[Trevor]

"Christopher S" wrote in message
...
My Adcom preamp has developed an intermittent fault, and any guidance on
how to fix it would be appreciated. Can't really afford a new
preamp/tuner.

Basically, a high pitch tone is produced through the speakers (old
school Heybrook HB1's....nice). The high pitch tone does not vary with
an change in volume. I can make the sound come and go by pressing and
releasing any of the buttons on the pre amp. Tone in/out for example.
Even jiggling the headphone jack plug will do it. The sound does come
back by itself though and the only way to make it stop is fiddling with
a button of the preamp. Any idea what is going on?

Sounds like parasitic oscillation. The cause of that is harder to guess, but
most likely a leaky or dried out bypass capacitor. The fact that it doesn't
change with volume position is a clue that it's not in the input circuitry.
I'd look at all the compensation capacitors on any IC's after the gain
control first.

Trevor.