モデル番号A1419 / Late 2012 / 2.9 & 3.2 GHz Core i5 or 3.4 GHz Core i7 プロセッサ

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I can't reset the NVRAM on my iMac 27"

Hi there,

Since two years ago I'm having random shutdown problems with my iMac. I've tried a lot of things but the last thing, because I think it can be a heat problem, was replace the HDD for a solid disc, it cools down the machine but randomly I've still having shutdown problems.

The main issue to me is that I can't reproduce the shutdowns so it's frustrating to give the machine to a technician listen from him the machine is fine...

Recently I've noticed that I can't reset the NVRAN (using CMD+OPT+P+R) because when I'm doing it the Mac wont start again and I'm wondering if that fact maybe can help for a diagnosis. What do you think?

Thanks in advance!

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Just so we understand, the act of doing the reset does not cause the system to restart on its own like it normally would. Instead you need to disconnect the power and then restart?

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I'm thinking your real issue is a bad power supply and/or poor power in your home.

I would first make sure your outlet you are using for your computer is 20 Amp (120-US) and you only have a your computer gear connected on that power circuit. I would recommend getting a good fill-in UPS if you note your lights flicker as thats a good sign of poor power. Even still you might want to ask your power supplier for a power meter for a week so you can see how your buildings power is. I would also make sure the outlet is properly grounded and back at the breaker panel make sure the ground service is properly grounded (check the ground line and the ground point its self!)

Only after checking your power service would I dig deeper into your computer. I would swap out the power cord with a known good one. sometimes the simplest solution is forgotten!

I would reduce what you have running to make sure you aren't mistaking a software issue as a hardware one.

Then I would look at the diagnostic LED's on the logic board while not ideal you can get an internal display cable extension hopefully you can catch the event. Otherwise I would just replace the power supply.

Update (02/01/2018)

Here's the needed part: iMac 27" Power Supply, 300W, Late 2012/13 and here's the IFIXIT guide you'll need to follow: iMac Intel 27インチ EMC 2546の電源の交換

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Thanks Dan, I would discard any problem with our building-home power because I have another iMac (same model) running without problems. I've tried the same place for a while and also the power cord.

In my wife's iMac I have a good UPS so I can try to put in mine to test.

This is for the random shutdowns but do you have any idea about the Impossibility to reset the NVRAM?

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I need to understand better what you mean by not able to do a NVRAM reset. If you can take a vid of what you are doing showing me what is different between a good and a bad reset.

I think its just the fact the system won't get out of reset mode thats confusing you here. Which I suspect is the power supply messing up and the downstream power.

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Ok, when I try to reset the NVRAM in other iMac, before the chime I keep pressed the keys and then the computer restart and chime again, and it keep doing it while you keep the keys pressed, right? When I do the same in this iMac (turn on, wait for chime, press keys) the iMac shutdowns but never start again, at least after some minutes with the power coord disconnected

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Thats what I thought!

This gets down to how your system power is enabled.

Apple uses a soft switch Vs a mechanical switch like your wall light switch. This logic is part of the power supply. The simple blister switch you press on the back of the system is a low voltage circuit which then turns on the AC circuit internally and the rest of the systems power services.

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Ok so I should try to replace the power supply, right? Thanks!

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Fran Fox さん、ありがとうございました!
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