How do I fix black screen?
Here are the symptoms:
-When you press the power button, the screen flashes white twice, then stays black. Otherwise, everything else starts up normally with the chime and hard drive sounds, etc.
-Shining a flashlight to the screen shows it still works (just no backlight?)
-I've reset the PRAM, no go
-I've replaced the inverter board but that didn't work
-It does work with an external monitor attached
I am currently running an extended Apple hardware test to see if it brings up anything specific.
Could something on the logic board be bad?
It's the screen connection to the board, screen or video card
Here is the screen guide, take it apart and check the screen connector, if nothing, try a a screen
If not, it's the video card or logicboard(I think this model has a MXM card)
I do not know much about the video card built onto the board or a seprate card, but I know this, if it's a ATI(AMD now) or NVIDIA, it's probably on a separate board, if it's intel, no, on the logicboard itself, you will need to take your iMac apart to know for sure about this
Here is the GPU swap guide