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Sherwood S7900A - Weak Channel ONLY ON FM?

MrBillinMinn

MrBillinMinn

New Member
Hello All,

I'm relatively new here, but have been getting addicted to picking up Vintage Audio stuff. I recently got what I think is a nice deal on a Sherwood S-7900A along with some mint Cerwin Vega R-26 speakers. They were very clean, and seem to work great except that when the Tuner is set to FM, one channel is very noticeably weak. On Phono, AM, or AUX both channels sound great.

Any thoughts on if this might be an easy fix? Again, it's only the FM setting where the problem occurs.

THanks in advance!

Bill
 
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If AM has good balance then I would look at Multiplex section for sure. Early multiplex chips from that era are famous for crapping out, especially the ones that look like spiders. Hopefully its just a bad capacitor or adjustment needed.
 
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I had similar problems in a KLH, it was a couple of bum caps on the MPX board. Completely different circuit I'm sure but similar age and symptoms.

also caps are cheap and easy to change, alignments and old chips not so much.
 
petes'picks said:
Goto post #19 in this thread, towards bottom there is a schematic for mpx section and pictures of the caps he replaced that I would target (C130 and C141). Also does it sound balanced if you switch it to mono, if it is then its definitely in the MPX section. With a meter just check if them tantalums are shorted.

Sorry forgot link. https://audiokarma.org/forums/index.php?threads/sherwood-s-7900a-repair-and-restoration.957298/
Click to expand...
Great info, Thanks!! Yes, when I am in FM mode, and select "right" on the selector, both speakers are quieter, and when I choose left, both speakers are louder (normal level). When I pick Mono, they are both even, and if I pick "rev" the problem moves to the left speaker. I think you are on to the issue and will be looking at that link, thanks a lot!! Otherwise it's pretty clean. I replaced all the lamps and it looks and sounds great.
 
If you look post #19 again you will see the Lowpass schematic below the Multiplex schematic. In that schematic you will see the signal path for each channel after the Multiplex circuit. Notice the left channel has C501, C502 and C507 and it mirrors the Right channel which has C503, C504 and C506 along with a few resistors and transistors. If you were to tune into a strong station and set your meter to AC you should be able to measure the amplitude of audio signal and compare the strength between both channels to find out after which component the volume is dropping. Its a pretty short circuit path after the multiplex so should be easy to find, if the amplitude is pretty much matched before the component (resistor, transistor or cap) then it drops off after that component compared to the left channel that would be your likely culprit.

The multiplex and Lowpas schematics are at the very bottom left side of page 19 here https://akdatabase.org/AKview/albums/userpics/10007/Sherwood S-7900a_S-8900a Service Manual.pdf
 
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