Apple iMac iSight A1145 Will not power up

Hi all

I have acquired an Apple iMac 20" iSight A1145

I have replaced the power supply as well as the DC inverter board but the unit will still not power up.

The condition was unknown before this as I also installed a 500 GB HD and also 2GB RAM

There are no LED's lit on the mother/logic board etc so my Q being is the mother board faulty?

I tested the PSU at the transformer output (5 pin plug) and I am getting 12 something volts between brown and the two blacks, however no voltages from the two grey? Correct or not?

I have not tested the multi pin plug at the mother board from the DC board as I cannot find any schematic diagrams as to what the connections are

I need help quick as I have ordered another mother board and have rebuilt this iMac at least 5 times in the last 48 hours! and it is driving my Mrs barmy! ha ha

Thanks