2011 27 inch iMac - Black Screen of death?

I can provoke the black screen by watching youtube videos on full screen but otherwise I have no problems so long as I limit the brightness to 6 notches or less.

I did take the iMac apart some time ago to clean out any dust and have a look at connections and noticed that the monitor cable where it is attached to the board doesn't look right: One of the clips that is supposed to secure the cable is bent so the cable holds itself in without being clamped in on one side.

Could that in itself cause the screen problems?

If so, is it an easy fix?
 
Hi there, I am NOT a tech savvy person at all! but have a 27" imac, 3 years old. Suddenly yesterday, the screen went black. System still running in background, no cursor in corner. I have read this thread, tried the SMC and PRAM resets, no joy, just makes a noise on resetting PRAM. It has a wireless keyboard and mouse. As always, terrified of the cost of a visit to the store!! Any help would be welcome :)
 
wild.child said:
Hi there, I am NOT a tech savvy person at all! but have a 27" imac, 3 years old. Suddenly yesterday, the screen went black. System still running in background, no cursor in corner. I have read this thread, tried the SMC and PRAM resets, no joy, just makes a noise on resetting PRAM. It has a wireless keyboard and mouse. As always, terrified of the cost of a visit to the store!! Any help would be welcome :)
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Do you see any video on the screen? Shine a bright light into the screen. Look at the top for menus, and you dock, or windows/icons on the desktop. You may need to look carefully, or make the room as dark as possible. The video on the screen could be very dim.
Because you can't really see anything much, make sure that you have shut off the iMac completely (disconnect the power cord to make sure), then start up. Wait at least a minute before checking for video. Again, shine a bright light into the screen, moving the light around at different angles and positions, just to try to see any icons or menus.
Do you see ANY images on the screen?
That would mean that you have video, but the backlight, which gives you the brightness on the screen, may have failed.
If you don't see ANY kind of images, even very dim on the screen, then you should try an external display, connected through the Mini Displayport. If you get video on an external display, and still nothing on the built-in display, then the LCD panel may have failed, or maybe the inverter board.
 
Short Answer.

Unplug the V-Sync cable going into the Backlight Board.

Long Solution:

I tried changing the V-Sync cable as Apple's troubleshooting suggested, but the problem remained. I have a second Mid-2011 that works fine and began using it as a parts machine to troubleshoot the bad one. Even though they are both Mid-2011's they use different backlight boards, I placed the incorrect backlight board in the iMac and the V-sync cable wouldn't fit, so I just left it unplugged. I had it run for hours on a youtube loop, and with my manually adjusting the brightness and no issue. So I assumed my backlight board was bad. I ordered a new one. Apple even sells them in a kit (part 923-0047) sending both types. I replaced my backlight board with the correct style and hooked up the V-sync cable. I then still experienced the black screen issue. Beyond frustrated I went to put in the new part of the wrong style because the other had worked. Then I remembered that when I did that, I had to leave the V-sync unplugged. I left the correct style of Backlight board, but unplugged the v-sync cable, and tada! No more issue. I can change the display brightness up and down without issue, and it hasn't even shut down on me.
 
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