Hello,
I have got a working iMac 24 a1200. Everything is OK GPU /LCD are perfect.
Now I bought a dead iMac 24 a1200 on ebay. It shows no picture but the fans are turning on and the white led is on, but the LCD doesn't show any picture.
When I turn on the iMac the start tone appears and the backlight turns on but the pixels stay black (black with little glow behind). I connected the iMac to another monitor - no input.
So I swop the LCD out of the ebay iMac and installed it in my working iMac 24. It worked - so the ebay LCD must be OK.
After that I turnd on the ebay iMac without the LCD but connected to an external monitor. The iMac started nornaly and showed a perfect picture on the external monitor. So the GPU is fine.
By the way all 4 control leds are turnd on when the iMac is runnig.
What could be the problem? Is it a logic board defect? Why is the GPU not communicating with the LCD?