Tried 3 PSU's still NOT turning on any diagnostic LED on logicbord

Hello There...

Problem:

- No diagnostic LED ON, on logic board...

Tries:

- Change the PSU, and the iMac turned on... but like an 2 hours of use.. it shuts itself down... and NO diagnostic led on.. AGAIN :( (I think that the new PSU was faulty)

- Change again PSU, but still doesn't turn on the iMac... :(

Whatelse can I try??? apart the PSU???