Chosen Solution
So i recently replaced my PSU en HDD of my 27 inch imac recently. I replaced the PSU because it wouldn’t turn on anymore and the HDD because my old one died. It is a verified apple HDD with temp sensor so all good there. But once i try to install anything it will get past the loading bar and than a white screen and… nothing. Just a permenant white screen. Until about 30 min later when is flickers and turns off. Any help would be aprieciated, kind regards Update (06/11/2019) Thanks already to everybody who helped. I tried the Linux LIVE USB and I think I can safely say the GPU is defective. It does load the linux OS fine at first. I get a visual interface and right about when the resolution changes to the native resolution the background turns black and graphical glitches show up. Also the screen kinda went on and off from time to time? Is this like a driver install thing? Anyway I can’t click anything in Linux though. I can move the glitched cursor but nothing else. So it is safe to say it is the GPU?
Try booting from an external USB drive with a known good system on it and let us know your results. Does the last four figures of your serial number (found on the bottom of the stand) match one of these? DHJQ, DHJW, DL8Q, DNGH, DNJ9, or DMW8 DPM1, DPM2, DPNV, DNY0, DRVP, DY6F, F610
Sadly it sounds like your system got hit by a surge or lightning strike. Thats why your power supply went and your logic board also got nailed as well.
Alright. For anyone that still c ares. It suddenly stopped working. But now i cant even boot into linux since the screen will become black. There is nothing on screen. The HDD is still clicking and the fans are still spinning but nothing happens. I will probably try a gpu reflow since it is my last option.
