Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1] 2 3 4 5 6 ... 9 Next   Go Down
  Print  
Author Topic: Hallicrafters SX-115 selectivity design eror  (Read 44514 times)
K9AXN
Member

Posts: 442


WWW

Ignore
« on: March 13, 2015, 05:57:00 PM »

This is a common complaint regarding the Hallicrafters SX-115

β€œThe 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
« Last Edit: April 19, 2015, 07:07:06 PM by K9AXN » Logged
N8CMQ
Member

Posts: 723




Ignore
« Reply #1 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!
Logged

N8CMQ   Jeff Retired...
K9AXN
Member

Posts: 442


WWW

Ignore
« Reply #2 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
Logged
N8CMQ
Member

Posts: 723




Ignore
« Reply #3 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!
Logged

N8CMQ   Jeff Retired...
G3RZP
Member

Posts: 1326




Ignore
« Reply #4 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!
Logged
K9AXN
Member

Posts: 442


WWW

Ignore
« Reply #5 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]
Logged
K9AXN
Member

Posts: 442


WWW

Ignore
« Reply #6 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]
Logged
WA1RNE
Member

Posts: 1010




Ignore
« Reply #7 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.
Logged
W1BR
Member

Posts: 4196




Ignore
« Reply #8 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.
Logged
G3RZP
Member

Posts: 1326




Ignore
« Reply #9 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.....
Logged
K9AXN
Member

Posts: 442


WWW

Ignore
« Reply #10 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
Logged
W1BR
Member

Posts: 4196




Ignore
« Reply #11 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
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #12 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.  Smiley
  
73 Tom
« Last Edit: March 20, 2015, 05:33:30 AM by W8JI » Logged
W1BR
Member

Posts: 4196




Ignore
« Reply #13 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
Logged
G3RZP
Member

Posts: 1326




Ignore
« Reply #14 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
Logged
Pages: [1] 2 3 4 5 6 ... 9 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!