Chosen Solution
iMac 27" 2009 i7 2.8GHz I bought this machine and the V-Sync cable was damaged and during transit from the seller it stopped working due to the munched up 4 contact pads on the flex connector. I temporarily fixed it by using crazy glue under the tracks and pushing the contacts back down onto the plastic strip. Cleaned off the contacts and got the display back. However, I had to take it apart again and next time I reassembled it would not work. I received a replacement V-Sync cable from the seller and tested it with a multimeter. Contacts all in good shape (under my microscope). Replaced it but I still have a black screen. Have reset SMC and PRAM, tried Command F2 to make sure I was not in display mode. Shining flashlight on screen it does not have an image so I do not think this is just a backlight issue. I also tried an external display and it also does not show an image. If the backlight board was blown I am assuming it would not affect external display but when I had the bad cable initially it also would not display external. Can someone confirm if a bad backlight board could cause these symptoms? What is the most likely culprit? Maybe the 4850 graphics card? Could the Display Panel itself be damaged from this cable being defective? But that wouldn’t explain no external display… Open to ideas….
You might check to see if the video/graphics card in the iMac is not fully seated. This would cause no display on the internal and external displays. I do not think that the display panel would be damaged but the glue might have done something that affected the panel in some way which seems very unlikely. I would see if the video card is loose. I once got dust between the graphics card and the slot and it caused it to have graphical errors. You might also want to clean the graphics card contacts and slot with compressed air.
