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Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor, ID iMac12,2

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Reboot and freezes when using USB / Wifi

Hi. I got a second hand iMac. The machine boots OK, i could install systems (tried 10.11 and 10.13). The Mac works correctly (seems), as long as I don’t use a USB scanner or try to access the internet.

When I do, I get all sorts of weird things: black screen and reboot (also happens some time when just using software), grey screen (most often), blue screen (once), other screen colors - in any case it freezes and will only boot again after resetting the PRAM (several "sound + reboot” for about 20 seconds). Replacing RAM did not help.

I have to decide if I try some repair or break the machine into spare parts. I think it’s a board problem, but which board?

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Have you tried running the onboard diagnostics yet? Restart your system and press the D key

Reference: Mac startup key combinations Let us know how it goes

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I did. Diagnostics don't show anything.

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With no error its a bit of a hunt and peck ;-{

I would start off with the power supply and then you’ll need to look deeper into the logic board circuitry.

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OK. but what do I look with the power supply? How to test it? Actually, same question with the board. I mean if it does not show from outside...

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@tcoulon - I would just replace it unless you have the skills to test it,

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OK, I unmounted the screen. The board of the power supply is full of white marks, I guess it's a bad omen... The other boards "look" good.

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Thierry de Coulon will be eternally grateful.
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