Chosen Solution
The iMac boots fine, works well but after 2 or 3 hours will suddenly completely cut out, as if the plug has been pulled out. If you try to restart it’s dead. If the unit is unplugged and left for about 15 minutes, replugging and booting works fine. The plug and cable are seated well, I’ve done a SMU reset, and a PRAM. Apple Hardware test was clean, no joy I’m guessing this is a faulty power supply, is there any thing else this could be caused by?
Faulty Power Supply sounds like the case. It could be overheating, does it get hot?
The problem was resolved by changing the power supply. I followed the ifixit guide for a 17" machine, mine was a 20" iMac. Basic fixings were the same but in slightly different places. The main difficulty was the main snap on connector for the power supply(black one) was routed under the chassis and I couldn’t get to the connector. Cutting the black wire from the faulty Power supply close to the PS as possible, made it possible, with tweezers, to re route the wire, under where the hard drive would be replaced, there was enough space under there, and it hasn’t caused any problem. All works well.
Doesn’t seem to be getting overally hot. I haven’t worked on many of the intel iMacs dont seem to have come through the workshop yet. But could be a faulty supply, does the videgraphics cut out or does the computer just shut down? Could be a faulty temperature sensor maybe?
