[sdiy] Troubleshooting a switch mode power supply

[Nils Pipenbrinck]

Hi list,

I have trouble with a piece of equipment that is powered from a switch mode
power supply. It blows fuses as soon as I power it up.

My guess is, that the chopper transistor is broken. Anything else on the pcb
looks good. I already checked the filtercaps and rectifiers.

I just want to have a second opinion before I replace it (it's kinda hard to
desolder it. The heatsinks are also soldered on the pcb. Of course it's
mounted in a way that you can't access the screw.. what a stupid design).

The device was on a very dirty mains for quite a while, and I wasn't aware
that the power switch is just for standby.

I have my rehearsal room in a second worldwar bunker. As far as I know they
never improved the mains, and there are lots of crazy and old equipment
connected. Spikes and transients all over the place. On bad days we can hear
all the dirt in our amplifiers. Maybe a really bad spike killed the tranny.
That's my guess.

Thanks,
  Nils