Weird PSU problem

sswats · 5 Jan 2011 at 12:07

Old computer was switching on but nothing comes up on screen.
But the PSU would emit a hissing/humming sound before i've even pressed the power button on the PC.

So I tried a Corsair CX400 from my other PC. But all i get is the PSU fan spinning fro a second then off. And then the PSU fan starts switching itself on for a second then off again with no input from me. And continues to do so even if I switch it off from the PSU switch.
Only if I disconnect it from either the mains or the motherboard does it stop.

But then I tried a cheap (as in £10) 300W PSU and it switches on with no problems and everything appears on screen.

I was planning on installing a good PSU, not this cheap one. Any idea whats causing this problem?

Tomcat825 · 5 Jan 2011 at 12:19

Have you tried switching the mains power cable?

sswats · 5 Jan 2011 at 12:23

Just tried it. didn't help.

95thrifles · 5 Jan 2011 at 12:30

what you describe with the CX400 sounds like a short, maybe try it outside the case, its possible if theres a short the built in protection on the cx shuts it down to save any damage, where as the cheap PSU prob doesnt have the protection built in

sswats · 5 Jan 2011 at 12:52

Thats what i'm thinking. Corsair has protection the other one doesn't.
It currently is outside the case.

Could a component cause a short. Like a graphics card, CPU or ram?
None of them are connected directly to the PSU but could they cause a short in the motherboard?