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Chris
 
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Default HELP: Fisher 500-T Receiver, Why Right Ch Weak, esp Bass ?

wrote:
HELP: Fisher 500-T Receiver, Why Right Ch Weak, esp Bass ?

I have a Fisher 500-T Receiver, probably around 1967 vintage.. plays in
stereo, but right channel definitely plays weaker than left, esp bass
response.. receiver has 2 - 3 large, original capacitors.. could these
be the problem? If anyone can suggest some probable causes and how to
diagnose, would greatly appreciate.. thanks for any help.


I used to work on these things back in the 60's. Fisher was
one of the first to go solid state and used what was known
as a totem-pole output stage in their power amp sections.
This is where the output stage used the same polarity transistors
(PNP or NPN) with the collector of one transistor was connected
to the emitter of the next transistor. With the the collector or
emitter of one tied to B+ and the reverse of the other tied to
ground. Fisher also used in their early sets, an interstage
coupling transformer between the driver and the outputs.

The connection of the emitter and collector was connected to the
speakers through a large electrolytic coupling capacitor to
isolate the speakers from the DC.

A number of manufactures used this approach because complimentary
power transistors had not been available at the time. They really
did not begin to show up until the early to mid 70s.

What you are describing sounds to me like this large electrolytic
has gone bad as I have had these very symptoms with that type of
failure.

Unfortunately this design, especially with PNP germanium power
transistors that Fisher used in their early receivers, was
horribly thermally unstable and would go into thermal run away at
the drop of a hat. This is what lead to the trashing of Fishers rep.

Kind of sad really, as their other stuff was pretty good.

Chris