Cursitor Doom wrote:
Hi all,

I'm returning to the scope smps I was working on some time ago before
fitting a new bathroom intervened. I've removed the smps board from the
scope for testing purposes.
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

You're running a switcher at way out of spec input voltage. The magnetics
making awful noisy or hissing sounds is not unexpected under those
conditions as it fails to startup correctly.