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MJC

In article , says...

On cranking up the supply voltage (230V here) I get to about 150V on
my
variac whereupon I can hear a capacitor venting; a clear hissing sound
like air being let out of a tyre. Trouble is, I can't see which cap it is
out of about the dozen or so electrolytics on the board. There's no
visible steam or smoke whatsoever. I've tried using a piece of thin
hollow plastic pipe as a stethoscope and moving around between caps, but
it's nowhere near precise enough to differentiate the cap responsible.
(I'm guessing it's an electrolytic; can't think what else could make that
sound).

Could it be an electrical discharge? Try viewing in the dark (with
precautions against contact!)...

Mike.