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Yamaha C-50: a learning experience

Buster Caps

Buster Caps

New Member
Hello! Amateur tech here with great appreciation for and admiration for the mentors on THIS site.

I have this Yamaha pre amp from the 80's needing attention. Problems include one channel dropping out, static in balance and volume controls, and the power lamp out. AK threads led me to the likely targets: relays, cold solder joints, and servicing controls.

The massive ALPS volume pot was especially resistant, but once removed from the board, I could separate the 'segments' just enough to flood with DeOxit. The relays seemed exceptionally 'touchy'. Probing with a plastic dowel would cause a 1KHz signal observed on my scope to disappear or drop voltage on one channel. I removed 2 relays from the Flat Amp circuits and burnished with 600 wet/dry paper moistened with DeOxit. Suffice it to say that these measures (and some reflowing) made a world of difference. Problems solved -- 90%!?!?

One problem remains. When powering up from cold, the output seems unstable for a few minutes. There is slight but audible volume loss which returns and holds (I guess as circuits "warm up"). I am wondering what could be causing this. What areas/components are suspect and how does one go about finding/testing them?? (I have several FLuke DMMs and a Tektronix 465 scope)
Also, while I was checking voltages around this pcb, I stumbled on finding voltage in the HEAT SINKs equivalent to that measured in transistors fastened to them !!?!? IS THIS NORMAL?

Thanks for any advice.
 
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Voltage on the sinks -- Yes that would be normal. The sinks are isolated from each other. The transistors have no mica isolators. Just heatsink compound. As far as the cold start issue.. Could be anything from a cold solder joint,dirty switch or relay contact.Even possibly a dried up cap.
 
Darn! I was almost sure you'd say that the 'cold start' signified transistor issues. Sounds like I can't really locate the problem component(s) systematically, (or even at all). Especially since output settles in a few minutes; not much time to run all over the board with probes. I will try to polish those relays again. Thanks for the reply.
 
Buster Caps said:
Darn! I was almost sure you'd say that the 'cold start' signified transistor issues. Sounds like I can't really locate the problem component(s) systematically, (or even at all). Especially since output settles in a few minutes; not much time to run all over the board with probes. I will try to polish those relays again. Thanks for the reply.
Click to expand...
Can't rule it out. But...It sounds like a solder connection issue.
 
Switches, relays, and volume control have been cleaned....Putting my jeweler's magnifiers on to re-examine the boards. I'm thinking that the power supply and flat amp circuits should be the initial targets. Also thinking I should do some scrubbing with IPA -(do you ever use denatured alcohol?)- before I get into replacing caps or transistors 'shotgun style'. Thanks, Avionic
 
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