Chosen Solution
Hi all! I have a late 2009 21.5" imac with an ati 4670 graphics card. One day it won’t boot anymore (white screen). If booting with the shift key helt down, the imac boots but without graphics acceleration. So I replaced the graphics card. Now after turning it on, the display remains black. An external display (TV) works fine. So at the moment I can do everything on my TV, even graphics acceleration works. But the main iMac display remains black. The backlight works. I triple checked if the fat display data cable sits tight and it seems ok. So what can i do now?
Sounds like the vertical sync cable is damaged or not connected correctly.
