Chosen Solution
Previously my iMac Built-in display randomly went black but once restarted, worked fine. Now mostly stays black except occasionally it displays perfectly at the startup but goes black in approx. 10-30 secs and sometimes it randomly displays perfectly for about 2-3 secs while I am using it. I now use an external monitor and works perfectly fine. Recently my local apple repair shop as part of diagnostic process, replaced the LCD panel with another working used LCD panel from Late 2009 model and it worked fine. Does this mean that I should replace it with a used LCD panel as a best solution? My LCD panel seems to work fine when it stays on but what could be the problem? Any suspected bad connection within LCD panel with this model? Any help is much appreciated. Thank you!
With the system turned on with the display now black, take a flashlight and press it against the display shining inwards at a sharp angle (15~20º from the surface) are you able to see the faint image of the display? This series of iMac uses LED’s for the backlight system. Given your symptoms it could be either the Displays backlight unit (within the display) or the LED Driver Board. As the system is winking in and out I’m leaning that your issue is really the LED Driver Board LED Driver Board Apple P/N 661-5311 and here’s the guide iMac Intel 27" EMC 2309 and 2374 LED Driver Board Replacement Using a volt meter (DVM) you should be able to diagnose which.
