Hello:

Sounds like a bad coupling cap used in conjuction with a resistor to
produce the oscillation.

Cheque the caps for DC resistance of at least 10 Meg ohms.
REgards

Sam

wrote:

I have a Fisher 600, not the quasi T model but the all tube model that
has developed a problem in the A channel.

I am getting a slight oscillation or high pitched squeal which seems to
happen through all selections (tuner aux phono etc) but is loudest, by
far on the phono setting, maybe because it goes through an extra stage
of amplification before going to the finals?

I fired up Soundforge and it appears that the frequency is about 9k or
so.

With nominal settings of my mixer and coming out of the tape outputs
with nothing playing and the Phono selected my meters in Soundforge show
the B channel at about -65db solid (normal and sounds like pink noise)
and the A channel at -51 db (NOISY) and wavering a little bit. I can
sort of hear the pitch changing a little bit.

BTW playing through the attached speakers on the 600 I hear the same
thing.

Things I have done:

I've disconnected all inputs.

Swapped the finals, 7189's in pairs.

Gently tweaked the pot labeled "Hum Adjust".

No difference so I put everything back the way it was.

Any ideas on basic troubleshooting before I try and find a schematic?

I'm an EE but the last time I screwed with tubes was in high school back
in 1975

I have a scope, meters, signal generator etc.

TIA!!!