A few times a day, my iMac just straight up turns off instantly. It didn't used to, but now it makes a popping sound when it turns off which worries me. It also seems to turn itself off more frequently. Apple hardware diagnostic checks out, so I don't think it's the RAM. I've never experienced something like this before, but I think it's a bad power supply, right? I have the setting "Start up automatically after a power failure" option turned on, and the computer does not turn back on after it turns itself off, it stays off. Is this normal for a bad power supply? If I unplug it and plug it back in to simulate a power failure, it turns off and turns back on by itself. The inconvenient part of all this is that I opened the iMac and installed my own solid state drive and upgraded the hard drive to a 3TB and upgraded the RAM to 16GB, so I think I have sufficiently voided my warranty. I supposed I can reverse the HDD and SSD mod and take it to the Apple store, but I don't have the original RAM modules so I'd have to leave those in. How can I test or make sure what the problem is before taking it to the Apple store? The console and system log have no valuable information in them.