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Author Topic: Hallicrafters SX-115 selectivity design eror  (Read 44499 times)
K9AXN
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« Reply #45 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    
      
« Last Edit: May 11, 2015, 08:59:45 AM by K9AXN » Logged
G3RZP
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« Reply #46 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....
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K9AXN
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« Reply #47 on: March 25, 2015, 10:03:36 AM »

A little humor helps Pete.  Thanks.

Kindest regards Jim K9AXN
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G3RZP
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« Reply #48 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.
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W8JI
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« Reply #49 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
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AC2EU
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« Reply #50 on: March 25, 2015, 02:16:50 PM »

The SX-115 selectivity horse is dead guys...   Roll Eyes
Can we give it a nice burial?  Cry
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W8JI
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« Reply #51 on: March 25, 2015, 03:31:45 PM »

The SX-115 selectivity horse is dead guys...   Roll Eyes
Can we give it a nice burial?  Cry


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.



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K9AXN
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« Reply #52 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          

 
« Last Edit: May 02, 2015, 07:46:46 PM by K9AXN » Logged
K9AXN
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« Reply #53 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
« Last Edit: May 02, 2015, 07:44:32 PM by K9AXN » Logged
W1BR
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« Reply #54 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

« Last Edit: March 26, 2015, 11:17:59 AM by K1ZJH » Logged
W8JI
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« Reply #55 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

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W8JI
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« Reply #56 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
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K9AXN
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« Reply #57 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
       
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« Reply #58 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   


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KG8LB
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« Reply #59 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 .
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