Answer to "iMac won't power on. How do I check this model's power supply?"Not for nothing, but my 24-inch mid 2007 iMac also went completely defunct in one power cycle. I actually brought it to an Apple Store where they replaced (according to the paperwork) power supply x2, backplane, and main logic board. They ended up returning it to me "no charge" and pulled the hard drive at my request. I'm following up to ask you if you have had success with your repair. I'm leaning toward replacement but if this problem turned out to be something simple and mechanical like the power switch, it would be totally worth my time to tear it down and repair it.