82 JC120 - desperado

[fdesalvo]

Long time - hope all is well with you guys!


Have an older 120 on the bench. Amp had a blown speaker and rectifier.  After replacing the rectifier and refreshing the 4 output transistors, there is still an underlying issue. One of the power amps has 30vdc present on the orange brown output pair.  No bueno. I’m out of my depth with solid state things like this, but took it in to help a long time customer.    Not sure where to look next. Any help appreciated!

[glass54]

Hi fdesalvo,
I have fixed a few of these in past decades.
We don't know which the orange brown pair is ie NO INFO on schematic.
I'd suggest a DC offset problem, so I would be suspicious of a leaky C16 or C49 (depending on your problem Power amp).
It's an aged 10uF 16V Electro. I would recommend a new 10uF 25V or higher Bipolar and replace both.
Next I would consider checking C24 (C63) and as best you can measure DC Volts around Q6 and Q7 (Q13 and Q14).
You have 2 power amps, and one good one, you are lucky  Go for it with a good multimeter.
Regards
Mirek

[fdesalvo]

Ty!  I have to bite the bullet and study up on these silicon demons lol.

[fdesalvo]

Apologies for the morose update, but I’ve decided to return the amp to my friend. I’m out of my depth here and with family conditions and my recent hand surgery, I just don’t have the cycles necessary to learn on this job. I appreciate the help however.