Computer SMPS hissing, then turning off

[bogdan2014]

Hello,
I have a computer switchmode supply that I'm trying to repair.
The problem is that it runs for about 10 seconds while putting out a strong hissing sound, and then it stops. I think the sound might be coming from the output inductor.
I've checked bad solder connection, bad caps, the secondary side rectifiers, all good.
From what I've seen, it seems to be a half bridge converter with the controller on the secondary side.

[T3sl4co1l]

Hissing is usually a sign of a poorly compensated control loop.  Often, the current limiting loop is thoughtlessly designed, and functions crudely (with hissing), or simply shuts down completely (a comparator and latch to disable the output).

Double check for shorts, non-functional transistors and diodes, bad caps, etc.

Tim

[gbrettell]

Sounds like a bad capacitor on the secondary ... does it smell like Tuna??

[james_s]

Capacitors would be my first thought as well, but he said he checked those so I'll assume they're all good.

I encountered a similar fault in the power supply of a TDS340 scope, it turned out to be a shorted rectifier diode in the output, one of the TO220 ones mounted on a heatsink. Says he checked those too, sure there isn't one you missed?

Have you measured the outputs while turning it on? That can provide additional clues.