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Modulator sick, Degnebbit.

T

Tom Wells

Pt 15 AM is sick. In question are two 6SN7 tube stages. Straight audio class A voltage amps, cathodes resistors bypassed for audio.
Of the 4 steps, there's 1 straight audio gain, one for bass and treble controls, and another which mixes RF from xtal oscillator with audio on grid of the triode, then another bufffer, I think.

ANYway, output dropped to 1mw, audio clumpy, scope waveform goes all "peaky or "spiky"
and cutoff at centerline show with less-than-full audio drive levels. Non-linear on sine waves, looking like bias is being applied.
No more asymetric +150 modulation with envelope growth, either, anymore.
Elimination of one other audio tube (by removal), and following driver stage shows the problem is present in one of the two
6SN7s. The audio of the monitored signal shows the fault present at this point. Occasional noises and accompanying changes in audio level,
which was not as apparent with the output tube operating.

I think I'm going to find one of the cath audio bypass caps died.
Tube swaps made no diff, all gain controls, drive control, bass and treble controls behave normally.
No low gain, squealing, or lack of boost/cut on tone controls, nor funny breakout oscillations.



This is the second part failure in 15 years.
 
If I hadn't seen a picture of that masterpiece, I'd be quick to suggest a Hamilton Rangemaster...but what you have there is the last of a vanishing breed. And if it lasts another 15 years after this fix, let the hits keep happening!
 
Tom Wells said:
Pt 15 AM is sick. In question are two 6SN7 tube stages. Straight audio class A voltage amps, cathodes resistors bypassed for audio.
Of the 4 steps, there's 1 straight audio gain, one for bass and treble controls, and another which mixes RF from xtal oscillator with audio on grid of the triode, then another bufffer, I think.

ANYway, output dropped to 1mw, audio clumpy, scope waveform goes all "peaky or "spiky"
and cutoff at centerline show with less-than-full audio drive levels. Non-linear on sine waves, looking like bias is being applied.
No more asymetric +150 modulation with envelope growth, either, anymore.
Elimination of one other audio tube (by removal), and following driver stage shows the problem is present in one of the two
6SN7s. The audio of the monitored signal shows the fault present at this point. Occasional noises and accompanying changes in audio level,
which was not as apparent with the output tube operating.

I think I'm going to find one of the cath audio bypass caps died.
Tube swaps made no diff, all gain controls, drive control, bass and treble controls behave normally.
No low gain, squealing, or lack of boost/cut on tone controls, nor funny breakout oscillations.



This is the second part failure in 15 years.
Click to expand...

From your description of the circuit, there isn't a lot that can be wrong. If changing the tubes doesn't help, it is probably a capacitor. It is far less likely that a resistor has shifted value, but not something that should be over looked. If one capacitor is bad, it might be time to shotgun replace all of them, since they are all presumably the same age and possibly from the same supplier. Since I know this is a homebrew piece of equipment, that presumption of component age and type may not be valid.

Anyway, you know the steps. It is "Troubleshooting 101."
 
Don't know the circuit, but don't overlook any 2 watt carbon resistor; notorious for changing value with age and heat.
 
Turned out to be an audio coupling cap gone bad, .5 mfd 200v Aerovox.
It's a round tubular cap, of unknown construction (black with white markings), is this a paper cap?
I doubt it, since I think I got this in a batch of expired mil-spec parts from Radiation, Inc, of Melbourne, Florida.
On that third 6SN7, the first half is audio triode with the
cathode straight to ground, no cathode resistor at all. The cap that coupled audio from the plate of that section to the grid of the second side died in such a way that it still looks and "acts" right when testing it with meter.
It still measures .5 mfd with a Fluke, it still charges up showing a "climb" when checking with resistance test.
Haven't put it on the old green-eye capacitance bridge yet.


I also changed out the 10000 pf cath bypass on the af/rf mixer stage, but that made no diff.
Same with the 235 pf coupling cap from the plate of the mixer 6SN7 to the grid of the 78 (same as a 6D6), no diff.

Beware of parts that measure "good" on a meter but don't work in a circuit.


AM 1620 is back on the air and I can worry about other things.
 
After a day of listening, I think that maybe that coupling cap had been soft-failing for a few weeks before complete failure.
I had been hearing occaisional pops and sometimes the output was a bit low, and audio had been a bit harsh.
All that is gone now.

Guess it's time to repair a blown brake line on one of the cars now. Always something broken waiting to be fixed.
 
Blown steel brake line came off the '72 Dodge without too much cursing, and I had a new line the right length lying around!

Now time to get a few hours sleep before the midnight shift, can't wait to see what will break at work tonight!
If things didn't break, I'd have no job. Blessings on the failed parts that keep my employed.
:-\
 
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