We had a brownout and the iMac shut itself off. It wouldn't turn back on. I unplugged it from the wall for thirty seconds and it turned on and shut itself off again. It would not run longer than 5 minutes. So I ordered the power supply.
The repair went smoothly. The only trouble I had with the disassembly was that I somehow got the idea the front bezel was two pieces and I tried to pull out what I thought was the inner piece. Fortunately, I didn't break anything.
After I put it all together, I was anxious to try it out. I hastily plugged in the cables, spun the computer around and pushed the power button.... nothing. Nada. Zip. Sinking feeling.
I plugged a lamp into the power strip to test it, the lamp was ok.
I imagined I would have to take it apart again, so I spun it back around and saw the power plug had come out from the back of the iMac. I plugged it in and pressed the button and felt a huge relief as the computer turned on and stayed on.
1. Make sure the plug is firmly seated in the back of the iMac when you're ready to turn it on.
2. Follow the tip in the instructions: Pressing on the LCD with the thumbs is like magic for separating the one-piece bezel.