Cydrome Leader wrote:
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.

No load could also cause weird noises- that board looks really old. If
that transformer is burned or arcing it's already trashed- more testing
won't make the problem worse, but giving it with no load or with no load
and at a weird input voltage won't help troubleshoot anything easier.