No lights on LogicBoard

Hi there, i have 2x iMac's. One is a intel 20" EMC 2133 and the other is a 20" EMC 2166.

The EMC 2133 works okay however randomly it just freezes!. THe only option is to shut it down with the power button. I have changed the HDD and installed a fresh copy of Mountain Lion, but it still causes me grief with the random freezes. I then purchased the EMC 2166 model on ebay. Got it for a good price as the PSU was U/S. Will the PSU from the EMC 2133 work in a EMC 2166 model when fitted? i tried it and there are no LED's. Now i am not sure if that is due to the incorrect PSU or maybe that the logicboard on the EMC 2166 is fried as well. Hopefully this doesn't sound confusing... any help or advise would be greatly appreciated. Love this site. Thankyou for all of the info you provide us all.