Can't power on, bad power supply or inverter?

Hi all,

After a power outage my 20" intel imac early 2006 model (EMC 2105) refused to turn back on. Twice previously when this has happened I could do an SMC reset by unplugging the imac, holding down the power button for a few seconds, let go and plug it back in.

That did not work this time. The machine was totally unresponsive. I opened the machine up to find the diagnostic LEDs but this model is an early model without any LEDs. I did a continuity check across the wire going to the power button as a sanity check and it responded fine when pressing it. I removed the display and hard drive to examine the PSU (model 614-0363) and it appeared physically fine.

I unplugged the PSU from the logic board and used my multimeter and measured 12v on the PSU between the black and the grey wires. The brown wire had like 0.8v on it but from my research online the function of the brown wire is to shutdown the PSU in the case of overvoltage and it does not play a role in turning the PSU on. The PSU just always outputs 12v to the motherboard.

At this point I tried to turn the machine back on and it worked surprisingly! It had a solid LED and played the startup chime. This was with the HD and display still disconnected. I connected the HD and was still able to start it up.

However, once I reconnected the display it refused to start up. As far as I know there are two possibilities for this:

1) The PSU, although it shows 12v from grey to black, can't provide enough power to startup the entire machine and is too weak.

2) The voltage inverter which provides AC power to drive the flourescent blacklight of the display is defective.

How can I tell which problem I have?