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



 
 
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  #1  
Old October 18th 05, 12:04 AM
[email protected]
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Default HELP: Fisher 500-T Receiver, Why Right Ch Weak, esp Bass ?

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.

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  #2  
Old October 18th 05, 05:58 AM
Karl Uppiano
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Default Fisher 500-T Receiver, Why Right Ch Weak, esp Bass ?


> wrote in message
oups.com...
> 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.
>

Just a wild guess. The electrolytic DC blocking capacitor on the "ground
side" of the negative feedback voltage divider has dried up. You could
re-cap the whole amp, but I'm betting that replacing the one from the right
channel and its counterpart in the left channel will make a huge
improvement.


  #3  
Old October 18th 05, 01:16 PM
Tim Schwartz
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Default HELP: Fisher 500-T Receiver, Why Right Ch Weak, esp Bass ?

Hello,

It is quite likely that some of the coupling capacitors, either in to
the power amp, preamp or tone controls have failed. The best way to
really troubleshoot this unit will be to feed in a sine wave to both
channels, and look a various points in the circuit with a scope probe. A
schematic would make this much easier.

A coupling cap that has gone (nearly) open will allow some higher
frequency through, but will certainly kill the base off. These caps, at
least the ones that usually fail are of the electrolytic type, and
usually 10uf or less in value.

Please note that a broken pot in the tone controls or filthy switches
and controls could show similar problems.

Regards,
Tim Schwartz
Bristol Electronics


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.

  #4  
Old October 31st 05, 08:55 PM
Chris
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Posts: n/a
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

  #5  
Old November 12th 10, 01:19 PM
razzbarey razzbarey is offline
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Location: Houston, TX currently in Japan
Posts: 1
Default

Quote:
Originally Posted by View Post
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.
These receivers have a capacitive coupled output. 2 of the large capacitors are probably the output capacitors. Easiest way to check is to look on the circuit side of the capacitor with an oscilloscope and compare the output side. Low one is the bad one. I would replace both to be sure of good signal and maybe bypass with a 1 or 2 uf film to improve the highs. Good luck!
 




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