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Audire Power Amp Oscillating

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The factory added input level pots to each channel of this stereo amp (a custom mod for the purchaser) when it was built years ago. Now one of the channels is oscillating. The oscillation affects the audio range and will vary somewhat in frequency and amplitude depending on the setting of the pot.

I swapped pots between channels and all was well... for awhile... then the same channel began oscillating again. The oscillation can be stopped by bypassing the pot. The NP electrolytic on the input has been replaced and the other components in that channel have been checked, so I see no obvious problem with the amp itself.

Any suggestions on curing the oscillation without removing the pot?
 

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try a plastic 0.1uF cap ( low ESR low ESL) on output

Also put ferrite choke around all wires to Pot prevent inductive common mode pickup from output stage. twisted wires help too as well as shielded wire..
 
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Thanks for responding...

Putting caps on the output has no effect on the problem, and the wires to the PCB are already twisted. The problem is very intermittent - usually worse when first powered on. When it acts up again I will try your suggestion with the ferrite choke.

Another possible clue is that sometimes shutting off the power to the "good channel" will change or even stop the oscillation in the other channel (the amp is a dual mono design).
 

I'm guessing in that case that they share the same power source. Over time the ESR of the electrolytic capacitors increases and I suspect you are getting some cross-coupling of signals through the power lines. Given that it used to work, my first suspects would be those 100uF/63V capacitors in the + and - supply lines, second suspects would be the main reservoir capacitors in the power supply.

Brian.
 

Thanks Brian...

The electrolytic on the + rail has been replaced. The one on the - rail checks good on my ESR & capacitance meters. The amp actually uses a separate power supply for each channel (dual mono). The reservoir caps in both channels are OK - all have low ESR and clean DC output.

The only thing the two channels really have in common is signal ground. Maybe there is an interaction there somewhere...
 

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