Roland JC120H squeel

[drewl]

Repaired a bad rectifier bridge on this JC120H and customer complained of it squeeling.

With all controls cranked the amp does oscillate- With or without anything plugged in.

it had an intermittent distortion pot which I fixed, and a ground lead for the rear panel distortion footswitch connects to an input jack with a wire soldered to a ring/washer.
Re-locating this away from the input jacks got rid of the squeal- BUT- it still squeels for a couple minutes until it warms up, then you can crank all the controls and it is fine.

So, for some reason until it is warmed up the preamp oscillates even with distortion off as you slowly bring up the volume pot.

Now most people won't have all the controls full up, and it is fine once it warms up, but I'm thinking a cap somewhere in the preamp is starting to fail or takes a bit to charge up.

power supply is good, clean, no hum or ripple and comes up immediately.Roland JC120' 84 (1).pdf

[g1]

Supply caps (in some areas of supply) can show no ripple and still not be doing their de-coupling job. Usually I'd expect motorboating but squeal is also possible.
Those +/- regulators I'd expect would remove ripple even if their output caps were no good. Do you have a cap tester?

[drewl]

yes, real nice cap tester.
Unfortunately the power feeds aren't on the schematic, I'll have to plot them out by looking at the board.

I monitored the de-coupling caps in that area and they all looked good, voltage came up and didn't vary.

[g1]

That's kind of what I was getting at, the voltage out of the +/- 15V regulators will probably come up fine and ripple free even with no caps there (C6 & C7). But you might get motorboating or squeal without them.
Sort of like the front end B+ node in those Fender hot rod style amps that go bad and cause motorboating or feedback issues, they look fine with no ripple, they just don't de-couple. Only a cap test or substitution will show you they are bad.

[DrGonz78]

The one time I had a Jazz Chorus amp squealing like that it had to do with the input jack not being insulated from the chassis ground. On most of those amps the jacks rely on plastic insulators to not create ground loops or something. I mean I heard of hum and hiss from a ground loop before but with the JC amps it was a nasty squeal sound. Also, those switching jacks get pretty dirty too.

[drewl]

Damn, thought I had it as C7 220uf was all funky/corroded.
Replaced that and the 10uf caps and with everything cranked it will squeel for 2minutes, after that it's fine.

[g1]

I also noticed some NP electrolytic coupling caps between stages which can also be problematic. The 2minute thing makes me suspicious of caps as you are, but I suppose it could also be a semi-conductor or other type of component. Freeze spray?