

1.) If the IF shifted, the BFO has to shift the same amount to keep the same tone. This means the relationship between BFO and IF peak frequency does not change one bit. 

2.) Two IF cans are at least 1.5- 2 kHz wide at -6dB in the 500 Hz position. How does moving the IF center reduce AVC detector back-feed from the BFO signal when the bandwidth of the two cans is so wide?

All I asked for was a simple answer so I could understand how you arrived at your conclusions. I can't understand the IF thing.

We can talk about Q, skirts, series components, and getting crystal filter shape factors from a few L/C circuits later. I would like to learn how moving the 50kHz IF 250 Hz reduces BFO leakage first.

Thanks,
Tom


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 27, 2015, 07:52:01 PM
Hi Tom,

I need to know whether you have ever owned or worked with an SX-100 Mark1 and or Mark2.  Can't answer the questions without knowing your depth of knowlege of both of those radios. 

Would like your favorite email address to speak off line before any further correspondence.

Kindest regards Jim K9AXN
       


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 27, 2015, 09:55:29 PM
Hi Tom,

I need to know whether you have ever owned or worked with an SX-100 Mark1 and or Mark2.  Can't answer the questions without knowing your depth of knowlege of both of those radios. 

Would like your favorite email address to speak off line before any further correspondence.

Kindest regards Jim K9AXN
       

Hi Jim,

This is a real simple basic technical question.

You say the IF was shifted to reduce BFO leakage back through a pair of moderate Q resonant circuits. How can that be possible?

1.) If the IF is shifted 250 Hz, the BFO has to move to match or the CW beat will be off pitch. The net change of BFO to IF center has to be nothing.

2.) The Q of the IF sections isn't all that great. The inductors are only in the Q<200 range, and the other components and resistors load it down even more. A pair of IF cans with no load resistance at all just coupled through 2.2pF has a couple kHz BW. So even if the BFo didn't move, the BW seems to be so wide as to not make a difference.

I'm trying to make sense out of the claim Hallicrafters moved the IF 250 Hz to reduce BFO back feed. Tell me what I am missing, and why or how it would change.

Thanks, Tom   




Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KG8LB on March 28, 2015, 04:25:24 AM


The quote is correct regarding the change from 50.50Kc to 50.75Kc.  What do you not understand???  Do you have another explanation?  I'm not interested in your opinion.  Electronic or physics facts are not subject to the democratic process.  We don't get to change the rules of physics by voting or with simple opinions.  

You were incapable of doing common simple electronic calculations regarding resonance, what is it that qualifies you to do even more complex calculus regarding the reason for the 50.500Kc to 50.750Kc change?  

I've been wrong many times and I don't loose sleep over it.  Before retiring spent many years doing design proof of concept presentations to mostly courteous people and a hand full of belligerent ones.  Kinda thought that was behind me.  After many years in that sand box I'm not bothered but bored.

Do yourself a favor and write your paper describing the workings of that filter in terms that anyone can understand.  I did and you claim it is "BS".  Write your own thesis and publish it.  I promise I will be a gentleman with all due respect and courtesy not attempt to tear it to pieces.

When I describe the nature of capacitors I can point you to research or engineering documents that fully support my assertions.  I also provide common tests that can be done in the common ham shack with common instruments to see exactly what I speak of.  

If you were doing electronic design in the mid 60's you would be very familiar with the ceramic issues.  Every engineering conference spent what seemed interminable time explaining the risks of using what are now class 2 and 3 ceramic caps.  The Hallicrafters and National engineers used that knowledge to design their radios.  I have found only one misstep by either, and it is those caps in the SX-115.    

The filter in the SX-115 is one of the simplest filters known to man.  I explained precisely how it works in words anyone could understand!!!  You did not understand and still don't!  Perhaps you would like to write a paper with your explanation.


Your original post:

"The article is incorrect as to the function of the capacitors being discussed, and it has a tone of disrespect for Hallicrafters engineers. Worse, it explains the circuit incorrectly. If we are going to point out a design error, we need to make sure we actually understand the circuit"  

Should this not apply to the author of your original statement??

I'm bored with this incessant attempt to find fault Tom.

Say what you will.  I have proved up the assertions with incredibly simple math and simple explanations and you simply do not understand it.

For the folks that own an SX-115.  You can improve that radio substantially by taking an hour to replace those 7 caps with POLYPROPYLENE not Polyethylene.

If you wish to speak to me off list my email is hallicrafters2000@k9axn.com I'm finished with this thread.  

Thank the rest of you for your courtesy and indulgence and good luck with those fine instruments.

Have a super evening -------- Kindest regards Jim K9AXN




 Jim thank you so much for the fine information . I have owned three SX-115s . I did the instructed alignment etc but always came away a bit underwhelmed  . They wound up on my over-priced/over-rated list . A drop dead beauty --but !   
 I have also owned a pair of late model SX-100 and found them to be real treasures and near the top of my favorites list .

  As a result of your information I will give the 115 a whirl when the next one turns up . Good bet it will be a keeper .


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 28, 2015, 08:23:39 AM
Hi Gary,

Thanks for the courteous response!  Makes the vintage radio experience so much more enjoyable.

Have a super weekend and good luck finding the SX-115.

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 28, 2015, 09:37:49 AM
Good morning Tom,

Have no idea what logic you used to come to your conclusions regarding the shift from 50.500 to 50.750Kc.  

I have written several papers intimately describing the design issues the Hal engineers mitigated when they developed the SX-100 Mark2.  The difference in performance is staggering.  Turns a marginal radio into a keeper.  The papers addressed individual issues and the final, one for CW operators and one for the feedback problem and several others for overload.

I would like to have answered your questions offline for obvious reasons but you insist.

The two reasons for the change are:

1.  To isolate the BFO from the AGC detector.
      
     Symptom:  Turning the BFO on generates AGC indicated by the S-Meter in the 50.500Kc Mark1.
     Does not happen in Mark2 50.750Kc.  Tom, get some skin in the game and test the radios yourself.  Also follow the
     yellow brick path.    


2.  To make the radio more pleasant to operate for CW folks.

The bandpass in the 50.500 radio while in the .5Kc position is from 50.250Kc to 50.750Kc for an audio band width from 250 cycles to 750 cycles.  That means that you can by tuning, select a CW tone from 250 cycles to 750 cycles while remaining within the I.F. band pass.

Now the 50.750Kc version.
The band pass for the .5Kc position is 50.500Kc to 51.000Kc rendering an audio band pass of 500 cycles to 1000 cycles.  
You can tune that range to select the pitch that you desire while remaining within the pass band.  Doesn't that sound like a more useful range and if not simply use the BFO adjustment?

If any of this is unclear just think about it.  I'm out of here.  I'm a 75 year old guy that's planning to do some serious hang gliding in upstate NY around Ellenville in my old 1960's haunts.  

I thank all that expressed courteous complementary OR OPPOSING positions.  Makes a great hobby even greater.

Also thank the folks who audited their SX-115, SX-117 and SX-100 radios to confirm the capacitors used.

Good luck and kindest regards Jim K9AXN
        


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 28, 2015, 09:42:21 AM
Jim,

The last guy I knew who said he was going hang gliding was in hospital for 4 weeks and off work for 6 months.

Take care!

73

Peter G3RZP


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 28, 2015, 09:51:01 AM
Hey Pete,

I'll be thinking of you while soaring with the feathered bros.  Been gliding forever, beats hanging around the gin mills swapping stories. 

I would like to thank you for your professional courtesies.  Keep em lit don't mean ale.

Have a great weekend!

Kindest regards Jim K9AXN   


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 28, 2015, 03:02:44 PM
Hmmm, hang gliding in the Catskills where it is still considered winter, I spent 2 years in that area going to school and working at resorts. They also have several mental health facilities up there these days where old folks can wind down from stress........You do seem to be wound rather tight since you started this thread and never expecting to get a serious challenge to your credibility. Let me know where you will be and I'll have my GF fly us over and check the scenery and conditions, as a former Russian Army piston powered fixed and rotary wing pilot she has plenty of winter weather stick time and her current job here takes her into all sorts of weather.

Last year you disappeared for 6 months or so........

Only someone who hasnt spent any time chasing CW DX would complain about a 250-750 Hz CW tone range. OTOH I suspect the only reason for the change was via customer request as in the 50's CW was a lot different than these days. Tom and I have serious 160 and 80M CW time under our belts digging in the noise and a high pitch doesnt cut it.

You make a lot of claims about papers and past work but you never elaborate nor can I find any mention of you in the various Halli archives.
Lots of people can name drop or claim others work as their own.

I still have very serious doubts about your veracity and accuracy concerning the SX-96, 100, 115, and 117 filtering and components. Also have a SX-101A but havent been into it since it was rebuilt before I got it.

Enjoy your vacation and all this can continue again.....


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: AC2EU on March 28, 2015, 03:13:06 PM
I'm not far from the Catskills and I wouldn't be hang gliding there on a bet this time of year!
Besides, the weather has been very unsettled here and sudden high velocity winds are common.

Good luck with that...  :o   ???  ::)


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 28, 2015, 05:03:16 PM
Hi Jim,

Your right, going to have to hold for a while.  Not good to be impatient.  Was up there last year a bit early just love NY upstate, spent many years up there during the 60's.  Joe Pepitone had a Gin mill on a mountain side near kingston.  Used to make bets on who could navigate the winding road leading to his place on a skate board.  Life is good.

Thanks for the note Jim --- Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 28, 2015, 06:07:15 PM
Quote
Jim thank you so much for the fine information . I have owned three SX-115s . I did the instructed alignment etc but always came away a bit underwhelmed  . They wound up on my over-priced/over-rated list . A drop dead beauty --but !  

Ive owned one for about 5 years and it performs like any other Halli using 50 kHz LC filtering, fine for the casual user and poor for someone used to mechanical and multipole crystal filters. The SX-117 cost reduced model is worse. Halli soon abandoned that scheme and went with crystal filtering.

Did you replace all the various paper caps in plastic and do a thorough realignment of all 3 of those sets? Did you check resistors for tolerance? The manual alignment instructions leaves a lot to be desired as do the Hammarlund HQ-170/180 versions which uses a 60 kHz LC IF with a rather elaborate selectivity scheme.


 
Quote
I have also owned a pair of late model SX-100 and found them to be real treasures and near the top of my favorites list .

Ive had a SX-100 MK2A also for about 5 years and find it rather mechanically unstable for good SSB and CW; the whole thin gauge chassis flexes. The earlier model was even worse

I also have a SX-96 and 117 which are only good for shelf queens.

Quote
 As a result of your information I will give the 115 a whirl when the next one turns up . Good bet it will be a keeper .

If you do a proper overhaul and alignment and stay away from questionable parts changing you will be in for a different viewpoint....but remember its basic limitations as it is not a high performance radio such as a 75S3C or tweaked R4C; a NC-300/303 properly overhauled is a better set in several areas and almost a decade older. My highly modified 75A4 runs circles around the SX-115. One station has a SX-115/HT-32B/NCL-2000 and the other a CE-100V/that 75A4/another NCL-2000 and can select either setup with a single switch for fast comparisons.

Carl

Carl


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 28, 2015, 06:23:21 PM
Quote
You fellers didn't slip one by me.  I asked Tom after his first belligerent post if he was your proxy, sounded too much like you --- he didn't answer. 

Anyone in the know would say you are totally clueless but I do respect Toms ability to tie you up in knots and completely ignore your continuous belligerence and cheap shots; a sure sign you have already lost the battle.

Quote
Forgot to add I still have the SR-2000 prototype that I worked with and the original Cyclone 2 upgrade which is actually a Cyclone 3 missing some yet to be finished circuitry.  I'm writing an extensive paper regarding the entire technical history from Genesis to death of the SR-400, SR-400A, and SR-2000.  You are going to be dazzled with the information in that paper.  When I finish I'll send you a note so you folks can critique it for me.

I have less interest in any Halli transceiver than I have in a Collins, Swan, Drake, etc which is ZERO. My only tube era transceiver is a NCX-1000 and that only has two tubes and isnt even close to being ideal for any mode but it does sound great on AM RX/TX and fits well on the bedroom desk. Just enough power and audio punch to work 75 and 40 in the wee hours when I cant get back to sleep.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 29, 2015, 08:09:29 AM

The two reasons for the change are:

1.  To isolate the BFO from the AGC detector.
      
     Symptom:  Turning the BFO on generates AGC indicated by the S-Meter in the 50.500Kc Mark1.
     Does not happen in Mark2 50.750Kc.  Tom, get some skin in the game and test the radios yourself.  Also follow the
     yellow brick path.    

Things work the way they work.

You are proposing that a 250 Hz IF shift on a 50 kHz IF was done to reduce BFO back eed through the two resonant circuits.

That makes no electrical sense at all, for the reasons I gave:

1.) If the IF moves, the BFO also has to move the same amount for the same pitch. The net result is the BFO is in the same exact place in relationship to skirts.

2.) The resonant circuits are wide as a barn door. A 250 Hz move is nothing, even if you changed pitch and did offset the BFO 250 Hz more and suffered with the 250Hz pitch change.


Quote
    I have no idea how you came to the conclusions regarding band passes, center frequencies, or any of your
     narrative.  Please slow down and follow the numbers below.

The bandpass in the 50.500 radio while in the .5Kc position is from 50.250Kc to 50.750Kc for an audio band width from 250 cycles to 750 cycles.  That means that you can by tuning, select a CW tone from 250 cycles to 750 cycles while remaining within the I.F. band pass.

Who would find a CW pitch pleasant in that audio range? A caveman returning from the hunt would drop his dinner and head for the hills knowing whatever it was is much larger the he, and he never knew his girl friend to sound like that.      
 
Now the 50.750Kc version.
The band pass for the .5Kc position is 50.500Kc to 51.000Kc rendering an audio band pass of 500 cycles to 1000 cycles.  
You can tune that range to select the pitch that you desire while remaining within the pass band.  Doesn't that sound like a more useful range?

That makes absolutely no sense at all to me. The BFO is tunable for about a 3-5 kHz range, depending on receiver model in that 50 kHz IF family.

This means, no matter what IF frequency is use, the pitch can vary with a front panel delta of at least 3 kHz. With proper alignment it can go from negative pitch (reverse sideband) to a few kHz picth, including right through zero beat.

The user can get any tone he wants, the IF shift does not change that one bit.

So, understanding the BFO has a ~4kHz adjustment range, they very clearly did NOT change the IF 250 Hz to change the CW pitch. That idea makes no sense at all, just like the BFO back feed reduction makes no sense.

73 Tom


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 29, 2015, 09:15:53 AM
Sturley's book goes into some depth about how changing the coupling affects the centre frequency and symmetry, all depending on which method of changing coupling is considered. The least change in centre frequency and the best symmetry are achieved by coupling variation by mutual inductance i.e. moving the coils relative to each other, as done in various Eddystone receivers and the BC453.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: AC2EU on March 29, 2015, 10:14:48 AM
While the SX-115 is a great classic "look", it's technology is still as old as the hills. As another person noted, many of the Hallicrafters are also MECHANICALLY unstable as well as electrically. We can repair/improve the electrical, but the mechanical problems remain.
I've repaired many different Hallicrafters transmitter and receiver models for people, but have no desire to own one myself.
Nostalgia is a strong draw for some folks and if this is what they enjoy, then I say go for it.  

My personal weakness is old military radios which have their own problems as practical rigs...there is just something about the ruggedness and history that I find appealing.
A similar irrationality must afflict Hallicrafters collectors!



Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 29, 2015, 10:20:24 AM
Good morning Gary,

Before I leave this thread I need to finish with some sort of conclusion.  Before you purchase an SX-115 you need to hear the whole story.  

I started this thread because a good friend who is home bound asked me to take a look at his prize SX-115.  He was the original owner and was attached to the radio.  His major complaint was "The selectivity is not as it was; SSB is no longer clear.  He also is not a CW person and may be hearing challenged.  I decided to take a look at it even though I was preparing to take a spring summer break.  

Couldn't do a full regression test so started with the selectivity system.  First look under the radio at the selectivity components found what is considered by designers to be a violation of good practices; Ceramic class 3 Z5U caps used in a tuned circuit carrying circulating current.  This statement will draw the wrath of a few people but I ask them to provide any documentation that supports their assertions.

There was some disagreement regarding what I found and some claims that I believe were unsubstantiated.

Primary was the filter was broad as a barndoor.  By today's standards some think it is but for the old timers it's fine especially for a radio they have come to love representing their roots in radio.  The band width in the SX-115 is wider than other 50Kc Hal radios because ceramic Z5U caps were used contrary to good design practices.  I checked to see exactly what influence they had on the performance of the filter.  They shifted the 1Kc resonant anchor point and lower skirt closer to the 2Kc point than 1Kc --- approximately 1.6Kc.  The other positions were skewed wider as well.  

Is this a catastrophic problem?  To me No, but to some who rely on the stated band widths it is troublesome when switching selectivity positions.  There are things that people say that can skew what other people think.  If your told over and over again your favorite radio is broad as a barn door, you won't try to make it better.  Ever wonder why the SX-115 has the worst reputation regarding band width of all of the 50Kc radios?  Haven't heard anyone harp on the SX-88 or others which have the same filter system!  

Changing those caps did indeed make a difference even though it was minimal to me.  The people who use the .5Kc and 1Kc positions will take exception to minimal.  The old timers that think the world of their radio will see hope that it's short comings can be rectified.

Nick, W9NVN, had the most precise and unambiguous analysis of the SX-115 in his post.

He stated several things that may be correct regarding the SX-115 and he did so with courtesy.  The most profound design error is the 6BA7 mixers followed by the filter.  I thought he was correct but now do not agree.

The 3db noise floor on the radio that I'm working with was -118db in the .5Kc position and approximately -120 on the others; unacceptable.  Since changing the caps including changing the 2.2pf top coupling cap to ceramic plate NPO the 3db point for the .5Kc and 1Kc positions are essentially the same at -125 and I have yet to replace the 2.2pf cap in the 1st I.F.

Nick, do you have the design notes for the 12AT7 change and if so why don't you post them?

Nick, I did and do not work with the radios earlier than the SX-150 except the SX-100 and SX-117.  My primary interests are the SR-400,SR-400A, and SR-2000.  This SX-115 is the first that I've had on the bench.  With that I can't say good or bad for the radios you speak of.  My statement about the Hal engineers was that they were as good and forward thinking as any having so much less financial support to work with.  

Gary, please forward your email address to me would like to speak with you.

I have to disclose that I deliberately added two errors to determine whether the impolite ones read responses.

One was that the shape factor was 2.1/1 and the second 2.2/1.  I also said the SX-115 that I was working with displayed the same shape factor that the charts painted which are approximately 4.4/1 which conflicts with 2.1/1 and 2.2/1.  I repeatedly asked them to look at the chart and received no response.  The shape factor of 2.1/1 and 2.2 /1 are not possible and in conflict with the 4.4/1 defined in the chart.  Had they looked at the chart they would have immediately taken exception with my assertion and we could have moved on knowing that we had a two way conversation.  

The following sweep is a linear presentation of the SX-117 and SX-115 filter.  I did not record the spectrum analyzer screen to finitely detail the exponential skirt shape --- that is a to do.  

SX-117     http://www.k9axn.com/_mgxroot/page_10876.html

SX-115     http://www.k9axn.com/_mgxroot/page_10881.html

Have a super day and Kindest regards Jim K9AXN  


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 29, 2015, 11:28:12 AM
Just had to clarify the concept.

MY QUOTE:
------------------------------------------------------------------------------------------------------------------------------
I have no idea how you came to the conclusions regarding band passes, center frequencies, or any of your
narrative.  Please slow down and follow the numbers below.

The bandpass in the 50.500 radio while in the .5Kc position is from 50.250Kc to 50.750Kc for an audio band width from 250 cycles to 750 cycles.  That means that you can by tuning, select a CW tone from 250 cycles to 750 cycles while remaining within the I.F. band pass.

Now the 50.750Kc version.
The band pass for the .5Kc position is 50.500Kc to 51.000Kc rendering an audio band pass of 500 cycles to 1000 cycles.  
You can tune that range to select the pitch that you desire while remaining within the pass band.  Doesn't that sound like a more useful range?

Note:  I said without touching the BFO dial.  If you wish to listen beyond the 500 to 1000 cycle range simply tune the BFO.

Tom's QUOTE:
------------------------------------------------------------------------------------------------------------------------------------
That makes absolutely no sense at all to me. The BFO is tunable for about a 3-5 kHz range, depending on receiver model in that 50 kHz IF family.

This means, no matter what IF frequency is use, the pitch can vary with a front panel delta of at least 3 kHz. With proper alignment it can go from negative pitch (reverse sideband) to a few kHz pitch, including right through zero beat.

The user can get any tone he wants, the IF shift does not change that one bit.

So, understanding the BFO has a ~4kHz adjustment range, they very clearly did NOT change the IF 250 Hz to change the CW pitch. That idea makes no sense at all, just like the BFO back feed reduction makes no sense.

73 Tom


Jim's RESPONSE:
-----------------------------------------------------------------------------------------------------------------------------------------
Your perfectly right however there is an operator issue here.

Your way requires the manipulation of the BFO frequency in order to use a pitch above 750 cycles in the 50.5000Kc radio because the natural setting of the BFO is 50.000Kc.  The useable band pass with the 50.500 is from (50.250 to 50.750 Kc).  If you wish to use a higher pitch than 750 cycles you will be required to alter the BFO frequency below 50.000Kc to accommodate the frequencies above the natural band pass which ends at 50.750Kc.  

With the 50.750Kc change the natural bandpass is from 50.500 to 50.100Kc.  This allows you to select a pitch from 500 cycles to 1000 cycles by simply tuning without changing the BFO frequency.

With the 50.750Kc radio you can switch back and fourth between filter, CW, and SSB positions without touching the BFO with no regard to what pitch you selected between 500 and 1000 cycles.

With the 50.500Kc radio you have to intervene with the BFO when changing CW and SSB positions when you have altered the pitch above 750 cycles.

If this is not understandable someone else is going to have to find words to wrap around the concept.

Have a good day Tom --- see ya

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 29, 2015, 11:45:44 AM
Replacing the 6BA7 is almost certainly to be a good start, although there may be a loss in Q in the input tuned circuits: pentagrid mixers can offer somewhat negative input impedances above 20 MHz because of transit time. From the viewpoint of dynamic range, the 12AU7 is better than the 12AT7: there are some reports that the ECC88/6DJ8 and Special Quality E88CC are even better. They are favourites of the audiophools though and can be hard to get.

The Pullen mixer has a following but nobody has published any results in terms of noise figure and intercept points. Examination of the circuit does not suggest that it is outstanding.



Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 29, 2015, 12:35:39 PM
Good afternoon Pete,

Have to do a warm weather escape --- weathers not cooperating out east.

-----------------------------------------------------------------------------------------------------------------------------------
Sturley's book goes into some depth about how changing the coupling affects the centre frequency and symmetry, all depending on which method of changing coupling is considered. The least change in centre frequency and the best symmetry are achieved by coupling variation by mutual inductance i.e. moving the coils relative to each other, as done in various Eddystone receivers and the BC453.[/color]
---------------------------------------------------------------------------------------------------------------------------------------

I agree with that if the secondary side is resonant at the same frequency as the primary.  The primary will see almost pure resistance regardless of the coupling.  

The sides in the SX-115 filter are resonant and in phase at the coupling point at all of the bandwidth settings.  This means the energy extracted from the primary would appear as primarily resistance very similar to mutual inductance between resonant circuits.

If you calculate the resonant point of each side of this filter simply considering the combined capacity and inductance, the center frequencies in each bandwidth setting come out skewed and almost exactly centered where they should be .  This would lend some credence to the notion that the coupling effect did not materially affect the frequency relationship.

Thanks for the comment Pete and have a great day.

Kindest regards Jim  


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 29, 2015, 12:45:40 PM
Pete,

Thanks for the heads up on the tubes.


Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KB4QAA on March 29, 2015, 12:48:01 PM
K9AXN,
I started this thread with a skeptical eye after seeing your website.  Following your last several posts, you have lost any chance of respect from me.

Your condescending attitude towards the Halli engineers and ego is overwhelming.

You incorrectly accuse others of calling the Halli radios 'trash" when you are the one who thinks every radio must be re-designed because they are unmitigated failures in theory and material.

I only hope that not too many hams are lured into your cockeyed world.

[p.s.  Your playing of mind games by deliberately introducing false data is particularly despicable in a hobby that is based in fact and truth.  Please go away and find a different hobby.]


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KB4QAA on March 29, 2015, 01:09:25 PM
....My opinion said the capacitors were contrary to good design practices so I checked to see exactly what influence they had on the performance of the filter.  

....Changing those caps did indeed make a difference even though it was minimal.  

Bottom Line:  And so dear readers, after having been dragged through over six pages of postings, K9AXN finally admits above that the basis of his thesis is false. The specified capacitors perform adequately and replacing them with modern caps far exceeding the original quality has minimal change to performance!!!

You may disgorge the fishhook from your mouths.  ;)


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 29, 2015, 01:57:49 PM
Quote
Bottom Line:  And so dear readers, after having been dragged through over six pages of postings, K9AXN finally admits above that the basis of his thesis is false. The specified capacitors perform adequately and replacing them with modern caps far exceeding the original quality has minimal change to performance!!!

You may disgorge the fishhook from your mouths.  Wink

I skip over a lot of Jims blathering as Im not wearing hip waders so I missed his shape factor comment or just ignored it as I do so much from him over many years of Jim watching.

OTOH I stated very early on that my own unmodified but overhauled SX-115 selectivity curves closely matched the manual with not enough difference to get excited about. Having worked with filters at National it didnt take me long to realize that theoretical or stand alone lab measured curves and the reality released in production rarely overlaid each other. Add 50+ years of aging and many not bothering to replace leaky paper caps and well out of tolerance resistors before publishing dreck (I learned that word at a tender age from my Jewish friends in Brooklyn NY) just muddies the waters and leads his daisy picking and dancing groupies to the Kool Aid.     

Carl
                                                                                                             


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 29, 2015, 03:27:32 PM
I have no problem with good science.  I have a problem with shifting IF's 250 Hz to reduce back feed into AVC, and with shape factors near 2:1 with a handful of Q<200 inductors at 50 kHz and multiple other claims.

The SX117 didn't reduce BFO back feed into AVC by shifting the IF 250 Hz. That's nichrome nonsense.

The SX117 actually reduced BFO back feed through modifications around the product detector that reduced coupling out the product detector grid 1. They actually attempted to neutralize the BFO leakage through a new capacitor.

The 250Hz IF shift is meaningless.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: N9WW on March 29, 2015, 04:29:27 PM
This is a common complaint regarding the Hallicrafters SX-115

“The Selectivity switch positions don’t seem to be effective.”

Simple fix for the problem:

To fix the problem, simply replace the Z5U Ceramic disc capacitors on the selectivity switch along with C73 and C86 with Polypropylene film capacitors and realign the 50Kc I.F.  If you would like to understand why, read on.

If there is any question in your mind that the Z5U capacitors need replacing, do two things.

1.  Measure the capacity of the capacitors.  The .01uf caps are easy to reach.  Measure them with the radio cold.  They will be very close to .0082uf not .01uf --- (-5%/Decade).  That is a result of aging.  Don’t think they age?  Remove one and place it in an oven at 280 degrees for about two hours.  Then let it sit for 48 hours and measure the capacity --- it will be .01uf like new.  Now it begins to age again.  The heating does not hurt it.  The baking process was used by Mfrs to adjust them to tolerance.

  
2.  Measure the caps while the radio is cold .0082uf, then warm the radio to operating temperature.  Operating temperature on the SX-115 is approximately 20 degrees C above room temperature.  Now measure the capacity.  It will be approximately .0073uf --- a long way from .01uf.  


The following link is a video that was shot while heating a .01uf capacitor on the selectivity switch from 22 degrees C to 55 degrees C.  We recorded the capacity cold 22C .0083uf then at the operating temperature 42 degree C point .0071uf.  Then heated it further to 55 degrees C.

www.k9axn.com/SX-115/SX-115%20ceramic%20temperature.avi

The values of those capacitors are Critical, they control the frequency offset for the five selectivity settings.  

The Hallicrafters engineers made a component selection error.  

I believe the SX-115 was designed before the SX-117 by a year or two and the SX-117 is part of the story.  The SX-115 was the flagship and the SX-117 was supposedly the poor man’s copy.  

One of their engineers apparently decided that the capacitors used in the BAND PASS FILTER circuits of the SX-115 could be ceramic class 3, Z5U capacitors.  

Every engineering or design document explicitly warned that their use in tuned circuits, timers, and analog applications were patently poor design practices.  Hal ignored or missed this warning and went ahead with them anyway.  How they missed is a mystery because the SX-115 is the only radio designed with the 50Kc I.F. system that used the ceramic capacitors.  The SX-100 Mark2 and SX-117 used expensive General instruments film capacitors.  The SX-88, SX-76, SX101, SX-96, and early SX-100 used paper or film capacitors that did not have the instability with voltage or temperature that the ceramic Z5U has.  

The Hal engineer chose .01uf and .0047uf 10% ceramic Z5U capacitors to fit out the BAND WIDTH switch.  

Note: The SX-115 is the only 50Kc I.F. radio ever built by Hal to use the ceramic Z5U caps to implement the Band width logic.  This was a mistake as you will see below.    

The ceramic Z5U is one of the most non-linear, volatile ceramic capacitors ever available.  The schematic calls for a 10% Z5U ceramic disc capacitor.  What does the 10% mean?  It says that at room temperature, the capacity will be within 10% of the stated value; that is, DISREGARDING APPLIED VOLTAGE which has a profound influence on capacity.  All bets are off when voltage is applied.  With AC the capacity begins to vary at less than 1 volt.  This would cause linearity problems although not obviously perceptible.  The vast change in capacity with temperature and aging however, will cause the center point in each of the band width center positions to shift.  

The 500 cycle selectivity position is set by alignment to center at 50.750Kc 750 cycles above the BFO; resulting in a band pass at 50.500 to 51.000Kc.  The 1Kc, 2Kc, 3Kc, and 5Kc center points are shifted out by the capacitors used on the selectivity switch; If the capacity changes, so too does the frequency offset center position.    

Data sheet information for the Z5U.  The Z and 5 are the low and high charted limits of temperature which are +10 to +85 degrees C.  The U states that the capacity will vary from +22% to –56% over the temperature range.  That's the charted range.  Anything beyond will vary radically more.

  

The Polypropylene film capacitors vary <=2% over a wider temp range and the capacity does not vary with voltage whereas the capacity of the Z5U will vary up to 60% over the voltage range, an additional 20% due to aging, and who knows what due to temperature change.  

HOW THE SWITCH LOGIC WORKS:

The .5Kc position:  The capacitors on the band width switch control the position of the filters center point and the resistors flatten the response.  Example:  The .5Kc position shorts around all of the capacitors.  This is the unaltered center of the band pass and the point that you aligned first.  It is 50.75Kc and is 500 cycles wide.  This covers 50.500Kc to 51.000Kc at the 6db point with the carrier at 50.000 Kc.  The 6db audio band pass is from 500 cycles to 1000 cycles.  
 

The 1Kc position:  Introduces all of the capacitors paralleled on the switch in series with the fixed cap reducing the effective capacity moving the band pass center upward and removes the short around, increasing the center frequency to 51.000Kc but does not add resistors yet.  However the change from short to capacitors does widen the band pass to 1Kc.  This expands the audio band pass to 500 and 1500 cycles.    


The 2Kc Position:  Removes a .01uf capacitor moving the center point to 51.500Kc and adds a 180 ohm resistor widening the band pass to 2Kc, creating a band pass from 50.500Kc to 52.500Kc at 6db rendering an audio band pass of 500 to 2500 cycles.

The 3Kc position:  Removes the .0047uf capacitor moving the band center to 52.000Kc and adds a 220 ohm resistor rendering a band pass from 50.500Kc to 53.500Kc for an audio band pass of 500 to 3500 cycles.  


The 5Kc position:  removes the last .01uf cap re-inserts the .0047uf capacitor and adds a 390 ohm resistor.  This moves the band center to 53.000Kc and widens the band pass to 5Kc creating a band pass of 50.500 Kc to 55.500Kc for an audio band pass of 500 to 5500 cycles.


The above paragraph says it all as to why the Z5U is a design error.  The variation in capacity with temperature, voltage, and age disqualifies it.  The band width center positions will not remain where they should be using the ceramic capacitors.


I hope this helps and hope you change those caps out for Polypropylene.  That is a fine instrument --- keep it that way.

  

MORE INFORMATION FOLLOWS:

If you should choose to replace those capacitors, Please measure them before you remove them.  The front .01uf is disconnected when the switch is in the 2Kc position and the second when in the 5Kc position.  Measure when cold and when it reaches operating temperature; after an hour.  The .01uf should measure .0082 cold and .00074 warm.  They don’t affect the 500 cycle position because it is a short around, however the other positions will overlap each other to varying degrees.

Here’s an interesting experiment.  After you remove the ceramic caps, put them in your oven at 280 degrees for 2 hours then take them out and let them cool and settle for 48 hours.  Now for the magic; Measure the capacity.  They will be .01uf like new and the aging process begins again; at 5%/decade.  

The baking process is how the MFGRS used to alter the capacity to the target value.  You would always find a 10% capacitor at =< 2%.        


The aging process combined with non-linear –5000 ppm Temp comp personality of the Z5U Ceramic will result in the .01uf cap becoming a .0074uf Cap and will do nothing but get worse over time.

Conclusion:

Replace the two .01uf and one .0047uf ceramic disc caps on each selectivity switch as well as C73 and C86 .02uf ceramic disc caps.  

C86 is a sleeper.  It is used to bypass the screen BUT is also a critical functioning member of the tuned circuit coupling the components to both sides of the second 50Kc I.F. circuit.  It must be replaced with Polypropylene.   Both C73 and C75 are exposed to +215 volts which further reduces their capacity after the accumulated reduction caused by aging and temperature.  

 

NOTE:  The four other paper looking capacitors in the SX-115 are not paper but high quality Mylar film.  If you have already replaced them with Ceramic, remove them and replace with them with Polypropylene.  C100 and C111 are timers for the fast AGC which controls the first RF amplifier.  Using ceramic caps relegates the fast AGC to a second rate performer.  All of those Mylar caps were deliberate choices used in critical circuits requiring linear time constants.  The Z5U is patently non-linear in every sense and should not be used to replace the film caps in this radio.

 

Kindest regards Jim K9AXN

  

COPYRIGHT  James Liles


Great article Jim!

Thanks for your contribution and knowledge on this subject.  Regardless of what some Trolls might say, your website is a great reference on the SR seiries of radios.  As somebody who personally knew Bob Orwin (W9YKA), I know he would agree if he were still around today.

73,
Jim N9WW


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KG8LB on March 29, 2015, 04:46:29 PM
Thank you Jim for the notes .
 Seems like the old 115 has other issues as well !  That said , I appreciate the later SX-100s all the more .
My email is my call sign @comcast.net

 


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 29, 2015, 07:47:00 PM
Hi Tom,

Your comment:
-------------------------------------------------------------------------------------------------------------------------------------
I have no problem with good science.  I have a problem with shifting IF's 250 Hz to reduce back feed into AVC, and with shape factors near 2:1 with a handful of Q<200 inductors at 50 kHz and multiple other claims.

The SX117 didn't reduce BFO back feed into AVC by shifting the IF 250 Hz. That's nichrome nonsense.

The SX117 actually reduced BFO back feed through modifications around the product detector that reduced coupling out the product detector grid 1. They actually attempted to neutralize the BFO leakage through a new capacitor.
The 250Hz IF shift is meaningless.
---------------------------------------------------------------------------------------------------------------------------------------

I said nothing about any problem with the SX-117.  You manufactured that.  I stated that the change corrected the problem that affected the SX-100 Mark1 and was fixed in the Mark2.  The SX-117 never had the problem because the product detector provided isolation.  

Did you understand the explanation that I provided as to why the CW issue was fixed?  Have to ask --- you said nothing about it.

I explained the filter issue as not a show stopper because it's effect was minimal for me but still a nuisance and one that would never do anything but get worse.  That explanation is:  Right now today, with the ceramic caps the attributes of the 1Kc position have moved closer to the 2Kc position than 1Kc rendering a center frequency position centered on 1.6Kc but with a bandwidth of 1Kc.  You just can't predict what kind of change switching the selectivity switch will make.  I call that minimal but others do not feel the same.  Why not fix it --- it's going to get worse with time??

The mixer problem I was not aware of until Nick spoke about it.  Would you throw that radio away because of that???  How many of these radios have been re engineered to fix problems just like that?  It's OK for your favorite radio but not the SX-115.  Was the Drake R4C modified extensively to make a better radio?  Was the Collins KWM2 modified to make a better radio and if so were those changes a design error or they just wanted to stay busy? 

The thrill of working with these vintage place holders is as much operating them as trying to isolate the short comings and research the reasons for them i.e. was it financial, selecting the features that delivered the most bang using the finances available, researching the history and lives of those who designed, built, and serviced them.

I unapologetically state that I'm a Hallicrafters junky.  I love those radios the more so when they misbehave.

Gary,
I asked for your email because you sounded happy to hear that there was some hope that you would at last find an SX-115 that was modified to work as advertised.  I'll be in touch.  I asked Nick for the information regarding the mixer change that he knew of.  

FOR KB4QA,
"What good does it do to tell a person their prize is a piece of junk other than to satisfy your ego"  was in response to Carl inferring the SX-115 was a junk radio.  You simply read it out of context.  

"Disrespect for the Hal designers"  --- That's the last thing in the world that anyone who knows me would think.  You have an agenda.  
  
How you think is a choice you will have to make based on your principals.  Your simply a poor judge of character.  You stand by quietly as these things unravel a thread.  I believe there's a word for that.


Have a good day all!

Kindest regards Jim K9AXN



  


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on March 29, 2015, 09:51:57 PM
Jim

The question comes down to whether the radios worked as designed when they left the factory. Saying Hallicrafters engineers were incompetent for using Z5U ceramic caps isn't going to hold water 70 years down the road. I would not trust the electrolytic caps either at this point. And I am sure that many of the carbon comp resistors are also out of tolerance.  

I wouldn't argue that Z5U ceramics aren't the best devices to be used in circuits that determine resonance or selectivity.. but the question should be whether they were suited for the application at the time of manufacture.

I am also confused in regards to the IF rejection for the BFO injection frequency. Moving the IF shouldn't have had any effect at the time of manufacture.  Unless you are trying to say that the asymmetric IF BW curves resulted in the BFO being located further down on the IF slopes for USB or LSB modes by changing the second IF and converter crystals??  None of that is clear in your past posts, and referring back to them doesn't answer the questions that were posed by a few other commentators.

Regards

Pete


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 30, 2015, 07:35:39 AM
Hi Pete,
Thanks for the courteous reply and I'll clarify every pint.

Your first question:
-----------------------------------------------------------------------------------------------------------------------------------
The question comes down to whether the radios worked as designed when they left the factory. Saying Hallicrafters engineers were incompetent for using Z5U ceramic caps isn't going to hold water 70 years down the road. I would not trust the electrolytic caps either at this point. And I am sure that many of the carbon comp resistors are also out of tolerance.
---------------------------------------------------------------------------------------------------------------------------------------  
 
MY response:
-----------------------------------------------------------------------------------------------------------------------------------
First I take aggressive exception that I said anything like INCOMPETENT.  THAT WAS MANUFACTURED BY ONE OF THE UNPLEASANT ONES.  PLEASE READ THE SCRIPT AND YOU WILL SEE THAT IS SO.  

The notion that I would call anyone incompetent or stupid is ludicrous, --- that's not who I am; especially the engineering team that I believe to be the most forward thinking of the times.  I said it was a design error with no prejudice tone; and it was just that.  Were the changes made to the Collins KWM2 design errors, or were the numerous engineering changes made for maintenance?  How about the Drake R2C, were the engineering changes design errors or maintenance?  There is a distinction and I don't say that with any denigrating tone!  We all made numerous mistakes in design at that time because of financial pressures or the need to beat the competition to market.

Did the radio perform as as defined when it left the plant?  The answer is NO it did not!  Why was it not called out?  Because the problem was not obvious at the time.  For example:  When it left the plant the .01uf cap was .01uf AT ROOM TEMPERATURE but at operating temp probably .008uf.  The most important problem at that time was the temperature constant of the caps which is from N5000 to N10000 for Z5U.  The accepted room temp to operating temp change is 20 degrees C.  It of course varies but that is the mean.

As you saw if you viewed the video snippet. the capacity changed from .0083uf to .0073uf due to temperature in a 68 degree room.  Those 3 capacitors and the coupling/bypass capacitor in the second transformer set PARTICIPATE DIRECTLY IN THE CIRCULATING CURRENT OF A  TUNED CIRCUIT and THE VALUES OF THOSE CAPACITORS WERE CHOSEN PRIMARILY TO ANCHOR THE SKEWED BANDPASS AND TO PROVIDE COUPLING.  As you can see for the 2Kc, 3KC,and 5Kc positions, the fixed load and increased coupling resulted in a very high K*Q.  They inserted resistors to reduce the K*Q to 1.1, slightly over coupled, otherwise severe over coupling would distort the wave shape.  Clearly visible in the sweep analysis photos.  See the url. http://k9axn.com/_mgxroot/page_10881.html    

The next consequence was they age down 5%/year.  This was known well before 1960 and the capacitors were defined as high K caps, then Class 3, now class 2 which combines class 2 and 3.  

The combined change in capacity to this date as measured in this radio is -28% in a 68 degree room and -38% in an 80 degree room.  

The values of those capacitors were chosen to skew the center frequency away from the carrier to accommodate the expansion of the band pass created by COUPLING AND THE RESISTORS that are in series with the secondary transformer windings were used to adjust the K*Q.  I asked the folks that disagree to simply do the calculations to prove they were right or wrong and received no answer.  So I presented the answers proving my assertions.  

DESIGN ERROR OR OVERSIGHT?
Long before that radio was designed the behavior of High K/Ceramic class 3 capacitors was well known.  The abhorrent negative and non linear temperature constant, the aging behavior, the change with applied voltage --- which is different with AC or DC --- with AC beginning at less than 1 volt DC higher, very low Q and high Dielectric absorption, Hysteresis effects (erratic changes in capacity), more but unimportant.

The research papers provided by manufactures and independent customer labs stated that High K capacitors were not to be used in Tuned, timing, analog, high level coupling, or ANY circuit where you cared about linearity.  Read the whole litany --- won't repeat it here.

Every designer and user of high K caps was informed regarding the rules the risks and where they could be used.

You won't like this but it has to be said.  PAPER capacitors do not have these behaviors nor do film only the ceramic ferro dielectrics used in class 2 and 3 ceramics.  

Somehow this thing slipped through the cracks and NO I SAID NO SUCH THING AS INCOMPETENT.  Check for yourself in your radios and I don't care what brand.  Are there any circuits that Ceramic class 3 caps are used in a tuned circuit?  That is an error.                

Your second question:
--------------------------------------------------------------------------------------------------------------------------------------
I am also confused in regards to the IF rejection for the BFO injection frequency. Moving the IF shouldn't have had any effect at the time of manufacture.  Unless you are trying to say that the asymmetric IF BW curves resulted in the BFO being located further down on the IF slopes for USB or LSB modes by changing the second IF and converter crystals??  None of that is clear in your past posts, and referring back to them doesn't answer the questions that were posed by a few other commentators.

MY RESPONSE:  
Thanks for the well stated question.  

First those band passes are not in and of themselves asymmetrical; only in reference to the carrier.  They are symmetrical in reference to resonant frequency of the band pass.  

Your exactly right except there is no reason to change the USB and LSB crystals.  The carrier frequency anchors the heterodyne scheme.  They simply moved the center frequency of the band pass 250 cycles away from the BFO.  This moved the skirt away forcing the BFO and band pass to converge further down the skirt.  It also enabled you to use a 500 to 1000 pitch for CW by simply tuning without ever touching the BFO.

The reason for the shift particularly with the SX-100Mark1 to Mark2 model is not complicated.

The Mark1 behaved like the usual radio that didn't have a product detector.  The Mark2 much like a product detector.  How did they do that?  Forward thinking!  

They increased the BFO injection level, lowered the agc sense voltage, extended the AGC hold time.  There's more but that's the primary change.

The Mark1 already had a problem with the BFO feed back to the AGC circuit.  When you engaged the BFO the AGC was engaged and clearly displayed on the S-Meter.  The Mark2 increased the BFO injection and intervention then became necessary; this was accomplished by moving the skirt away from the BFO frequency.  The second reason we described above --- the CW issue.

Oh forgot something,  Transformers are not unidirectional.  Signal can pass both ways.   May help with the feed back issue.

Hope this helps, if not let me know.  

Have a great day Pete and thanks again for the dignified response.  I won't edit this thing --- if you find any glaring error just send a note.

Kindest regards Jim K9AXN  


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KB8E on March 30, 2015, 10:48:47 AM
Don't know anything about Hallicrafter designs, but in following this thread I must admit surprise that moving an IF frequency 250 Hz would have any significant impact on BFO backfeed. Is it even possible for a 50 kHz LC IF filter to have skirts steep enough that a 250 Hz shift would make a difference?

Not looking to get flamed, just missing something

Sam


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on March 30, 2015, 11:03:23 AM
Hello Jim

Well, I certainly don't wish to become embroiled in a peeing match. But, I do have some questions.

In my opinion, the BFO offset from the filter's response curve would have to be optimum for the IF frequency. If the filter is moved 25 kHz, than it seems intuitive that the BFO injection frequency would be moved an equal amount. Unless the filter shape factor has been greatly increased, than I don't get it. Perhaps I am a bit obtuse :)  I also don't quite understand why a passive two-port LC filter should not be expected to  have pass band characteristics that are not the same, regardless of direction?

Pete


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on March 30, 2015, 12:38:56 PM
Hi Sam and thanks for the question,

The skirt was moved from 250 cycles to 500 cycles away from the 50.000Kc carrier --- double the distance.

Your comment:
Is it even possible for a 50 kHz LC IF filter to have skirts steep enough that a 250 Hz shift would make a difference?  

Your under selling that filter.  Remember it's a 50Kc not 9Mc.  

Look at the shape factor chart.  

Let's try this --- some logic.  If there were only three things changed in that circuit, the BFO output increased, the 50.500 to 50.750Kc center frequency which shifted the skirt from 250 cycles to 500 cycles away from the BFO, and the AGC sense level lower (More sensitive) resulting in the feedback problem being reduced to a level that did not engage the AGC what would you think.  

Did you understand the CW issue?  If you do not agree please let me know.  If you do please say so.  I need to wrap words around these things that clarify the narrative or have someone else do so.

Pete:
I'm sorry but I don't understand your question.  Please email directly I must be wording the thing wrong.

Have a great afternoon

Kindest regards Jim K9AXN.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 30, 2015, 05:36:50 PM
Quote
KB4QA,
"What good does it do to tell a person their prize is a piece of junk other than to satisfy your ego"  was in response to Carl inferring the SX-115 was a junk radio.  You simply read it out of context. 

More of your BS Jim, you tell so many lies you even start believing them.

Quote
First I take aggressive exception that I said anything like INCOMPETENT.  THAT WAS MANUFACTURED BY ONE OF THE THREE UNPLEASANT ONES.  PLEASE READ THE SCRIPT AND YOU WILL SEE THAT IS SO. 

So now youre trying to lump three of us that are on to your incompetence into having made a statement that likely doesnt even exist. Trying to read thru your rambling replies is really getting tedious.

So when does your "vacation" start? We really need a rest from you for 6 months or so.



Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on March 30, 2015, 05:55:31 PM
I honestly can't understand what Jim is trying to say.

Any technical stuff makes no sense, unless it is buried someplace in the barrage of personal commentary.

From what I gather, Jim thinks the bandwidth of two LC stages is as narrow as, or perhaps narrower than, the bandwidth of the entire system. I was hoping he could explain how he reached his conclusions, but I have a feeling this is like nichrome science.

I have Hallicrafters and Nationals on my desk. I take no offense what others think about either, but the NC300 and 303 are clearly far better performers.

The only thing I'm trying to do is chase a little harmonic distortion out of the NC303 product detector. It sounds like not enough injection level. But it still is one of the better old receivers.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 30, 2015, 07:08:50 PM
I concur about the NC-300/303 and that judgement came about decades after I left National; I didnt appreciate their engineering back then. Unlike the SX-115 they got excellent performance from the 6BA7 and didnt copy Collins who really made a mess of it in the 75A3 and 75A4. I wound up using the 7360 back in 65 in my A4 along with several other changes and I still use that radio. If I did it today Id use the 6ES8 in a Pullen which Ive done to a few sets.

I forget what I did with the NC-300 PD but upping the injection rings a bell, Id have to dig out my notes.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 30, 2015, 10:54:43 PM
Do you have any solid data on the Pullen mixer, like SSB NF and TOIP? Pullen doesn't even mention such parameters in his book. In a similar vein, some people quote an enr for the 7360 of 1500 ohms which is fairly obviously wrong - at least as a mixer.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 31, 2015, 08:30:48 AM
I dont believe Ive ever seen a 7360 ENR quoted and the one for the 6ES8 is 150 Ohms. The Squires Sanders SS1R used a 7360 without a RF stage and on a near perfect 50 Ohm single band antenna it did very well. Put it on a tribander, trap vertical, wire, etc and it became a birdie paradise. National had one on loan and it looked great in the lab but on the club TH6 at 90' it completely fell apart.

With a 7360 in both conversions, and a 6GM6 in the RF stage the useable sensitivity and dynamic range of my 75A4 increased tremendously based upon side by side comparisons with an all stock A4 and a lot of DXing and contesting. Never did a full overload/TIOP test but 10M CW NF went from 14dB to 6dB when I converted the second A4 in 1984 and ran the tests at work with a HP 8970A. The A4 was never known as a sensitive radio on the higher bands and overloaded easily out of the box.

Ive never really cared for lab tests of various overload parameters as they rarely, if ever, hold up on good antennas under contest conditions, DX pileups, etc.. Sort of reminds me of EPA fuel economy quotes.

Carl


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KG8LB on March 31, 2015, 08:54:42 AM
Do you have any solid data on the Pullen mixer, like SSB NF and TOIP? Pullen doesn't even mention such parameters in his book. In a similar vein, some people quote an enr for the 7360 of 1500 ohms which is fairly obviously wrong - at least as a mixer.

 Great question  !


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 31, 2015, 11:13:10 AM
Quote
Do you have any solid data on the Pullen mixer, like SSB NF and TOIP? Pullen doesn't even mention such parameters in his book

The only "solid data" I have on the Pullen is that it does what was claimed in the old magazine articles.

Ive used 6SN7, 6J6, 12AT7, in various boatanchors so I didnt have to change sockets or build adaptors. Sensitivity is highly improved where needed, overload performance seems much better in casual use and it does pull some HFO circuits where a simple change of tube type to say a dual triode for an oscillator and a buffer stage mitigates the problem.

If/When I get some spare time I might do a 6ES8 swap into the SX-115 or 75A3 and run a thorough set of tests; Ive had my own HP 8970A since 2002 and use it often; plus a decent set of other gear for the IP3 and other tests.

Carl


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on March 31, 2015, 12:28:52 PM
Carl,

Why a variable mu 6ES8 rather than a 6DJ8 for a mixer? Are you looking to apply AGC? The higher gm should give a lower enr than a 6ES8.

Looking at various published figures, it looks very much as if a 12AU7 is about as good on IMD and dynamic range as a 7360.....

73

Peter G3RZP


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on March 31, 2015, 04:34:09 PM
Both tubes have almost identical 12500 transconductance with the 6ES8 having the interesting ability of running on the AGC line, the only dual triode from the US Im aware of. The 6DJ8/6922 is an audiophool favorite and priced accordingly plus my experience with them in TEK scopes and in several VHF projects of the day was they had a relatively short peak performance life and then became just another TV front end tube.

Carl


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on April 01, 2015, 09:16:58 AM
Good morning Sam,

Your Question:
Don't know anything about Hallicrafters designs, but in following this thread I must admit surprise that moving an IF frequency 250 Hz would have any significant impact on BFO back feed. Is it even possible for a 50 kHz LC IF filter to have skirts steep enough that a 250 Hz shift would make a difference?
Sam
--------------------------------------------------------------------------------------------------------------------------------------
Answer:
The thought is understandable but It's a 50Kc system and it's in only one half of the filter making it even less logical.  Best thing is to let you do a fairly simple test from the operator's panel.  That will make more sense than me trying to explain.

If you have an SX-100, light it up, short the antenna points to prevent any signal from interfering with the test.
For the duration of the test I will assume the radio has been properly aligned.
 
Set any band preferably band 3
 
BFO on and at “0”

RF gain “10 to set the S-meter to “0”

Band pass to 1Kc

Now look at the S-Meter.

For the Mark1 it will normally be off the peg to some degree.

For the Mark 2 it will be “0”

The BFO frequency will be 50.000Kc for both.

Now increase the BFO frequency while watching the S-Meter.

You will note as you progress the S-Meter will rise to S5 or higher and remain there.

Now set the band pass to .5Kc

Do the same thing starting at BFO “0”

You will now note as you increase the BFO frequency the S-Meter will peak then dip.

What happened as you increased the BFO frequency is the BFO signal passing through the right filter segment backward on its way to AVC detector, followed the .5Kc filter character, dipping at the far skirt.  You can measure the band pass and steepness of the skirt with the result.
  
In the Mark1 the S-Meter was already off the peg and as you increased the BFO frequency it increases further then dropped following the band pass.  The S-Meter reading clearly indicates the BFO was already well into the skirt frequency when at "0".  

In the Mark2 the S-Meter was at “0” not off the peg and as you increased the BFO frequency it peaked then dropped following the band pass.  Note the S-Meter was not already increasing.  There was a space between the BFO and skirt.
 
Now attach a frequency counter to the BFO and vary the BFO frequency while watching the S-Meter.

Pay special attention as to where the BFO frequency begins to affect the S-Meter in the Mark2.  Then you will clearly see why the additional 250 cycles was implemented.

This will provide an extraordinarily good view of the effect a 250 cycle shift in frequency affects the band pass.  This is self-explanatory --- much better than only words.

The original design attributes were to use the radio with the BFO at “0” because the manipulation of the BFO in the positive region would provoke feedback.  The Mark 2 essentially accomplishes this but the positive region is still not a good place to operate.


If you are not yet convinced that it is reverse feedback, clip C76; you can tack it back.  It's easy to get to.

 Finally, by creating a CW audio pass band of 500 to 1000 cycles rather than 250 to 750, it enabled the use of CW with the BFO at “0” by simply tuning for tone; not manipulating the BFO.

Have a great day and if this is not clear please let me know.

Kindest regards Jim K9AXN  


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KG8LB on April 01, 2015, 12:54:53 PM
Jim ,

 Once again , many thanks . That was a fine explanation .


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on April 01, 2015, 01:47:26 PM
Thanks Gary,

Genuinely appreciate the response.

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on April 01, 2015, 01:56:26 PM
The same result could have been had by moving the BFO frequency in the original design. Many CW ops prefer lower beatnotes due to hearing loss, and the fact that it is easier to differentiate between signals (with the same frequency difference) as the audible beatnotes are lowered in frequency.

From what I am reading, this only serves to mask an existing problem by shifting the audible range for the recovered CW beat frequencies... without addressing the actual cause relating to BFO blowbly--whether resulting from poor shielding, inadequate bypassing, or the use of an unbalanced mixer in the product detector.

Again, as I had originally questioned, does this go back to the slope of the Hallicrafters LC filter response being inadequate for proper BFO injection frequency?

I don't understand the referral to "feedback."  Is this a poor choice of wording, where filter blowby might have been more suitable?  Again, shielding, a balanced mixer in the PD, and bypassing should have taken care of the problem.

Jim, if what you are claiming is true, then Hallicrafters produced a real turd, and subsequent design modifications only served to put lipstick on a pig.  And going back, what did changing the disc ceramic capacitors actually improve, considering that the design was an fiasco according to your own posts?


Pete


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on April 01, 2015, 03:07:04 PM
The radio doesn't have a product detector and a lot of folks like that radio.  The change does not prevent you using the BFO to alter the audio range.  The change enables you to Use Cw with an audio range of 500 to 1000 cycles without touching the BFO.   

I appreciate you comment and thanks for taking the time to respond.

Kindest regards Jim K9AXN   


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on April 01, 2015, 07:21:54 PM
Last time I'll address the caps Pete.

You ask if those caps were a problem. 

I described the effect they had on the filter.  I repeat, the 1Kc position would become approximately 1.6 Kc wide and the center frequency would not be where it belonged.  The 2Kc, 3Kc, and 5Kc positions would also be skewed.

That said, everybody has a different opinion regarding the consequence or effect.  I consider this problem minimal because I don't work competitive radio; I care more about pleasant audio.  Someone that works CW using the 1Kc bandwidth would probably say the filter is too wide calling it unacceptable.  A person that works some competitive SSB radio expecting the narrowest intelligible voice would use a different radio saying the radio is broad as a barndoor.  The people that use AM will say this is great because they don't care where the center frequency is and the bandwidth would be wider than 6Kc.  So there you have it. 

If you wish to go over the details and calculations send your email.  I'll be happy to work with you.

I have to move on.

Have a great night.

Kindest regards Jim K9AXN




Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on April 02, 2015, 07:40:49 AM
Jim ,

 Once again , many thanks . That was a fine explanation .

Except it was technically inaccurate, and doesn't explain the "theory" that the IF was shifted 250 Hz to reduce BFO back feed.

The AVC level peaks and dips are actually because the BFO gets into everything, including early IF stages. That is a shielding and bypassing issue. Since the BFO has multiple paths into the AVC detector, all through different paths that cause various electrical delays in the BFO signal, as the BFO is moved the random phase of the multiple paths changes. This causes a level change with frequency. It's a little bit of several different things mixing.

This effect is actually very common with multiple signal leakage paths, especially into selective systems with several stages. It is nothing new or unique, and far more complex than one transformer section.

The bottom line though, is the claim Hallicrafters shifted the IF 250Hz to reduce BFO leakage is obviously wrong. If we shift the IF 250 Hz, on a coupled pair that is a few kHz wide at 6 dB (even in the 500Hz position), the change is meaningless.  Even if there were a change in level, the BFO has to be moved to the ***exact same relationship *** to the signal to have the same tone, so nothing changes.

73 Tom


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on April 02, 2015, 08:02:47 AM
[   Even if there were a change in level, the BFO has to be moved to the ***exact same relationship *** to the signal to have the same tone, so nothing changes.

73 Tom

Except that Jim also stated that the range of audible frequencies was CW beatnote range was also increased, limiting the lower frequency that is  preferred by most CW operators. It is still masking a problem, and doesn't make any sense from that perspective. Simply moving the BFO knob would have accomplished the same results.

Pete


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: AC2EU on April 02, 2015, 08:31:20 AM
OK, no more need to bury the horse...it's been beaten into the ground!  ::)   ;D
Bottom line, when restoring an old radio, use the best appropriate modern caps for the intended purpose.
No doubt there will be some improvement, but At the end of the day, it was still designed half a century ago.
If you want something more accurate, buy a new rig. Geeesh!


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on April 02, 2015, 10:07:41 AM
Thanks Jim,

I'm out of here.

A good day to ya

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on April 02, 2015, 10:51:53 AM
>If you want something more accurate, buy a new rig.<


Maybe. I just wouldn't assume that a new rig is necessarily better than some of 30 and 40 year old ones just because it's new.....

In the same way as it is not necessarily a good idea when management decides that 'change is progress'...


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on April 02, 2015, 11:10:02 AM
OK, no more need to bury the horse...it's been beaten into the ground!  ::)   ;D
Bottom line, when restoring an old radio, use the best appropriate modern caps for the intended purpose.
No doubt there will be some improvement, but At the end of the day, it was still designed half a century ago.
If you want something more accurate, buy a new rig. Geeesh!

Exactly what I suggested eons ago...

Pete


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W8JI on April 02, 2015, 11:18:52 AM
OK, no more need to bury the horse...it's been beaten into the ground!  ::)   ;D
Bottom line, when restoring an old radio, use the best appropriate modern caps for the intended purpose.
No doubt there will be some improvement, but At the end of the day, it was still designed half a century ago.
If you want something more accurate, buy a new rig. Geeesh!

I thought there might be some technical basis for the claims. There is often something to be learned when something does not make sense, but perhaps not in this case.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KG8LB on April 02, 2015, 01:56:32 PM
OK, no more need to bury the horse...it's been beaten into the ground!  ::)   ;D
Bottom line, when restoring an old radio, use the best appropriate modern caps for the intended purpose.
No doubt there will be some improvement, but At the end of the day, it was still designed half a century ago.


   And could have worked better then . With available components at the time  . This was not Hallicrafter's "bargain priced" offering .

>If you want something more accurate, buy a new rig.<


Maybe. I just wouldn't assume that a new rig is necessarily better than some of 30 and 40 year old ones just because it's new.....

In the same way as it is not necessarily a good idea when management decides that 'change is progress'...

   So true !

 


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on April 04, 2015, 09:42:03 AM
Quote
OK, no more need to bury the horse...it's been beaten into the ground!  Roll Eyes   Grin
Bottom line, when restoring an old radio, use the best appropriate modern caps for the intended purpose.
No doubt there will be some improvement, but At the end of the day, it was still designed half a century ago.
If you want something more accurate, buy a new rig. Geeesh!

I thought there might be some technical basis for the claims. There is often something to be learned when something does not make sense, but perhaps not in this case.

There was nothing wrong in 1960-61 with the SX-115 design or choice of components. This complete thread is about one persons often confused beliefs and trying to change that is in the same category of our voodoo nichrome believer who has been preaching his nonsense for over 25 years.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KG8LB on April 04, 2015, 10:06:07 AM
Aside from the fact it could and should have been better , especially considering the price . ..... ;)


   Jim's comments had more substance than the bashing that followed .

 


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KB4QAA on April 04, 2015, 10:55:59 AM
Aside from the fact it could and should have been better , especially considering the price . ..... ;)
 
That was not the subject of this thread.  At all!


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on April 04, 2015, 11:04:59 AM
 
Quote
  Jim's comments had more substance than the bashing that followed .

Thats YOUR opinion and a few other groupies that talk but dont do the work.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KG8LB on April 04, 2015, 12:30:37 PM
Aside from the fact it could and should have been better , especially considering the price . ..... ;)
 
That was not the subject of this thread.  At all!

   Actually , that it could have been better with proper parts selection is what Jim had claimed . Jim made his points well and politely .

There were plenty of comments that were not the subject of the thread .


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: W1BR on April 04, 2015, 12:43:44 PM
Aside from the fact it could and should have been better , especially considering the price . ..... ;)
 
That was not the subject of this thread.  At all!

   Actually , that it could have been better with proper parts selection is what Jim had claimed . Jim made his points well and politely .

There were plenty of comments that were not the subject of the thread .

I think it was shown that the radio would have worked, as designed, when it left the factory by the info that Tom supplied. Whether a disc ceramic cap, with Z5U dielectric, would be out of tolerance 60 years later is as moot as leaving wax paper caps in place. No parts last forever.

I would not argue that is good practice to replace vintage parts with modern components that are more stable, especially when the cost is usually only pennies.

Comments of a technical nature shouldn't be taken as personal assaults.  From what I read, Jim welcomes open dialog.

Pete


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: G3RZP on April 04, 2015, 03:03:25 PM
I would NEVER, as a professional engineer, have tried coupling tuned circuits with ceramic capacitors unless they were NP0. I would have used silver mica, stacked mica or polystyrene - probably the latter, but the types made with multiple leads to the foils to keep the Q up.

However, the skirt selectivity with multiple tuned circuits cannot ever compare with the shape factor of a crystal or mechanical filter because the elements just don't have the Q. Add to this, the answer to the question "How long is this thing supposed to work for?"

Performance good enough 50 or more years ago may well have maintained said 'poor by today's standards' - or it may not. Although a 1938 National HRO kept in a reasonable atmosphere might well still meet its original spec, which isn't generally quite good enough for today's conditions. If one is designing for a 10 year life under commercial pressure, different considerations apply.

Prior to my retirement from a semiconductor manufacturer, the company had had at one stage a 5 transistor array. Normal price, about 80 cents in quantity. One customer was paying $250 a pop - they needed a 30 minute guaranteed life when operating at 250C ambient. And 2000 pieces a year..

Different considerations.....


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on April 04, 2015, 03:17:04 PM
Gary,

It' great that you have the courage to speak but it will never go anywhere.

It will go quiet if there's no attention.

Kindest regards Jim K9AXN

   

 


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: K9AXN on April 04, 2015, 03:28:41 PM
Hi Pete,

I agree that the it's not possible for a discrete component filter to ever compare to a crystal filter.  It's absurd to thing otherwise.

The reason that I mentioned the 2.1 then 2.2Kc shape factor is to prompt a couple of the guys to at least look at the published charts.  They would have seen immediately that my comment was wrong.  Then we could have moved on.

Have a super day and thanks for participating.

Kindest regards Jim K9AXN


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KG8LB on April 04, 2015, 04:18:25 PM


Comments of a technical nature shouldn't be taken as personal assaults.  From what I read, Jim welcomes open dialog.

Pete

  I agree 100% . Jim was very open to comments . It seems there were some non- technical personal jabs tossed at him that were of little value.


Title: RE: Hallicrafters SX-115 selectivity design eror
Post by: KM1H on April 04, 2015, 06:26:32 PM
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I agree 100% . Jim was very open to comments .

About as open as ISIS is to the Pope.
As long as you agree with him 100% your golden and one of his groupies. Any deviation and youre insulted, lied about, degraded and totally confused by his disjointed and rambling replies.

Quote
It seems there were some non- technical personal jabs tossed at him that were of little value.

It comes with the territory, you sling the muck then expect to get a face full in return.