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Boots, but goes to a black screen

I've just had gotten another replacement logic board for my 2009 25.1" iMac. I press the power button and I hear the chime. The LCD screen lights up white, then goes black (yet I can tell that it's still backlit). The official tech manual claims the logic board has 4 LEDs at the bottom, but there are only 3, and all 3 are lit up. I've tried resetting the PRAM, but nothing happens. I've connected an external monitor, nothing changes and there's no video on either screen. I've tried connecting an external monitor without the LCD screen attached, There's still no video, and only LEDs 1 and 2 are lit.

It's my assumption that there's something wrong with the onboard video card. If not, then I'm absolutely stumped. I've been getting my logic boards from ebay, and have had the same issue with the video card.

I have another logic board, but with a functioning video card and no LVDS port. (Some genius ripped it off. It functions fine with an external monitor attached. Every now and then I'll see a seller offer a repair for the LVDS port. I'm tempted to try that. If I could, I'd just swap video cards, but Apple uses glue to hold them in place.

Any ideas or suggestions would be greatly appreceated.

Thanks

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Here's an update. With only an external screen plugged in, only Diagnostic LEDS #1 and #2 light up. There's no picture on my external monitor. This time around, with the LCD screen plugged in, Lights 1, 2, and 3 are lit, but the LCD screen lights up bight white and then dims to black. Also the hard drive and fans don't spin while the LCD screen is attached.

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Okay, this will make no sense, but God finally fixed my iMac.

The diagnostic LEDs in my case were completely useless in my case. When the external screen was finally working, only light 1 and 2 were lit. For a few times, when only the LCD screen was attached, all 3 LEDs were lit, but I had no video.

The turning point happened when I moved two of my RAM chips from the back slots to the front slots. (I didn't bother using all 4 ram chips at the time.) Just for giggles, I tried to reset the PRAM with just the external monitor attached, and it strangely worked. On top of that, I got a picture. The iMac booted like normal. Then I shut it down to try with both monitors.

It worked! But this time around, it booted to the emergency partition. I ran Disk Utility just in case, and restarted. It booted normally again. Both screens were mirroring. Then I started to put in the screws to secure the LCD screen to the frame, and it died. At that point, I feared that I might had fried my latest logic board.

I took the LCD screen back off to see the damage. As far as I could tell, nothing was fried. I adjusted some cables in case the LCD screen was pressing on them. The item that was sticking out the furthest was a small hack made by OWC to help the iMac keep tabs on the temperature of my replacement hard drive. I moved the small piece of padding that was on the original hard drive and moved it closer to the hack. I rescrewed the LCD screen down and I booted normally again.

Thanks for reading

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chilbig estará eternamente agradecido.
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