My iMac had a checkerboard screen, AHT reported an error with the video ram. I replaced all the caps which changed nothing. I pulled the LB out and pulled the heat sink off the video chip, cleaned up all the excess thermal compound and started it up - everything was working perfectly, so I went to grab a cup of coffee (2-3Min), when I returned, there was no power. I didn't have the LB screwed in, it was just propped up in the case, also only had the video, inverter and power supply plugged into the LB. No fans, sound... There was no smell or movement of the parts and neither the video or the processor were hot.

The power supply is OK (12,5,3.3V) leaving the dc/dc - without the LB plugged in the HD will spin and I get the proper volts at the connector. But when I plug it into the LB and power on the #1 LED blinks and the power supply shuts down... I have reset the PMU. I suspect the caps, I checked all them a hundred times but I can't get an accurate reading on the caps because they are in series. I would hate to have to replace them again

Does anyone know why the trickle voltage would make it shut off? Any ideas where I should look first? Troubleshooting ideas?