Rubicon 355 positive rail voltage on case?

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‘Morning all.

I have acquired a Rubicon 355 with the incredibly rare Loudmouth remote bass controller.

I plumbed it in and it sounds... a little splash and with worse stereo image than I expected, but I’m digressing.

It sparked against the car body work when I was installing it and it turned out that the heat sink has positive rail voltage from one of the full range channels on it(~21 volts).

I don’t know much about these sound freak amps from back in the day, is this normal?

Does anyone have a schematic I could trouble them for?
Thanks Perry, I've narrowed it down to the PS.

Pressing on the board, hard, in the vicinity of the rectifier diodes causes the problem.

If the retaining bolt in front of C21 is removed and the board lifted slightly the voltage goes away.

I cleaned up the insulating film under the transistors and noticed that the heatsink compound by the screw hole was a sort of gray rather than white, I measured it with the ohmeter and it seemed conductive.

I cleaned it all up and applied some fresh stuff and the problem seemed cured until I torqued down the bolt again.

I think there must be an invisible hole in the film as the problem now only occours when pressing hard or torquing up the screws.

I put a spare mica insulator under D13 and torqued it down and the voltage on the case fell to almost zero, however pressing hard in and around the general area can result of leakage of a few volts in the negative to the chassis.

I think the film is compromised and I should replace it, is it just ordinary mylar film?

Would there be any mileage in using individual mica or sinkpad insulators?


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It's not mylar. It's Kapton MT (not the HN that's on ebay).

There is plenty extra. Cut some that's not being used and put it there. The component may have a burr on it so check it before reassembling.

The gray isn't conductive in any significant way. It's a tiny bit of aluminum oxide from the screw hole that's mixed with the heatsink compound.
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