"DaveM" wrote in message
. ..

Since, by your own observations, the subsonic filter switch could be the
problem, or something in the immediate vicinity of the switch. What kind
of switch is it (slide switch, toggle switch, rotary switch)? If you can
easily disassemble it to get at the contacts, you might find that they are
very tarnished or even corroded; caused by airborne contaminants.

It's a plastic push-button PCB-mounted switch, not easy to disassemble at
all. I'd worry about breaking the plastic parts which are, I'm sure, quite
irreplaceable. Not particularly accessible either as there is a display
controller board above the tone control board where the switch is located.

Also sounds like the volume pots are still dirty. Use a cleaner that
lubricates as well as cleans. A tip... when you spray the cleaner into
the pot, turn the set so that the residual cleaner can drain back out.
That allows the cleaner to float out any contaminants as it drains.

I was using zero residue cleaner, I'll pick up some lubricating contact
cleaner and give it another go. There appears to be a hole in the back of
each pot where you can see the shaft turn, plus a small (1-1.5mm) hole on
one side. Maybe a blast of compressed air after cleaning wouldn't hurt
either...

I have also found transistors to cause that exact problem. I surmise that
the wire bond from the external leads to the silicon inside breaks and
becomes extremely intermittent, and can be very difficult to find if you
aren't wary of this kind of fault. Lightly tap on each semiconductor in
the area and see if the audio is affected.
Actually, just about all types of components can become intermittent
(microphonic) in this manner, so using an insulated tool such as a plastic
or nylon tuning wand, tap components and circuit board to see if that
affects the audio.

This amp is totally discrete and there are no soldered wire connections (all
wound) , so I think (hope) the fact that BOTH channels are messed up
indicates a simple dirty pot problem... I did check voltages at maybe a
dozen transistors, both left and right channel from PS through the pre-amp
and amp and they were all really close to expected at idle. I don't know if
+/- 1V is significant on a 43VDC rail or not... anyway there are quite a
few transistors but nothing unmanageable and as there are no proprietary,
obscure, or out of production IC's in this dog, whatever I did is completely
fixable. I just hope that it doesn't take me 30 or 40 hours to figure it
out.

I may try just bridging the "open" pair of contacts on the subsonic filter
switch with a jumper wire and see if that helps.

Thanks for your help

Dave


Good luck...

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters
in the address)

Life is like a roll of toilet paper; the closer to the end, the faster it
goes.