This has got me stumped...

[Cairnsey]

I'll make it brief:

GF's granny's PC won't power up at all - nothing happens when you switch it on. OK PSU I thought so I took out the 20pin ATX connector and used a paper clip to short green and black and all the fans spun up and the HDD etc kicked in. Not the PSU I thought. So to rule out the FPanel power button I shorted the power switch pins on the mboard with no joy - motherboard I thought. So, I took the PC home and replaced the mboard but still no joy - no power. The board I replaced it with has a little green LED which comes on as you'd expect but for the life of me I can't get it to boot. Is it possible the PSU won't boot a mainboard but everything else? or have I got two dodgy boards (even though I was 99% sure the board i put in worked)

or anything else?

Cheers

 

[RoastChicken]

Yea i think it is possible to power everything else have you tried putting a completely new/ PSU that works in and testing that?

I'd test the old board with a powersupply you know that works.

becuase obviously the old PSu isn't powering the Board.

 

[DigitalReaper]

If you dont have a spare PSU handy you could jumpstart the PSU and check all the rails are working. You'll probably need a multimeter for this, but you could use a spare LED for 3.3v and 5v, and a fan for 12v. Just twist some spare wire around the pins on the fans molex (and make sure they don't come off and short )

The little green light means you have +5v standby, HDs usually use +12v AFAIK for the motor so the normal +5v and/or +3.3v rails could be dead. Test both at the normal molex connectors and the mainboard connector as they could be powered by different circuitry despite being the same voltage.

Also did you use the same CPU?