2011 Apple 27 iMac bad/dead video card fix - Baking fun

I got a free iMac 27 computer that had the dreaded black screen and reboot, the one I got looks like someone had a go on it and had no idea how to take it apart, causing damage to connectors. First thing I did was to fix those so I could see what the real issues are.

With the connectors fixed it's time to see if it will boot, I use a Kali USB thumb drive to do this, if I can get into the OS then I can do some further diagnostic analysis but not here, I got the GRUB boot loader but when the desktop tries to load... reboot...  this points to a bad video card and it's well documented that is a very common issues with these.

This picture shows the symptom, black screen and reboot..

Lets fix it

Take the display off, if you don't know how, check out YouTube, lots of video's on how to do this but if this is out of your comfort zone, best not to attempt it.

Mainboard removed and heat sync for the CPU and Video card removed, both will get new thermal past. the old past was dried out and probably not helping much

Clean off all the old thermal past and prepare the video card for baking, I set the oven for 390f and baked the card for 9 minutes.

Reassemble everything back and make sure you plugged everything back in, I take pictures of all the connection before disassembly to have a record of what was connected and where. good thing is that you really can't mess this up as each connection is unique for each end point. 

TOP TIP: Before starting the boot up process, reset the NVRAM, Command + Option + P + R, wait for the boot chime and hold them down until you hear the second boot chime. let it boot.

I had a new install of High Sierra from a working Mac, so this install started out fresh. everything seem to work and I was able to get to the desktop.

Post OS boot I had some fans running full up and high temps, a couple of issues, one was the HDD fan was full speed due to the SSD. iMac's from this area had the HDD temp sensor built in and when changed from the Apple HDD the loss of that sensor makes the fan spin a full speed creating noise and disrupting the intel air flow. I found a free program called SSD Fan Control that lets you override the fan and set it down to 1100 RPM, that solved that problem but I still had over heating issues. The glass cover was destroyed, I used some painters tape to seal the edges, I think this helped with the internal air flow and the temps are stable and fan noise is very low.

I let the system run overnight and it's still alive. This was a fun repair of an older iMac that can still be used for some tasks.