Model A1225 / Mid 2007 and Early 2008 / 2.4, 2.8, or 3.06 GHz Core 2 Duo processor

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Is my LCD toast or is it my power supply?

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.

Answer this question I have this problem too

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When booted and using the external display can you discern the desktop on the iMac by shinning a flashlight at an angle on it?

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Mayer: As I'm sure you know, you're onto something! So, I just booted the computer up with the external display connected and powered externally. I also plugged the ribbon connector into the built-in LCD, but NOT the 12-pin power cable. As you suggested I can see, with a flashlight, the working desktop on the built-in LCD. Is it the backlight? I have certainly run this thing practically 24/7 for 6.5 years!

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I have this same problem and looking below it would seem as of this comment Mike's problem is not quite solved. I'm new to this forum so not quite sure if this is right way to go about it but I have posted the info I have gathered in an answer below and hope that someone might be able to provide some insight. Thanks

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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

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Visual inspection of the PSU yields nothing obvious. Do you mean test the PSU output to the LCD while disconnected from the logic board (with a multimeter)? If so, any idea where I might find what the proper output levels are? Any thoughts on the little loose bits tumbling about in the LCD? Thanks for your help!

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power outputs should be in the spec sheets available from apple. go to apple support and put in your model serial number . We used to be able to get it for you but unfortunatley you need to insert the full serial number we used to ask for the last 4 digits but this is something we cant do now so you can go here and get the info yourself http://www.everymac.com/ultimate-mac-loo...

Hope this helps

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Thanks a lot Jim!

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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...

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Early 2008 - MB325LL/A

I know the part numbers for my computer (through powerbookmedic). Just was hoping for some comfirmation that it is the LCD. As far as I can see this screen has an embedded inverter, which I believe means I need to replace the whole screen. Is that accurate? LCD is LM240WU2 (661-4685).

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I was able to get my hands on 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.

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Something else that I've just noticed is that the picture on my external monitor is very drab. It only seems to be producing blues and yellow. When I take a screenshot and view it on my MacBook Pro all the colours display correctly. Could this whole problem actually be the video card? I've read online that the video card actually sends the signal to the backlights to turn on...

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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.

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External monitor is powered from the wall, not from the iMac.

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You could try a display that is powered by the iMac itself. That would exclude the power supply as the issue. Only problem is that most people don't have that kind of screen laying around.

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Yes, that is exactly my problem. No extra 24 inch screen laying around. Basically I'm trying to get an idea of whether, based on the symptoms, I should try replacing the LCD first, or the power supply first.

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I'd say it's your LCD. With the sounds coming from your display and the fact that an external one works fine I think that is the smartest thing to do. What you could also do is going on the Apple forums and asking it there. Some people from apple are also on there. There is also a lot of people there with more experience than me.

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Thanks for your advice Mr. I'm also leaning toward LCD. Hoping to see a consensus. I will try the Apple forums as well. Thanks again.

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Mike will be eternally grateful.
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