Chosen Solution
With the system back together see if either of these diagnostic tricks helps in identifying your problem: Plug in an external display in does it show your desktop?Shine a flashlight into the screen at a sharp angle can you see a faint image of your desktop icons? If either of these work your backlight is having problems not the LCD or GPU. If the external display works but you can’t see anything with the flashlight trick you have a cabling issue between the LCD and the GPU or the power cable is bad.
Hey guys, how did you end up on this? I have exactly the same issues with my iMac (same model). Sometimes there is no Image, but I can hear the chime, another time I get a picture, but without chime. Then I get to see the progress bar while booting up. It gets filled up to approx. 50% and the screen goes black. The strange thing is, that once in a while the Mac boots up completely and works fine until I restarted or I switched it off. By the way, Apple hardware test says the system is not facing any issues?! I also assumed the GPU may be faulty, but by booting the iMac into recovery mode I get a video signal and I am also able to connect my external monitor via thunderbolt/ miniDP to extend the screen. I have no idea what to do :/
Just to keep you guys updated. Meanwhile I replaced the PSU, which didn’t have any effect on the described issue. So the PSU is not the culprit here. As a proper startup happens in approx. 1 of 100 startups, it is not that easy to et access to those crash log files. Does anyone know another way? Maybe I can use the hdd to boot my macbook from? One more thing: If I dismantle the Imac and get closer to the upper part of the logicboard there is a sizzling noise somewhere around the CPU area (marked in the following image)
I am not 100% sure, but I think it could arise from the component the arrow is pointing towards (What is this anyway?). So while trying to proceed further in identifying the issue, I am looking for a replacement board as well. Does anyone know if all A1418 boards will fit into a late 2012 iMacs case and also do have the same connectors (display, fan, PSU, speakers etc.)? How about a late 2015 non 5k board in particular? Cheers
