Fisher 500c phono stage channel output variance

[MadcapLaughs]

Hi all,

just wanted to see if anyone had some ideas on why my fisher 500c still has a stronger right channel when using the phono stage? It almost seems to be a phase issue possibly, mono lps sound balanced, but stereo records present a much stronger and clearer right channel, I need to turn balance knob to 9 o’clock before both channels are even. I will list the options I have ruled out at this point:

Swapped and replaced all phono and driver tubes with no effect

Replaced phono stage capacitors

Swapped turntables and speaker sources

The issue is completely absecent when using line stage and tuner sources on the receiver

Unit has strong and balanced 7591 Westinghouse tubes

Cleaned rca input jacks for the phono inputs

When playing mono records on the mono phono stage input, there is no stronger channel noticeable

 

[Tim D]

You might get more and targeted help in the Fisher area..... There are experts in the 500C there that can give real guidance on next steps to narrow down your issue.

I'm not one of the experts, but here's what I'd do on my own first:

You should check the voltages on both channels of the phono section against those shown on the diagrams. (Fisher schematics usually have the expected voltages printed for important parts of the circuits.) What I'd be looking for is variances between channels starting with the phono section. If you find differences then you can chase down why - sometimes it might be because bias is off upstream. (Be careful around high voltages. If you aren't comfortable about working with the high voltages - find someone who is.)

Have you checked the resistors in the phono section? Many resistors can be checked in-circuit (with the unit totally de-energized of course). If the resistor reads higher than 10% of nominal value I'd replace it. If it reads low then I'd lift one end and check it again to be sure it isn't low because there is a parallel path around the resistor.

Have you cleaned the selector switch and checked that it has perfect continuity to both channels?

Check that the tube pins for the phono section really are making good contact. Even changing tubes - if you have a bad or loose socket you may not solve the problem with a different tube. You may need to re-tension the sockets and clean them with de-oxit. If you still don't have a good connection then you might have to replace the entire socket.

 

[gadget73]

one option is to build an inverse RIAA network and feed both channels from a common signal generator. Check to see if the output is equal on both sides at the input to the line stage.

 

[Catmanboo]

Dumb question, but did you swap ends at the phono inputs? May be in your tt's. When in mono mode, that will go away, as both input signals are combined prior to amplification .

 

[RS Steve]

Always make sure the tape monitor switch has been deoxed and working correctly. Many of my Fisher's had channel imbalance caused by that switch, although it would be the same on every input, not just phono.