IMAC 27" random screen blackout

Question

my 8 month old iMAC 27" quad core machine recently started to "black out"  Screen goes black for no reason and the only remedy is to hit the power button - usually twice.  This will bring the system back to where it was, all open APPs etc. will be exactly where you left them.   I also tested the speaker volume up/down keys to see if the "system" was still functioning during a screen black out.  THe CPU etc. is still avtive as the speaker volume when maxed out will "beep" at you. 
I called tech support - they had me do some special resets of power control and PRAM - no luck
I took the system into Apple store for Genius Bar appointment - 2 hours of running the mac in the store, of course it did not black out....however, they ran other diagnostics and ran a bunch of utilities, graphics etc. to max the RAM and CPU load, heating up the system but still nothing happened.  they suggested I reistall the OS by doing a hard boot and holding down the command key to prompt me to re-install the OS from the "cloud".  This did not solve the problem either.
I then took the system back to Apple and told them to fix it or replace it.  They kept the system for 5 days, and replaced
item 661-5948 Board, Logic, 2.7GHZ, Quad Core i5
item 661-5967 card, video, AMD radeon HD 6770M, 512M
item 661-6615 LCD panel 27 inch with BLC
and labor: TOTAL COST $1183.32
This was all under the first year warranty so I did not have to pay for this out of pocket.  If you look at the list of repairs, they effectively gutted my iMAC except for the HDD and enclosure.  Ahhhhh at last I thought I would be back to a fully functional iMAC.   Two days after getting home and just browsing the internet the blackouts started again.
Once the system blacks out the first time (which appears to be a completely random amount of time) it will then black out more frequently after hitting the power button. 
It's time to call Apple again and demand a new system, wondering if the iMAC LCD crystal / video components are suffering from a manufacturing defect....as it seems the new components they replaced are suffering from the same issue.
APPLE - would appreciate you recognizing this problem and working on resolution.
thanks,
Share holder and completely converted MAC power (family) users!!!!

Answer 1

Hi, I hope you find this information useful:
A few years ago, we purchased iMac 27" 11,3 (Mid-2010) (with i7 2.93Ghz, ATI 5750 1GB VidRam, etc.) for our offices and software development team. They were originally shipped with Snow Leopard and worked as expected and without any problems. However, a year later, we upgraded to Lion and all of machines immediately exhibited the same problem: the screen would go to sleep by itself and would require a hard reboot or sleep on/off. (Only the screen was asleep and the rest of the iMac continued to function; this was confirmed by screen sharing to the machine.)
Due to the severity and impact on our business, we immediately contacted Apple and their genius bar staff in New York/SoHo. They were unable to determine the root cause, so they replaced all of the logic boards, video cards, and screens, and got us back online asap. This resolved the problem, but we were curious to know why. After days of rigorous testing, we concluded that the issue is caused by a bug in MacOS. Specifically, faulty Kext files (Kernel Extension files extend the functionality of the MacOS Kernel) for the ATI drivers.  So, why did the replacing the hardware work? We think that the Kext files that were released and distributed with Lion (and later MacOS versions) correctly supported the newer hardware revision of the replacement components, but not the older ones.
If you're experiencing this problem, short of replacing the components or waiting (for another 3+ years) for Apple to hopefully fix their OS software, you can lessen the effects and be able to recover faster by doing the following: (Please bear in mind that these are not permanent fixes.)
Turn on "Hot Corners" and assign one corner to "Put Display To Sleep". When your screen goes to sleep, move your mouse (which still works) to that assigned screen corner for a second, then move it again. This will wake your screen. You may have to do this a couple of times, but it's much faster than a hard reboot or a sleep on/off for your iMac.
Turn the brightness of your screen to a low setting (10%-15% of the total brightness).
Turn on screen sharing, so you have a last resort method of accessing the machine.
If you're not convinced that it's a MacOS software problem, then do what we did and see for yourself; install Windows 7, Windows 8, or Linux via Bootcamp or as the only OS on the iMac. Those operating systems and drivers worked flawlessly for us; the bug does not occur and the screen does not go to sleep, but if you dual boot and switch back to MacOS, the bug reappears because the iMac is loading faulty Apple MacOS software.
Good luck!