iMac dead even with the diagnostic LEDs on

I recently bought an iMac 24 inch A1225 on Ebay with a dead logic board. The seller wrote he had the iMac at a repair place and they told him that the power supply would be fine, also the LCD. Their must be something with the power going from the logic boatd to the graphic card (in his description).

i swapped the logic board and I had the same symptoms. The iMac won' t turn on at all, no fans, nothing. The diagnistic LEDs show me that the logic board reveives power and that the grafic card communicates with the LCD. But the screen is still black and there is no life in this machine even with the LEDs on.

I removed the power supply and it looks good. I did not test it but my guess is the power supply...?

Any idead what might it be before I buy a new power supply?

I also tried a third logic board (i have two spare ones) with no results.

In my opinion the iMac does not receive enough power. Maybe I am wrong.

Any help would be much appreciated.

Thank you!