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 Post subject: SX-28A AVC amplifier question
PostPosted: Mar Sat 19, 2011 1:40 pm 
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Joined: Mar Sat 27, 2010 7:06 pm
Posts: 48
Location: Montrose, Colorado
I have just finished restoring an old Hallicrafters SX-28A and in the final IF alignment ran into a problem with the alignment of the AVC amplifier. Following Bill Feldman's alignment procedure, while backing out the slug on T6 the voltage across the AVC output starts to go negative as expected, but suddenly jumps up to -120 volts, which is the same voltage as is on the primaryof T6. All resistance and voltage measurements on the tube are in spec, I have changed out the transformer with a spare, and I have gone over the circuit until I am blue in the face. But I must be overlooking something. Is there someone out there that has experienced this problem or that can give me a hint of what the cause of the problem might be?


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 Post subject:
PostPosted: Mar Sat 19, 2011 2:00 pm 
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Joined: Oct Sat 20, 2007 3:36 am
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Location: Southern NH, 03076
Have you changed the tube or used a scope? Sounds like the stage is oscillating. Hopefully all paper caps have been replaced and resistors checked for tolerance throughout the set.

Factory ground connections can also be tempermental.

Carl


Last edited by Burnt Fingers on Mar Sat 19, 2011 2:31 pm, edited 1 time in total.

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 Post subject:
PostPosted: Mar Sat 19, 2011 2:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Athens, Greece
Indeed, it's oscillating. Check your bypass caps in that stage.


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 Post subject:
PostPosted: Mar Sun 20, 2011 2:39 pm 
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Joined: Mar Sat 27, 2010 7:06 pm
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Location: Montrose, Colorado
Thank you, gentlemen, for your replies. Yes, all paper capacitors and most resistors were replaced during restoration, and all resistances are in spec including those measured to ground through the tube socket. Voltages are also as indicated in the manual.

The odd thing about this is that when the restoration was initially completed, I did both RF and IF alignments, with no problem other than the fact that band 6 did not work. This problem was fixed by small changes in the first and second RF sections only. I was in the process of repeating the alignment when the previously mentioned problem occurred. By the way, the voltage surge has knocked out both the first and second rf amps and possibly the first IF amp. Fortunately, it looks like the transformer coils are ok.

Is it possible that excess drive on the grid of the AVC amplifier tube would have caused the oscillation?


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 Post subject:
PostPosted: Mar Sun 20, 2011 3:52 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Athens, Greece
No.

Check to make sure your bypass caps were all installed correctly. Could be that someone installed that AVC detector transformer with one of the leads reversed.

Also T6 doesn't have slugs, it's tuned by a couple of trimmer caps.

Not sure how you're measuring this voltage, or even why, but I'm betting that the act of hooking a meter or scope to the stage could be contributing to its oscillation.

The proper way to adjust this stage is to inject a S-9 signal or so, monitor the AVC voltage applied to the grid of the 6AB7 firs RF amp (NOT at the grid itself, rather at the decoupling resistor). Tune the pri and sec of T6 for max negative voltage at this point. This sould also correspond with a dip in S-meter reading.

I don't know who Bill Feldman is or what his proecdure is...


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 Post subject:
PostPosted: Mar Sun 20, 2011 4:13 pm 
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w3jn wrote:
I don't know who Bill Feldman is or what his proecdure is...

Bill Feldman wrote a long article in "Electric Radio" on the SX-28 several years ago. Bill passed away a few months after his article was published.

The article covered the restoration and alignment of the SX-28 in great detail, including the alignment of the crystal filter and AVC amplifier. His methods deviated somewhat from the alignment instructions in the Hallicrafters manual. Of course, the Hallicrafters' alignment instructions for the crystal filter are less than crystal clear! Bill's simplified instructions are much easier to follow.

_________________
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 Post subject:
PostPosted: Mar Sun 20, 2011 9:59 pm 
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Joined: Mar Sat 27, 2010 7:06 pm
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Location: Montrose, Colorado
In both of the SX-28A receivers I have (one a parts rig) and in circuit diagrams, T6 has one slug in the primary (high voltage side) and none in the secondary. You may be thinking of T5, which is the transformer in the noise amplifier, which has two adjustable caps.

I also thought I might have wires reversed in the AVC transformer, but they were hooked up in the same way in both sets which led me to believe they are ok. I can check that out, though.

Feldmans procedure for adjusting the AVC is very similar to that in the Hallicrafters manual. The biggest difference is that when aligning the IFs, he has you screw the slug all the way down to eliminate any interference with IF tuning. So his adjustment of the AVC starts with the slug all the way down. However, that should not be an issue here.

All comments so far seem to point to bypass caps and or ground connections. I will have to go back and check those one more time.


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 Post subject:
PostPosted: Mar Mon 21, 2011 5:39 am 
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Location: Athens, Greece
Grrr... you said SX-28A, and I downloaded the SX-28 manual. Then after your last post I downloaded the SX-28A miil manual and IT has T6 with variable caps on the pri and secondary. Finally looked at the SX-28A manual and it indeed does have single slug tuned transformer. Sorry for the confusion - my fault!

In any event, does this oscillation happen when you are attempting to adjust this transformer to its resonance, or at some other time? Does adjusting any of the other IF transformers, or the noise amp transformer, affect the oscillation?

Finally, have you looked at this with a scope to verify the oscillation? The frequency of the oscillation can tell you a lot.

Did you mess with, molest, or replace C61, the 250 pF coupling cap to the AVC and noise amps?


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 Post subject:
PostPosted: Mar Mon 21, 2011 3:28 pm 
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Joined: Mar Sat 27, 2010 7:06 pm
Posts: 48
Location: Montrose, Colorado
Thanks for your last post. Let me try to answer your questions.

1. The oscillation (if that is what it is) occurs after I begin adjustment and the voltage across the AVC bypass cap (C64) begins to go negative as it should. As I continue adjustment to maximize the bias, the voltage suddenly spikes up to well over 100 volts. So yes, the problem appears to occur as I am approaching resonance on the transformer.

2. I have not tried reproducing the problem with adjustments to the IFs or noise amp. Also, I have not taken the time yet to look at the oscillation with my scope. Right now I am struggling to get the rest of the radio operating again, since both RF amps and both IF amps are now dead as a result of what happened. Fortunately the RF and IF transformers are in tact, so the radio is fixable, but I have not found a fix for those problems yet.

3. I did not mess with or change C61 before the problem occurred. It is buried in the first IF transformer can (T1). I did mention before that I did swap out T6 with a spare that had in a parts chassis but had the same problem with that transformer in place as well.

4. I can get an IF signal to the AVC amp by injecting 456 kc signal into the grid of the mixer. So at some point, if I can't quickly find the problem with all of the amps, I can take a look at the AVC amp by itself. As you would expect, I have isolated the AVC output from the RF amps and Mixer. That is sort of like closing the barn door after the horse got out!!!!!

Thanks for your continuing interest in my problem.


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 Post subject:
PostPosted: Mar Mon 21, 2011 5:16 pm 
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Location: Athens, Greece
I'd start by getting the RF and IF stages working, and back off the T6 adjustment so you get correct AVC action. If you have a scope, now is the right time to use it!

I'd suspect that 250 pF cap, but check the voltage on the grid of the 6B8 to be sure it's not leaking.

Not sure what you mean by the RF and IF amps dying because of "what happened". Do you mean the high AVC voltage? That should'nt have wrecked anything. I think you have other problems. In any event, you can just yoink the 6B8 while you're troubleshooting the rest of the radio. That way that stage won't oscillate or cause problems.

Did I mention you should look at this thing with a scope :D


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 Post subject:
PostPosted: Mar Mon 28, 2011 4:48 pm 
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Joined: Mar Sat 27, 2010 7:06 pm
Posts: 48
Location: Montrose, Colorado
Good morning,

It turns out that the mixer, IF stages, and detector are working OK. I initially had some questions about the gain I was seeing in the mixer and first IF, but I am getting the correct detector output with a measured input to the mixer and the waveforms are clean, so those sections are ok. That leaves the two RF amps that I have to get working yet. Both of those amps were working very well before I had the problem with the AVC amp, but are now completely dead. I have done the obvious checks of resistance and voltage, tubes have been checked and replaced, and the transformers are ok. So, I am still scratching my head over this one at the moment.

Yes, I have been using a scope extensively to trace signals in the RF and IF train, but since I don't have the RF sections working yet, I have not spent much time looking at the AVC amp itself. In the meantime, I have isolated that section so as to not interfere with other troubleshooting.

I initially questioned the 250 pf cap in the grid circuit of the AVC amp. I am getting a good strong signal (per the scope) on that grid from the mixer, however, so the cap is not open. It is also not shorted. Whether the value is right or not is still a fair question. I have not taken the time yet to tear into T1 to get at that cap and measure it.
The voltage on the grid of the 6B8 seems ok, the tube checks out as good, and I have swapped out the tube with a spare. The signal on the cap of the 6B8 is not a clean one, of course, since it comes directly from the plate of the mixer and is thus not "cleaned up" by any transformer.

Thanks again for sticking with me, and I will let you know how things progress.

By the way, with a W3 call sign, what are you doing in Athens?

Hank


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 Post subject:
PostPosted: Mar Mon 28, 2011 7:47 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2719
Location: Athens, Greece
Working :D

Have you checked the DC voltages, no signal, AVC on, RF gain at max? AVC off?

This radio has 2 AGC loops. You need to focus on grid voltages on the 2 RF stages and IF stages. If they're considerably negative, you still have an oscillation going on somewhere that's being rectified and passed on to the AVC chain.

The concern I have about the 250 pF capacitor is leakage, which could upset the bias on the 6B8. Compare voltages with the manual and report back any variations.


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