I had a computer back that I built a couple of years ago, it wouldn't turn on. I suspected the PSU, so I disconnected it and tested it with my PSU tester and found that it was dead.

Luckily I didn't jump in with fitting a new one I thought I'd try a 'known good' second hand one first. Turned it on and got a nice smell of burning and said some naughty words.

Scratched my head and had a close suspecting a Mobo fault. Turns out the owner had tried to fault trace it himself and he had shorted the mobo out by connecting the power switch lead to the wrong header. Anyway a short rewire and a new PSU and it's alive again. I'm well annoyed about my test PSU though.