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Old 10th Jul 2020, 11:45 pm   #1
mrrstrat
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Default Another Hallicrafters Sky Champion S20.

Split from this thread:-

https://www.vintage-radio.net/forum/...d.php?t=164495



I am working on a S-20 (not the "R") as well. I am having the same problems you are having.

The question I have first is: how did you get the little box housing the caps for the BFO tube? I cannot figure out how to remove it. I have replaced all caps and resistors. It nearly worked when I got it, but aligning the radio (messing with T1 and T2) one of the 600 Ohm dog bone resistors went up in smoke. I need to get the little BFO box off so I can replace the components.

Then I noticed the BFO plate is NOT connected and was clipped - from where I don't know as time has erased where it looks like it came from.

Can you post a pic of the interior with lots of closeups in the 6K7/6L5 and the BFO 6J5? I just need to see where each wire goes .
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Old 11th Jul 2020, 2:16 am   #2
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Default Re: Hallicrafters Sky Champion S20

Quote:
Originally Posted by usradcoll1 View Post
There is a world of difference between the S20 and the S20R. The S20R has two I.F stages and a different dial drive setup. I have to review the schematic to locate the other differences. The S20R is the last one made before the start of the war.

Dave, USradcoll1.
I can attest the S20 and S20R are two different radios. Lots of info on the S20R, nearly nothing on the S20. I stumbled into this thread and am hoping to learn as the OP is on how to get this set running
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Old 11th Jul 2020, 5:58 pm   #3
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Default Re: Hallicrafters Sky Champion S20

Quote:
Originally Posted by mrrstrat View Post
I am working on a S-20 (not the "R") as well. I am having the same problems you are having.

The question I have first is: how did you get the little box housing the caps for the BFO tube? I cannot figure out how to remove it. .
Hi there on my radio on the underside of the chassis there are two small nuts with lock washers. The small bolts that hold the housing are miniature carriage bolts with no obvious head on the top. After unscrewing the nuts it is possible to lift the housing just enough to change the components underneath. Very tight for access don’t burn your fingers with the iron!
Good luck
Lynton
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Old 11th Jul 2020, 6:01 pm   #4
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Default Re: Hallicrafters Sky Champion S20

Quote:
Originally Posted by mrrstrat View Post
I can attest the S20 and S20R are two different radios. Lots of info on the S20R, nearly nothing on the S20. I stumbled into this thread and am hoping to learn as the OP is on how to get this set running
Hi Dave
Yes I agree that they are totally different radios. The point I was making was that the BFO circuit was very similar and I think uses the same valve(tube).

Lynton
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Old 11th Jul 2020, 6:55 pm   #5
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Default Re: Hallicrafters Sky Champion S20

Quote:
Originally Posted by LyntonP View Post
Hi there on my radio on the underside of the chassis there are two small nuts with lock washers. The small bolts that hold the housing are miniature carriage bolts with no obvious head on the top. After unscrewing the nuts it is possible to lift the housing just enough to change the components underneath. Very tight for access don’t burn your fingers with the iron!
I kinda figured this out. I loosened the screw and ended up drilling out the rivet on the bottom to remove the box (yea, I know NOW I should have looked underneath to see it was only held on by two small bolts).

I found out why there was problems when I got under there: the 250pf had its wire broke at the socket pin. And the 83-year old cap looked like it was brand new! I never saw one in pristine condition. I replaced it with a HV mylar 250pf, the 0.01 was replaced, and the 100K with a 1-watt metal oxide. I did assemble it back and one anchor holds the box rather well (until I find another one).
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Old 11th Jul 2020, 7:15 pm   #6
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Default Re: Hallicrafters Sky Champion S20

Guys, I am not hijacking the OPs thread: I just have a couple of similarly related questions. I am at the same point he is on my S20 and right now we are the only two people in the world I know with these radios and in the same condition.

The BFO on this radio appears to have been cut at some point many years ago. There is no sign of where the wire went. I do have a schematic but no nominal voltages of what the set should be when functioning properly (as many service manuals have but is not present in any manuals and documentation I have found on the S20).

I included pix of mine rebuilt: 100% of all caps and resistors. I highlight the section where the BFO tube wire is (I labeled it DISCONNECTED).

From the schematic I have it looks like it needs to attach BELOW the 30K resistor (pencil pointing to the place). The wire was cut and wrapped around another wire by the top of the 30K resistor (pencil again pointing to location).

I need a good pic of where the two light tan wires go - please post or send me one if you please

I am going to determine if the IFs are working: I think the OP was down this path as well.
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Old 12th Jul 2020, 3:13 am   #7
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Default Re: Hallicrafters Sky Champion S20

Radio appears to work: I was able to tune in a LOUD AM station. Volume is still lower than I think it should be.

I ended up getting lucky and adjusting the many variable caps manually by ear to establish the radio is able to work (RF, IR, mixer,..). And it is extraordinarily quiet and clear!

I could not figure out how to do the first steps of the RF alignment from the Rider documentation (are you supposed to hear a tone?). For my other radio you hooked at meter to the cathode of the FM detector and tuned 8 coils for "maximum deflection". The S20 instructions do not specify how you determine what the maximum resonance is.

How did you do this step?? Hooking up the signal generator was not the big deal: it is how you determine if the 455Khz resonance is determined I could not figure out from the documentation.
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Old 12th Jul 2020, 11:07 am   #8
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Default Re: Hallicrafters Sky Champion S20

Normal method is to inject a modulated signal at the IF. Connect an output meter in place of the speaker, or a 'scope across the speaker, and adjust the IF Transformers for maximum audio output.
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Old 12th Jul 2020, 1:23 pm   #9
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Default Re: Hallicrafters Sky Champion S20

I am guessing they are doing that very thing with injecting signal directly into the cap (grid) of the 6L7. This radio of course has a transformer on the speaker - without the transformer the output from the plates is around 230VDC. The speaker is 3.2 Ohms and during this part in the alignment procedure does NOT produce any sound whatsoever. I am assuming I do not have to remove the speaker from the transformer to measure it with a multimeter or a scope.

The only time I heard a tone was when I was tuning to an antenna tone when the AM band was 'tuned for loudest tone'.

I included the enigmatic Rider instructions.
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Old 12th Jul 2020, 1:35 pm   #10
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Default Re: Hallicrafters Sky Champion S20

If the output from your RF Signal generator is modulated with an AF tone, you should hear that tone from the speaker. If you don't you have a fault somewhere. Try swinging the RF frequency a bit in case the IF stages are off tune.

Otherwise you may have a fault with the audio stages. Try injecting an AF tone directly into them and listening for it at the speaker.
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Old 12th Jul 2020, 2:06 pm   #11
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Default Re: Hallicrafters Sky Champion S20

I was thinking the same thing: The tone was not here and I did both a sweep of the signal generator and even switched to different frequencies (in the event a harmonic was to be detected). I got NO sound. I changed the T1 tuning throughout the entire range and tried different combinations to no avail - no sound.

My thought process was that I needed to see if the RF was resonating properly, then check the IFs. I tried various things to inject the signal into the radio and detect evidence of a working radio with my scope. During this process I heard a faint station poke out of the speaker and see if I could pull it out by simply tuning it in with the RF/IF slugs.

I ended up finding a strong station in my area and then tuning the T1/T2 transformers to make it as strong as I could. I then did the band 1 procedure and did not find the tone either. But I did tune the three (oscillator, mixer, antenna) pots to make the station come in pretty good.

This showed me that the RF-->IF stages were capable of working as I did get a station reasonably well. To my knowledge if the RF and IF oscillators were not working at all I could not get the station tuned. And the detection is working as I get a sharp and clear channel without background noise. In fact the station I tuned to is FM station sharp. Even stranger is that the station is very close to what the dial shows it should be (around 1490khz). I can only get another station somewhat adjacent to this one. None others will tune and I am lead to believe that means the RF frequency may be wrong.

I had a very similar process with my SX43: initially I had a tuned in channel using this "method" and was able to later fully align it. I am hopeful that this radio has the same situation as well.

So next step is to get the RFs to be 455Khz and then move forward to tuning the bands.
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Old 12th Jul 2020, 2:11 pm   #12
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Default Re: Hallicrafters Sky Champion S20

If you can receive a station, then obviously the receiver is working to some extent.

Are you sure that your signal generator is working correctly and generating a MODULATED signal? Try it on a known working receiver by injecting a signal at the aerial socket.
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Old 12th Jul 2020, 2:19 pm   #13
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Default Re: Hallicrafters Sky Champion S20

Excellent question and yes: I am using a calibrated and upgraded Heathkit IG102 (1970s model) and have it set for modulated tone. Interestingly enough the SX-43 requires that the un-modulated signal to be injected into the stator gang and the voltage of the FM detector cathode be polled for a DC voltage deflection. I used this signal generator to fully align my SX-43 a couple weeks ago. And I can get a tone with the S-20 but it seems to be an associated harmonic of the frequency I have the signal generator set to and not the actual frequency. The tone I get I have to search for by manually sweeping the signal generator with the modulated tone enabled. A most strange clue indeed.

I tried to repeat this on the S-20 but those two radios are different radios altogether. In fact, I did not find a signal change in any of the tubes I could interpret similarly as the SX-43 procedure for alignment of the RF oscillator.
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Old 12th Jul 2020, 2:31 pm   #14
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Default Re: Hallicrafters Sky Champion S20

Some receivers have their IF stages aligned by tuning for maximum agc voltage or maximum S Meter reading. This can be done using an unmodulated signal.

It's much more common to inject a modulated signal and tune for maximum audio output.

If you're injecting a modulated signal and can't hear it, then either the signal generator is faulty, which apparently it isn't, or the IF stages are off tune. You could try injecting the signal further on in the IF chain.

You do have the receiver set to AM?
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Old 12th Jul 2020, 2:58 pm   #15
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Default Re: Hallicrafters Sky Champion S20

Agree with this and yes: the S20 is only AM and I have it on the AM band. I am an electrical engineer, but not a RF/radio expert (audio and amplifier expert). So I did try to inject the tone in different places (2nd IF, HF,..) to detect it. This is a common method in debugging multi-stage and multi channel guitar amplifiers. I have taken the time during the Covid lockdowns and self-isolations to learn about radios .

My best guess is that the RF oscillator is way off and not at 455mhz. And possibly why other channels cannot be tuned in but a few is the fact the error in the RF oscillator precludes any other stations than the two I can get. Possibly it is at some relative harmonic that favors the 1400-1500 Khz stations perhaps?

Yesterday morning I was at the phase of scrutinizing my replacement of ALL of the caps and resistors without a good incremental test periodically during the process (which I did with the vastly more complex SX-43). Some of the exact values of those components I could not get (50K resistor, 25K resistors, 0.05uF caps) and replaced them with values within 10% or ganged resistors and caps to get the value. for 25Ks I used 24K resistor, for 50K I used 51K, and for 30K I used 27K. All smaller order resistors were either ordered in their exact values or I made the value. The same with the capacitors - I took no other drastic artistic liberty in values. The 0.05uf values were replaced with the 0.047uf values, for instance.

Any key circuit components were matched within what I would thought the typical component variation of the day allowed (in 1938 when the radio was built).

And I checked the tubes for a third time with a TV-7 tester. The tubes are all strong and rated beyond NOS and I have multiples. I learned from my recent and new fix-it activities with radios that even a great tube may not work in a radio circuit - you have to pop it in and try it. This was a major lesson for me and adds a layer in troubleshooting. Tube rolling is part of the process I have used. And the BFO section before I fixed it burned up three premium 6J5s I had (ala tube rolling to see if things 'got better'). On the topic of the BFO: I still have to get the loose wire for this installed but the BFO switch turns it off completely and does not affect the operation of the radio when it is not connected - I am trying to get a good picture of another S20 to give me an idea where the lead dress for this re-connection should go.

I guess I am really back at the phase to interpret the archaic document to understand what they meant in it for starting the alignment process (everything else after this one sentence I understand fully):

"Remove 6L7 grid cap. Connect signal generator through 0.1-mf condenser to the grid of 6L7, and signal generator ground to chassis of set. Set signal generator to 455 kc. Adjust trimmers on T1 and T2 for exact resonance."

How is this exact resonance to be determined? The Riders documentation leaves this method out. I included a picture of the 6L7 and its grid cap.

The grid of a 6L7 is the metal top, so I have been clipping the signal generator through a 0.1uF cap to it. Since the voltage is under 5v, I even tried directly hooking it up to the tube cap (no 0.1uf in series). No sound. The set might be that out of alignment.
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Old 12th Jul 2020, 3:16 pm   #16
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Default Re: Hallicrafters Sky Champion S20

By AM I meant distinct from SSB ie BFO off.

Only the changing of very low value capacitors in the RF and IF stages is likely to affect alignment.

The "archaic" alignment instructions look perfectly understandable to me. We are talking about a vintage radio, but such alignment instructions would have been valid right into the transistor era.

Seems to me like you have an impossible fault with contradictory facts. You'll have overlooked something somewhere.
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Old 12th Jul 2020, 3:41 pm   #17
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Default Re: Hallicrafters Sky Champion S20

I totally agree with you lol. Sometimes part of the resolution process I have found is to re-articulate the issue to another engineer, symptoms, and what you did to try to fix it and then the reason jumps out.

I am pretty confident in the rebuild as I took out a component and replaced it right away.This is a simpler radio than the SX-42 or SX-43 for sure. And I did assume I did something wrong and re-evaluated the replacement and circuit a couple times yesterday. In my SX-43 I actually had to correct a couple of wiring deviations I found a previous owner did many decades before I got the receiver. It was not capable of working before I corrected it and used the Hallicrafters, Rider, and Sams documentation as a guide.

Myself not so familiar with the alignment process (and 35 years separates me from core studies on RF from school) I do not understand how the determination of exact resonance is to be accomplished: the text does not say 'listen for tone' or 'measure for deflection at X point'. My newness to radio repair makes this a most cryptic prescription. Certainly the techs of the day - or someone more familiar with radio repair - could better contextualize this.

I also agree: I am overlooking something fundamental. I want to look closer at the 6L7 - this seems to be a re-occuring nexus of focus in this project. The 1/3 watt 600 Ohm dog bone resistor in this location originally got smoked before I declared war on the dog bone resistors in the set and removed all of them.
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Old 13th Jul 2020, 12:29 pm   #18
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Default Re: Hallicrafters Sky Champion S20

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Originally Posted by mrrstrat View Post
The BFO on this radio appears to have been cut at some point many years ago. There is no sign of where the wire went. I do have a schematic but no nominal voltages of what the set should be when functioning properly (as many service manuals have but is not present in any manuals and documentation I have found on the S20).

I included pix of mine rebuilt: 100% of all caps and resistors. I highlight the section where the BFO tube wire is (I labeled it DISCONNECTED).

From the schematic I have it looks like it needs to attach BELOW the 30K resistor (pencil pointing to the place). The wire was cut and wrapped around another wire by the top of the 30K resistor (pencil again pointing to location).
That cut wire and the one it was wrapped around it might be the BFO injection capacitor (C31 a twisted wire capacitor) a capacitor in that circuit position was often made up of two insulated wires wrapped together AKA a gimmick capacitor, perhaps you could confirm either way?

Lawrence.

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Old 13th Jul 2020, 12:33 pm   #19
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Default Re: Hallicrafters Sky Champion S20

Described in the parts list as 1uuF twisted pair.
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Old 13th Jul 2020, 1:22 pm   #20
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Default Re: Hallicrafters Sky Champion S20

I am pretty certain that this wire was misplaced - This definitely is the plate wire and responds to states of SW2 (the BFO ON/OFF switch). It would appear that the S20 actually uses the SW2 switch to interrupt the power to the plate (thereby effectively turning it off).

The pic enclosed shows the wire with a tag on it (in the event I put this radio away long enough to forget why it is disconnected). I put a length of red heat shrink tubing on it to further insulate it.

This wire I discovered was not connected when I noticed the BFO 6J5 was not hot like the others and the BFO switch appeared to do nothing. Tracing and following the switch I noticed this wire pulled off to the right and single wrapped around the 6K7 suppressor grid wire with about 1.5cm extended as if twisted and placed out of the way. And the looks of it appeared to have been there for many, many decades.

I think it belongs at the bottom of the 30K resistor R6 where I am pointing the mechanical pencil at (Riders documentation enclosed to show where this location is at in the schematic) as this point gives a 250-ish plate voltage and appears to be correct in the schematic. Problem is that it is about 10cm of wire needs to be added to get to it and I wanted a pic of where this wire originally went to make sure the lead dress was correct in any placement I am selecting.

C31 is indeed a twisted 1uf - I'll check out if it is indeed a gimmick capacitor (or an actual 1uf - I didn't remember seeing this whilst swimming around in the inners of this radio). And I'll check if the wire makes sense to be the function as you indicate: I did DMM the wire against the BFO socket and verified it is indeed the pin #3 (plate) wire. But I am new to these type of circuits and still figuring out radio-stuff.
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