Sorry for the double posting, earlier posting can't be edited...
Hi all
Been following this thread as well as e.g. Imac display issues following Yosemite upgrade and hoping to find a solution for my problems with my mid 2011 iMac 27" equipped with Radeon 6770m.
I have been having problems with the "pixels" since Yosemite and a temporary fix have been tips from here related to "reduce transparency" and "cleaning" the screen with a Finder window.
Since 10.10.3 and the Photos app the problems escalated. The Photos app caused my iMac not only having pixel problems but also crashed the entire UI.
I contacted AppleCare and produced log files as well as images and videos of the problem.
The iMac crashed after seconds to minutes after Photos app was started with pixelation problem occurred.
When I looked at the log files I found that it was only the UI that crashed, log files indicated that the machine was up and running "normally" besides the UI.
Events labeled as "GPU.RESTART" with event GPU Reset happened during the time from the freeze until I hard rebooted the machine.
2015-04-16 20:39:02,000 kernel[0]: Trying restart GPU ...
2015-04-16 20:39:09,000 kernel[0]: [1:0:0] GPU HangState 0x00000000, HangFlags 0x00000004: IndividualEngineHang 0, NonEngineBlockHang 0, FenceNotRetired 1, PerEngineReset 0, FullAsicReset 1
<Kernel_2015-04-16-203950_iMac.gpuRestart>
I also found in the log files for Photos
Application Specific Signatures:
Graphics hardware encountered an error and was reset: 0x00000000
Apple also asked me to install Lion and see if the problems occurred there as well. I installed a fresh version of Lion and after 24 hours the problem didn't occur there. I upgraded Lion to Yosemite as well as a fresh installation of Yosemite and as soon as I started the Photos app, the machine crashed.
The respons I got from Apple after a couple of days was that it was a hardware problem with my iMac but the couldn't conclude whether it was the GPU or the logic board. Since the Radeon 6770m was not covered by the extended warranty the advisor couldn't give me any additional help.
I contacted the local service point and the price estimates I got was approx $100 just for an estimate and then additional $300-350 if it was the GPU or $800-900 if it was the logic board.
I've been reading about the concept of "baking" faulty boards and having the to work again after a time in the oven, e.g. Re: Red and green square dots?
Since I had some time of this weekend I thought, why don't give it a try.
NOTE! It's highly risky and I can't recommend anyone doing this but I just wanted to share my experience.
I thought that if I destroy the GPU it's not worse than having it fixed through the local service point so why don't give it a try.
I opened the iMac and removed the logic board in order to gain access to the GPU. Removed the GPU and cleaned the board. During this time i pre-heated the oven to 170 (celsius). Put the board in for roughly 7 minutes and after that opened the oven door and let the board cool down before I moved it.
Applied new thermal paste and put the iMac back together. Started and kept my fingers crossed...
Everything seemed to be working as before with the difference that I could start and use the Photos app. I also removed "reduce transparency" so the machine is back to the "normal" state.
The machine has now been running more than 24*3 hours without any problems.
Again, not something that I can or would recommend but I just wanted to share my experience regarding this problem.
PS, thx everyone contributing to this thread and others related to this subject.