Chosen Solution

UPDATE 1: When I boot the computer up with the external display connected and powered externally, AND with the ribbon connector plugged into the built-in LCD, but NOT the 12-pin inverter cable I can see, with a flashlight, the working desktop on the built-in LCD. UPDATE 2: I was able to get an LM240WU2 LCD (that I was assured was working - can’t vouch for that though) and popped it in my iMac. This resulted in the exact same problem as before: halfway through the startup chime the computer powers down. No picture is seen on the LCD. When I press the power button on my iMac it begins to start up, but halfway through the start chime it loses all power and needs to be unplugged before the power button can be pressed again. The screen always remains black. What I have figured out so far:

  1. First diagnostic LED is lit. Upon pressing power button second LED lights up, but halfway through the startup chime the third LED lights momentarily then the whole thing dies completely leaving no LED lights on until I unplug it and plug it back in.
  2. iMac will boot up and run with the LCD completely disconnected, but not with it connected.
  3. When an external display is connected the iMac starts up and runs reliably. External screen shows picture, but the display can’t display all colours, only blues and yellows. Is it possible that the power supply no longer has enough power to start up the big LCD but runs everything else fine? Or is it more likely that the LCD screen is the culprit considering the little bits I can hear in it? I have had this happen twice before, and both times, after many restarts everything booted up again and ran for another 4-5 months before happening again.

The fact that your not getting past the second of 4 lights leads me to believe your only receiving partial power from the power supply . Inspect the power supply for blown capacitors.Test the power output at the power supply disconnected from the board and make your determination from your results against the proper output levels. If you get proper readings from the power supply and it boots OK with the LCD disconnected i would tend to agree that the problem is the LCD. But once again make sure your getting a proper test from the power supply before replacing the LCD .Hope this helps

Mike, let make sure of your exact machine so we get the correct part. Please go here and put in your serial number and let us know your finding. The serial number is located on the bottom of your stand. http://www.everymac.com/ultimate-mac-loo…

Are you powering the external screen from the iMac itself or from the wall? Its unfortunate that you didn’t go to Apple when this happend the first time, they would probably have been able to fix it.