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White screen on a Franken-iMac - can't boot

Greetings from Bulgaria. I’m new to this, so please bear with me. I recently bought two 27” iMacs mid-2011 (A1312) with missing parts in order to try and build one working computer for my wife to use (bought them separately from flea markets - 60 USD in total after haggling). The iMacs are the same model - only the CPU and the GFX cards are different (one is i5 and the other is i7). I know that there is a great chance that some of the parts are faulty and I don’t know the history of the machines and why they ended up in a flea market, but it is my tiny project and I want to do my best to make it work.

Block Image

I took all the good parts and combined them into one machine. Тhe display cable connecting to the logic board is the only broken link, but I’ve already ordered a new one and waiting for it to arrive. For the time being I am using an external display to check if the system is working. I installed 8 Gb RAM and an SSD I had lying around. There is no OS on it, but I have an old DVD of Mountain Lion which I intend to install. The system does not boot though - even from the DVD.

Block Image

I hear the chime and LED 1, 2 and 3 light up. This means that there is power and the logic board communicates with the GFX card (dare I assume that there is no problem with them?). The screen is white-grey with vertical stripes.

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I tried resetting the PRAM (Holding cmd+opt+P+R) - it restarts, but nothing changes. 15 seconds after the power is on, the fans start spinning at 100% but I assume that it is because of the missing thermal sensor on the HDD - the one on the optical drive is there (I intend to buy a cable with sensor but for the time being it should work without it too). I am not sure about the origin of the problem.

My next step will be to swap the GFX card with the one from the other iMac, but I would be very grateful if you share your experience with me. Any advice is more than welcome.

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The vertical stripes on Macs from 2011 with AMD 6xxxM GPUs tend to suggest that the GPU has died. In the 27” iMac from that era, the GPUs use the MXM-B standard, which means you can replace the card with a standard one; Nvidia GTX 7xxM cards that came out from Dell/Alienware laptops tend to work pretty well for that. Once you replace the GPU with a non-Apple one, you won’t have any boot screen any more, so you’ll want an HDD with an OS on it: if you are running macOS High Sierra on that drive, and you have a Nvidia GTX 7xxM card installed, it should be supported out of the box by the OS. Before bothering with the GPU, though, make sure that the HDD is actually working by plugging it into a computer and seeing if it functions. If it’s dead, just replace the drive.

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Thank you for the comment - it really helped me. I tried booting from the HDD of an old MacBook Pro I have and it started booting. It seems the other HDD was no good. The external display shows graphics, but the image is still striped. I will try swapping with the other GFX card I have.

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have you tried to boot into safe mode?

  • Press the power button, and then press and hold the shift key when you hear the start-up chime.
  • Release the shift key when you see the Apple logo and progress bar.
  • Once the Mac has gone into Safe Mode, go to your Applications folder to uninstall the apps that you no longer use, then empty the trash.
  • Restart the Mac.

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Hi, Laura. Thanks for the comment. As I wrote in the description, I cannot boot into Safe mode or anything, because there is absolutely no OS installed on the HDD. I am trying to boot from the Mountain Lion installation DVD, but I cannot do that either. The white screen persists. I think the problem is not in the OS or the HDD.

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@laurayuan - Even still you should have gotten something on the screen Vs a white screen as you've shown.

About the screens on your Mac as it starts up but the assumption is what the internal screen shows not an external as the default screen is the internal.

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Petko Slavov estará eternamente agradecido.
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