Yamaha C-85 phono weak left channel?

AlexanderEli

Active Member
Hello all,

Having some bizarre trouble with my beloved C-85.. when playing records, the left channel is pretty weak on both phono inputs and at all cartridge load levels. When I switch the R/L input cables, the weak channel stays on the left, so I don't think it is the turntable. When I switch to mono output, the problem goes away, so I assume it isn't the output level, and maybe even more confounding, when I switch to a different source on the "Tape 1" input, the problem goes away, so it seems isolated to the phono section. Any ideas what this might be? Is there any testing I should do before I dissect it?

I have never done any DIY audio repair, but I am interested in learning! :)
 
C85s have been known for cold solder connections especially the voltage regulator pass transitors and the copper buss bars and RCA jacks. Weak phono could also be a dirty phono selector switch. Open it up. And tap around the board with something non-metallic. Help locate a bad solder connection if there is one.
 
My C-85 developed a similar problem. One of the fine AK tech folks found bad solder/connection and remedied the problem. The cost was well worth the results.

Good luck,
Roger
 
C85s have been known for cold solder connections especially the voltage regulator pass transitors and the copper buss bars and RCA jacks. Weak phono could also be a dirty phono selector switch. Open it up. And tap around the board with something non-metallic. Help locate a bad solder connection if there is one.

Thanks for the advice! I have opened it up and looked around, when I do the test you suggest, should I pull out the circuit board first? Or will I just see the part move when I tap on it from above?
 
do the "stress" test while listening through the preamp.
the eraser end of a lead pencil is good.
when you disturb the part with the bad solder joint the circuit is restored and the change in the signal is obvious
 
do the "stress" test while listening through the preamp.
the eraser end of a lead pencil is good.
when you disturb the part with the bad solder joint the circuit is restored and the change in the signal is obvious

OK, this I can do! Should I do this on the parts side or the solder side of the board?

Are there any voltages I should be cautious about?
 
OK, this I can do! Should I do this on the parts side or the solder side of the board?

Are there any voltages I should be cautious about?

As long as you keep one hand in your pocket and use a plastic or wooden probe with the other hand.
You should have no issues. However there may be 120 volts AC back by the power transformer and extra utility AC sockets on the rear panel.
 
Wouldn't you know it, I power it on, set everything up with an open top and pencil to poke every individual component, and the problem was gone.. no weak channel. So I poked every darn component at least twice, and had no fluctuation in sound.. I am at a loss. The symptoms are consistent with a bad solder joint, but the problem is unfindable! I suppose next time it happens I'll pop it back open and try again.

I'd like to replace the important RCA jacks though, any advice for what brands fit the holes best?
 
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