Solution: iMac 2011 display goes black, switches off during operation
Anyone who has an iMac from 2011 may know the problem that the display simply turns black during operation. However, only the backlight goes out and if you light a flashlight on the display, you can still see the information on the screen meaning the pixels themselves are still controlled.
Since that was the case with my iMac, I lived with it (display off and on again brought it usually back to life - shift + control + escape and wake it up again), but at some point it became so much that I decided to bring it to a repair shop. Since Apple does not support devices from this year anymore, I've been looking for a freelance repair guy. He ran the iMac in test mode for weeks, measured power supply, inverter and display and could not find anything. Of course NVRAM and PRAM Reset did not help either.
Only the second professional was able to find the fault: The cause for switching off the background illumination are faulty LEDs in the display. So he disassembled the display and replaced the faulty LEDs and it is running perfectly again! Apparently, the voltage increased so much due to the faulty LEDs that a safety shutdown of the lighting array was made. It did cost me around 280 euros.
This makes the decision of course a little bit difficult: for 200 euros, there are also used panels, but the have the same age and are therefore likely to suffer from the same mistake. Or you can import a new panel for a lot of money from Asia. I opted for a repair because the iMac has nothing else and runs very well with a SSD. In addition, scarce 300 euros are far below the price of a new 27 "iMac, which comes with SSD at least to 2,200 euros. Although that one comes with a Retina display, I would love to own 