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My guess would be the cable... if it were the logic board then nothing would work with an external monitor either.
 
Update

Thanks Andre. I just hooked it up to do more looking around and the desktop looks fine (on an external monitor). I'm inclined to replace the display cable. has anyone tried that before. I've seen the pbfix instructions and it says very difficult--any opinions? I'm no electronics whiz but I am resourceful and follow instructions well (when necessary). 🙂
 
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