No power after PSU replacement

Hi,

I've got a iMac that doesn't power up. I open it up and I noticed the diagnostic LED don't lit up and according to other guides if the LED 1 doesn't lit up means that the motherboard don't get a proper voltage feed from the PSU. I checked the voltages with a DMM from PSU and there was no voltages coming from the PSU either connected or disconnected from the motherboard. So I thought the problem was the PSU and I ordered a new PSU from ebay and replaced it.

At first I checked voltages of the new PSU before connecting it to the MB and were right. I connected it to the MB and checked again and everything seems right again and the LED 1 lit up (according to other guides this is how it should be). Then I tried to power up the iMac but suddenly the LED 1 turned off without any other sign of "life".

After few attempts pressing the power button, connecting and disconnecting the AC plug, disconnecting the power button from the MB and trying to power it up shorting the power connector leads, now when I connect the AC plug the LED 1 lit up for less than 1 second and then turn off and the iMac won't turn on in any way.

At this point I suppose the MB is dead. If any of you can confirm this or give me some other suggestion it will be really appreciated.

Thank you a lot.