Chosen Solution
Hi, I bought a new PSU for my iMac, changed the PSU, restarted iMac. Got the loading progress bar, and get to the iMac logon screen. I then enter password, press enter button, iMac restarts again! Please help, I also changed HDD and Apple hardware test is OK. The graphic card was replaced once before. After replacement it worked about 2-3 years. After this period the iMac itself began warm up and from that moment the problem occured. I had a suspicion our AC power source itself was the issue, because we had 253V instead of 230V at the power outlet. So for this reason I replaced PSU in my iMac but it did not help. Update (01/21/2019) Let me describe the systems history for you. When first self-restarts began, the iMac was sent to authorized service in 2016. The AMD graphic card was replaced. After repair the iMac worked well until now. But one day it began to overheat so high, that I couldn’t hold my hand on it, so I turned it off I didn’t time to investigate what happened. After that I turned it on, it overheated again. When I turn on the iMac I get introductory sound, system loading (Apple logo and progress bar) and after that login screen appear, I put password and hit enter and then screen goes black, iMac restarts and the same situation repeats again. Or, during the loading screen iMac turns off, and turn on again a restarts and loop starts again. At that time it was found the city’s substation had a problem, for a long time was pushing out high voltage (250-256 Volts). It’s OK as it should be now. So I bought new power supply for my system. When I replaced (bought it on IFIXIT) the same error still occurred. So eventually I have replaced hard disk. After that the iMac worked perfectly for a few days, it was even more times restarted correctly. Today I have turned it on and the same problem appeared again. When I run Apple hardware test there was no errors as result. Thank you.
Lets try going into Safe Mode. Press and hold the Shift (⇧) key when you restart the system. Reference: ● Use Safe Mode to isolate issues with your Mac ● Mac startup key combinations Are you able to get to your desktop? If you do run Disk Utility - First Aid and restart normally. Update (01/22/2019) Sadly, I’m starting to think your logic board is damaged. There is additional power control circuitry on your logic board which smooths out the voltage and lowers it again. Given the AC power problems you had I thinking this is damaged as well. At this point this will need deeper skills to diagnose than what we can do here. You’ll need to find the schematics and board views for your logic board as well as the needed test tools to diagnose the logic board, as well as the ability to fix it (micro-soldering and access to the needed parts). You’ll likely need to find someone skilled near you. The other option would be to replace the logic board. Here’s one source TheBookYard UK - 2011 iMac logic boards, Apple P/N’s 661-5948, 661-5949, or 661-5950
