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Old 5th Aug 2020, 3:29 pm   #141
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Default Re: Another Hallicrafters Sky Champion S20.

For the previous tests I left the LO 6J5 valve out: should I replace it for this test?

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If that's with 50mV Peak to Peak from the generator on g1 of the mixer then it looks like the mixer is amplifying ok, if that's the case leave the generator settings as they are and transfer the 'scope probe to g1 (the top cap) of the IF amplifier and see what's what there.

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Old 5th Aug 2020, 3:33 pm   #142
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Default Re: Another Hallicrafters Sky Champion S20.

NO!

The IF stages don't need a local oscillator to work.
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Old 5th Aug 2020, 3:34 pm   #143
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Default Re: Another Hallicrafters Sky Champion S20.

I did try this with and without the LO valve is: on the grid cap of the second 6K7 I get absolutely no signal detected.

* EDIT: Schematic added for ref *
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Old 5th Aug 2020, 3:37 pm   #144
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Default Re: Another Hallicrafters Sky Champion S20.

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For the previous tests I left the LO 6J5 valve out: should I replace it for this test?
No, leave the LO 6J5 valve out for this series of tests.

EDIT: Post crossed.

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Old 5th Aug 2020, 3:50 pm   #145
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Default Re: Another Hallicrafters Sky Champion S20.

Measure the DC voltage on the top cap of the IF amplifier.

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Old 5th Aug 2020, 3:54 pm   #146
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Default Re: Another Hallicrafters Sky Champion S20.

1.87VDC measured

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Measure the DC voltage on the top cap of the IF amplifier.

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Old 5th Aug 2020, 4:07 pm   #147
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Default Re: Another Hallicrafters Sky Champion S20.

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1.87VDC measured
Now measure the DC voltage on the cathode.

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Old 5th Aug 2020, 4:21 pm   #148
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Default Re: Another Hallicrafters Sky Champion S20.

4.62VDC at 6K7 cathode.

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1.87VDC measured
Now measure the DC voltage on the cathode.

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Old 5th Aug 2020, 4:34 pm   #149
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Default Re: Another Hallicrafters Sky Champion S20.

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4.62VDC at 6K7 cathode.
Is that with the RF gain set to maximum?

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Old 5th Aug 2020, 4:49 pm   #150
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Default Re: Another Hallicrafters Sky Champion S20.

Its 35.74VDC when RF gain turned all the way down and 4.61VDC at max RF gain.

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4.62VDC at 6K7 cathode.
Is that with the RF gain set to maximum?

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Old 5th Aug 2020, 5:13 pm   #151
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Default Re: Another Hallicrafters Sky Champion S20.

Just a comment.

This is the valve which previously had no HT on the anode. See post #69.

I thought this would be a break through, but it wasn't.

The lack of HT was traced to an open circuit winding in the IFT. The IFT was replaced, but there was still no HT on the anode. Apparently the original IFT and its replacement had both been miss-wired. I don't think we were given any further details.
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Old 5th Aug 2020, 5:20 pm   #152
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Default Re: Another Hallicrafters Sky Champion S20.

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Its 35.74VDC when RF gain turned all the way down and 4.61VDC at max RF gain.

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Quote:
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4.62VDC at 6K7 cathode.
Is that with the RF gain set to maximum?

Lawrence.
That's ok, about 3 volts -ve grid bias at max gain.

So, assuming the HT is adequately de-coupled, you have 2 volts peak to peak in across the primary of the 1st IF transformer and nothing across the secondary but there appears to be DC continuity between g1 and the AVC line, check the resistance of the secondary anyway to make sure it's not shorted out, other contenders could be the secondary miles off tune with reference to the primary or no AC grounding for the secondary.

EDIT: Post crossed.

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Old 5th Aug 2020, 5:26 pm   #153
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Default Re: Another Hallicrafters Sky Champion S20.

I'll check this all very shortly: switching tasks but will post replies. Thanks for the help on this
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Old 6th Aug 2020, 4:46 pm   #154
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Default Re: Another Hallicrafters Sky Champion S20.

I believe you are referring to the second stage: this one I did replace with another one I bought. Thats when things got better.

I do have a bunch of IFs of different types: I have something marked as a "Bell Industries 455 KC I.F. Input & Interstage". Is that one suitable for the first IF stage?

I enclosed a pic - I would rather replace it all together like I did the other one if it would be an improvement.


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Just a comment.

This is the valve which previously had no HT on the anode. See post #69.

I thought this would be a break through, but it wasn't.

The lack of HT was traced to an open circuit winding in the IFT. The IFT was replaced, but there was still no HT on the anode. Apparently the original IFT and its replacement had both been miss-wired. I don't think we were given any further details.
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Old 6th Aug 2020, 5:50 pm   #155
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Default Re: Another Hallicrafters Sky Champion S20.

Please ignore me. It was a passing comment.

Carry on as suggested by Lawrence.
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Old 7th Aug 2020, 1:36 pm   #156
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Default Re: Another Hallicrafters Sky Champion S20.

On post #154: does that IF Can I have sufficiently replace the one I have for the FIRST stage IF? The box says its a "455 KC I.F. Input & Interstage" type.

The second stage on my set I totally replaced with a HV 455Khz IF and appears to work.
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Old 7th Aug 2020, 1:43 pm   #157
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Default Re: Another Hallicrafters Sky Champion S20.

As suggested by Lawrence in post #152 check the resistance of the IFT windings. It's not unknown for the trimmer caps connected across the wings to go short circuit if over tightened.
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Old 26th Aug 2020, 2:47 am   #158
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Default Re: Another Hallicrafters Sky Champion S20.

I got a signal tracer in my equipment herd a few days ago and used it today. It took me about 10 minutes to determine the issue is in the 6Q7-6F6 area of the set. Using the tracer I was able to poll the dual diode input (from the second IF) of the 6Q7 (after both IFs) and actually get great volume, excellent tuning, with stations on medium wave everywhere, and even band 2 I was picking lots of activity.

I was also able to establish the BFO works great (I put it back where I originally found it on the plate wire of the 6L7). The demodulation and feeding the audio section are the issue and the weird tuning issues I reported was because of the problems with the 6Q7/6F6 sections. I had no problems through the entire range of the set on long wave and two of the bands using the signal generator as my detector/audio source. The fact I received around 1300Khz happens to be because the strongest station in my area is on this frequency and I tuned the IFs manually using this station as a signal. I am glad it is this as I suspected the fixed coils or tuning capacitors may have been the issue.

In looking closely I did see a couple key components were of incorrect value (a sharp eye focusing on where the issue lies thanks to the signal tracer). I also am using a TV-7 to scrutinize the tubes in question and maybe select a better set than what I am using.

Everyone here on this forum has been a big help on this rather long journey.

I plan to revisit the injection of a 455Mhz modulated signal and see if I can see if anything changed after some of the things I did improved things. I fully expect the signal tracer to reveal the missing tones and until I correct the poor demodulation behavior will not pull in very much on the set. It is functioning much better and MUCH louder than it did before. I also corrected something in the wiring to the schematic for the S-20 (and a cap value change from what was there originally to what the schematic showed).

I am assuming that if I can get a clear and loud signal at the grid of the second 6K7 grid and plate and it is still loud and clear at the double diode input of the 6Q7 and very quiet and lacking station selectivity at the 6Q7 plate then my IFs are in working order (maybe needing a nice tuneup). The trace of the signal goes down hill after the 6Q7 input (just after the second IF). If looks to me as if the demodulation is not working properly as the demodulation probe in my signal tracer shows the set is capable of acquiring and tuning in stations (with full BFO functionality). I tested each point from antenna input to the output speaker transformer/speakers. Things die shortly after the input into the 6Q7.

I used a restored and fully functional Heathkit T2 and a premium after market demodulating RF probe. Simple and fast! It is a keeper tool in my tool kit.
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Old 26th Aug 2020, 8:59 am   #159
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Default Re: Another Hallicrafters Sky Champion S20.

Looks like things have improved. Previously in post #143 you couldn't even get a 455kHz signal through the first IFT, but now off air signals are getting through the IF stages and reaching your signal tracer.

If the IF stages are working correctly you should be able to peak the signal by adjusting the trimmer caps in the IFT's. If they're anything like the IFT's in a SX24 adjusting each screw will give a very definite peak.

I believe that many posts ago you checked out the AF amp and output stages by touching the wiper of the volume control resulting in a buzz from the speaker? That being the case the fault is in the detector area and there aren't many components to check there.
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Old 26th Aug 2020, 1:20 pm   #160
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Default Re: Another Hallicrafters Sky Champion S20.

The set didn't make sense in what I was seeing - I am around anywhere from 25 to a couple hundreds radios at various times and this one was the paradox of them all. My only critically tuned oscillator projects before was building tube theremins and they are very tame compared to super hets.

Apparently I did have a problem with one IF that I gutted and replaced the insides with a 455Khz 2nd stage IF I bought on eBay. But I was still not able to hear the modulated signal tone of a 455Khz RF signal - before or after this repair.

My hypothesis is at this time aside from the bad 2nd stage IF the set IS able to receive fully and tune normally (as evidenced by the signal tracer probe at various areas in the circuit before the 6Q7 detect/AVC/1st audio stage tube and 6F6). With that I believe the reason for the behavior is a lack of sensitivity of the demodulator to effectively extract the audio portion at sufficient levels for the audio stage to use. Only the very strongest stations work.

Might be a case of a less than optimal tube as I have seen in my SX-43 restore. Or a combined issue of a bad substitution I made as I did correct a couple things in the circuit that did not match my schematic (resulting is performance going up 50% better) and a couple silly resistor mistakes I made. I had a 300K resistor where a 300K was needed. Never made that mistake before but working on 5 different sets at the same time I must have picked up the wrong resistor. Normally I give it a second look and measure it to make sure its around nominal.

Today I plan to look for other such over sights and focus on the 6Q7/6F6 area. I am certain the speaker and transformer are fine and have the means to completely test them by replacing the two out of circuit with my signal tracer (it has an ability to be used as a host transformer/speaker for just about any tube audio output out there).


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Looks like things have improved. Previously in post #143 you couldn't even get a 455kHz signal through the first IFT, but now off air signals are getting through the IF stages and reaching your signal tracer.

If the IF stages are working correctly you should be able to peak the signal by adjusting the trimmer caps in the IFT's. If they're anything like the IFT's in a SX24 adjusting each screw will give a very definite peak.

I believe that many posts ago you checked out the AF amp and output stages by touching the wiper of the volume control resulting in a buzz from the speaker? That being the case the fault is in the detector area and there aren't many components to check there.
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