SOLVED! (and it's not the other hardware!)
I had the blackscreen/blackout issue (not the backlight, that's a different issue) after I added RAM. I had added 2 x 4Gb and kept the 2 x 2Gb in the original slots. I now had 12 Gb.
The blackout issue started shortly after this.
After reading the post (below) I took out the 2 x 4 Gb and replaced them with 2 x 2 Gb from another Mac I had lying around. This gives me 8 Gb with 4 identical DIMMS in the slots. The Black screen did not occur again, no matter how I try to recreate it changing the brightness, screensavers and/or sleep mode.
I think the RAM solution makes sense, so far working for me, problem looks solved. Try it and let us know! Thanks Sunfiresaturn!
Sunfiresaturn including Apple logo changes for feature 126 Apple image including Apple logo changes for feature 126 Apple image ends here
Jan 19, 2014 5:03 PM
Re: 27' iMac (2011) Screen turns off
in response to Ashadur
[DocumentBodyStart:b21f0b0e-bbb6-4d8f-827a-58c825f45ac6]
All of your symptoms were indentical to mine and I tried EVERYTHING to fix it.
My realization?
RAM
I have 4 slots open in my 2010 - iMac
2x2GB DDR3-10600
2x4GB DDR3-10666
For some reason, my iMac couldn't handle both together. At first I thought it was the 10666, but then tried both RAM types separately, and both work.
I guess you shouldn't pair 10600 with 10666 - or at least different ram sizes.
I am going for 4x4GB 10666 now.
I hope this helps everyone because I was losing my mind for a short while.
iMac is happy and on full brightness.
Life is good.