[SOLVED] iMac black screen, external display good

[geranium]

Can anyone shed a little light on this one? I've got a Mid-2010 iMac 12,2 on the bench.

• It boots and runs until the screen goes black (anywhere from 5 minutes to several hours) - seems like it may be heat dependent
• The external monitor continues to display normally
• Once black, there is no backlight and no signal at all (flashlight on screen shows no image)

One possibility is that the video card is not completely out of action, and that the external DisplyPort output is working while the internal DisplayPort output is not. But wouldn't the backlight still be illuminated if that were the case?

Other possibilities I can think of:

• LCD temperature sensor is overheating and shutting down the display
• some part of the LCD is failing

It passes the short Apple Hardware Test, can't do the extended test as the display shuts down.

Any hints appreciated.

Thanks.

 

[geranium]

Just cracked it to see if it is packed with dust, but it looks pretty clean from a cursory peek.

 

[overburnz]

could be the display overheating, could be graphics card too, try to listen to the fans inside to hear any overworking of the fans.
my bet goes to the graphics card

 

[geranium]

I take back what I said earlier. There actually is a signal - the backlight is gone. I just couldn't see the ghost image until the glass was off - it's very faint!

Off to find a backlight board...