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Model A1312 / Mid 2010 / 3.2 GHz Core i3 or 2.8 & 3.6 GHz Core i5 or 2.93 GHz Core i7, ID iMac11,3

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Having problems with 21.5" i3 2010 distorted blank/rainbow pixel scre

Hi @mayer I am having a similar problem to what Sam reported. My iMac is an A1311 (EMC 2389) -- mid-2010 with 21.5" screen. Can you offer any tips on how to solve this issue? (Or whether it is solvable in the first place?)

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Btw, I have posted a pic of my screen here: https://imgur.com/HokjRey

Mayer: raven, I have not seen that type of pattern before. Hook it up to an external monitor and let us know what you get.

Raven: Thanks [/User/13051/mayer|@mayer] I will give that a shot and report back.

Raven: Hi [/User/13051/mayer|@mayer], when I plug in an external monitor, the iMac still freezes before the login screen, but instead of a fuzzy screen with a rainbow of random color pixels, there is just a uniform blue screen (on both monitors). Any advice on what to do or suggestions on what the diagnosis could be? Thanks in advance

Mayer: Try booting in Safe Mode by holding down the Shift key on start up and see if there are ant changes. Next try holding down the option key and booting from the repair partition.

Raven: Hi [/User/13051/mayer|@mayer], I am able to boot into Safe Mode, the problem is that there is no graphics acceleration, so most applications are unusably slow. I can also boot into the repair partition, but I've already tried doing a clean reinstall of macOS (both the latest version of High Sierra, and the version that my iMac originally came with, Snow Leopard), but the problem remains. Ideas?

Mayer: raven, does your serial number match one of those in my original answer? How much RAM do you have? Have you ever changed the hard drive?

Raven: [/User/13051/mayer|@mayer] Nope, my serial # ends in 6DAS. 4GB of RAM. Yes, I recently replaced the factory 500GB HD with a 1TB non-SSD HD, using the iFixit guide and recommended tools. Everything else is factory original.

Here are the specs for my iMac: iMac "Core i3" 3.06 21.5-Inch (Mid-2010)

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Raven I moved all our conversation to a new question as it was getting to long to tag onto an old one.

raven, are your fans racing? Was your drive an SSHD or just a standard HD?

With that external monitor hooked up, insert your original system install disk right after you power on and hold down the C key until the Apple appears. Let me know your results.

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Thanks @mayer.

Nope, fans are running normally. I did a fair bit of research before replacing the hard drive, and learned about the proprietary hard drive firmware that Apple utilized for temperature monitoring. When replacing the hard drive, I attached a 2N3904 transistor to the temp monitoring cable, which acts as a temperature sensor (for reasons that I don't fully understand). This prevents the fans from going into failsafe mode / running at maximum speed. My hard drive was a standard 1TB Western Digital HD, not an SSD or hybrid SSD/HD.

As you can see in the attached pic, I am able to boot from the original Snow Leopard install/recovery disk that came with my iMac, but *only* if I hold down the Shift key while booting -- similar to booting into Safe Mode in macOS. Otherwise, if I try to boot from the install/recovery disk without holding down the Shift key, it freezes with a solid blue screen before getting to Snow Leopard installation.

https://imgur.com/a/3BEnkgX

What do you suspect might be the problem?

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Tell me more about this transistor you used, possibly a link, What hard drive was in it and what you changed it to. This one is not easy. Let's see if @danj has run into this too.

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Here are a couple links about using a transistor as a substitute for the internal HDD temperature monitoring, when replacing the internal HDD:

type of transistor used at hard drive thermal sensor inside iMac a1224

https://www.haught.org/2015/11/16/imac-2...

https://marco.org/2011/05/12/owc-imac-ha...

https://everymac.com/systems/apple/imac/...

I'm pretty confident that the hard drive is *not* the problem. @danj has suggested in other posts that the this problem is caused by faulty/failing VRAM on the internal ATI Radeon HD 4670. When I remove the Apple extensions (*.KEXT) related to the Radeon card from /Systems/Library/Extensions, I can boot up normally (i.e., without needing to boot into Safe Mode), but there are is no graphics acceleration.

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Raven - It looks like thats the case. Time for a new GPU card.

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@danj All I can say is ARGH.....

@mayer Thanks for your patient help

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raven estará eternamente agradecido.
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