回答“Logic board or power supply?”Hi FAlbano! Hi Everyone! FAlbano I'm curious to know about your iMac. Did it work fine after changing the power supply? Or did you find out what the problem really was? I have a 2013 iMac 27" and it has started randomly going off. No warning it just goes black. And refuses to turn back on for some time. Moreover, how long it stays on seems to depend on how long I've left it off If I try to start it again after say 10 minutes, it turns on but shuts off again in a minute or even during startup. If I leave it for a few hours then it might work for half hour or so the next time. When I left it alone for more than 24 hours it worked for more than an hour before it shut off. I'm quite puzzled and it's become impossible to work on this machine. Thought I'd like to have some clues before I take it to the service people. Don't want them to dupe me into unnecessary part replacements. All advice is much appreciated. Thank you all!