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iMac(s) Black Screen Diagnosis

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Hello all. I just recently acquired two imacs (27 and 21.5) at an auction. Neither of these iMacs currently have hard drives in them, but I attempted booting the snow leopard install dvd on both machines (just to test them out). Upon doing so I was unable to see anything (black screen from the beginning) but both macs chimed (as expected). So, I plugged in an old lcd into each of them and turned them on and loaded the install disk on both of them successfully (on the external lcd albeit). I am kind of lost as to what to do next, I really do not want to have to take it to an apple store and pay an exorbitant price, since I am somewhat comfortable repairing computers and there are numerous youtube tutorials for repairing imacs, but I do not know what is wrong with them or how to diagnose the problem. I have reset SMC on both, and I have taken the ram out and put it back in both of them. So what do you all think- Display?? I am (with my limited knowledge) guessing it is not the graphics card since I was able to run an external monitor. If anyone has any suggestions as to what may be the cause, and/or how I should go about fixing it, your help will be greatly appreciated.

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Could be display (a whole assemble or some parts of it). If the external display works, then VGA card should be good (unless it is half broken - no display but external).


I don't know how VGA cards are fixed to MB in iMacs (is it soldered directly to MB or as detachable card), but if it ca be replaced with a known-to-be-working one, you could have some solid conclusions done.


On the other hand, read this who knows... I remember some time ago a member had complained on his iMac video performance troubles (VGA overheating).

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