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Author Topic: Hallicrafters SX-115 selectivity design eror  (Read 44478 times)
KM1H
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« Reply #30 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
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G3RZP
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« Reply #31 on: March 22, 2015, 04:21:35 PM »

Carl,

What is the difference between testing for Q and DF?
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KM1H
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« Reply #32 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
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KM1H
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« Reply #33 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
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K9AXN
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« Reply #34 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.
 
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K9AXN
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« Reply #35 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
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G3RZP
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« Reply #36 on: March 23, 2015, 01:16:38 AM »

Q and DF - not opposites but reciprocals. So you only need to measure one of them.....
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K9AXN
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« Reply #37 on: March 23, 2015, 07:36:32 AM »

Hi Pete,

Thanks for the semantic correction.

Kindest regards Jim K9AXN
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K9AXN
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« Reply #38 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
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K9AXN
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« Reply #39 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
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W9NVN
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« Reply #40 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?Huh 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
« Last Edit: March 24, 2015, 04:09:41 AM by W9NVN » Logged
K9AXN
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« Reply #41 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    
« Last Edit: May 11, 2015, 08:32:46 AM by K9AXN » Logged
W8JI
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« Reply #42 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

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G3RZP
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« Reply #43 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'.
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« Reply #44 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
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