Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor

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Voltage regulator in iMac 27" - screen blackout.

1. Is there a voltage regulator in this iMac? If so, where and how it can be tested for damage, if not that what is the next thing to look at?

2. Can it be replaced?

Update (08/25/2017)

Long story short:

(Chime normal at startup)

  • I did not open the iMac - Apple did. They cannot reproduce the blackout.
  • Fans were dusty - Apple blew the dust off (when the iMac was there first time)

- Second time - They "thought" it was the video card and replaced it but came to the conclusion is not that but my upgraded RAM

  • It now has the very original RAM - blackout persists!
  • It seems like it is having personality fits: today it worked, (just blacked out as I'm writing this), yesterday it did work (even hot) but 2 days ago screen was going in blackout every 2 seconds
  • It reacts to Shift-Ctrl-Eject, but not always stays on
  • My voltage is going up - from 123.5 (earlier) to 124.9 (as is just blacked out)
  • I have a secondary screen attached - so I can finish my work in case blackout happens (as it did)

So, the only variable is the voltage.

Can you help? Please!

Update (08/25/2017)

My external monitor - Thunderbolt connection to HDMI cable

  • I turn on my iMac but the external monitor turns on when I see the password screen
  • It will not run on If I try to run diagnostic - (D task)
  • it will respond to "Sleep" command"either Shift-Ctrl-Eject or my screen shortcut
  • when the iMac screen blackout, the external monitor does not react

Update (08/25/2017)

So, back to my original question - is there a voltage regulator in this iMac?

Update - Kind of (09/14/2017)

Last advice from Apple Store was that the memory was bad. I returned the original memory in it in order to take it back to the store, but I was busy for about 2 weeks. Now, it seems like the issue is gone. Or, at least it did not happened in the last 3 days.

The voltage fluctuations does not seem to matter like I previously thought. So, the possibility of LED driver board to be bad is slim now.

I still have no explanation why it did not happen at Apple Store (while having the upgraded memory installed).

I assume the fault is with the memory slots (I guess that can be an issue too) and then is not worth to be repaired. The price is half of the computer when was new.

Anybody any ideas on this?

Update (09/15/2017)

The pic of modules

Block Image

Block Image

Answered! View the answer I have this problem too

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Please give us you symptoms including boot tones and what the diagnostic LEDS are showing.


So your external monitor works all the time? Where are you getting these voltage reading from and why and do you have any background in repairing electronics?


I have a Digital Multimeter - I test the voltage out of my electrical outlet


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I have been repairing Apple products since 1979 and never heard of anyone measuring voltage fluctuations from the outlet. Do you have some electrical engineering background? Or do you just have a little bit of knowledge, enough to be dangerous? If you don't have that background, lets see if we can get into actually trying to pinpoint where the issue is and fix it if possible.

You are aware that there was an Apple recall on this model on the GPU? Did Apple mention that to you. Did you take it to an actual Apple store or an ASP and how long ago was this.

BYW Apple has now remove the information or the recall as they are no longer honoring it.

That article was here:

Here's the info on it. Does your serial number match one of those recalled?

Does your external monitor continue to run when the screen blacks out? Did Apple leave the new GPU in the machine or did they put the old one back?


If your external monitor is working, the problem is not with the GPU and we need to look further up the chain.

Will Apple honor the recall? They didn't even tell you about it and it took a class action law suit to get them to fix it in the first place, they never notified anyone about and they expired the program and removed evidence of it ever existing. Apple is not run by a man with integrity like Steve Jobs anymore. But is run by a greedy corporate entity with an army of attorneys. So, NO their not going to help you. It now a 2011 machine and Apple considers it a "legacy machine" and won't even work on it now.

Use a flashlight and shine it on the screen at an angle and see if you can discern the desktop when it blacks out.


I suspect you issue is a failing component in the LED driver board.

iMac Intel 27" EMC 2429 LED Driver Board Replacement


LED Startup Sequence

LED #1 = Power available.

If no LED is visible:

• Verify AC power source

• Verify known-good AC cable is connected

• Verify cable connection between AC inlet and power supply

• Verify cable connection between power supply and logic board

• Verify power supply

LED #1 + LED #2 = Power available, and system is powered on.

If second LED is not visible when power button is pressed:

• Verify power button connection to logic board

• Verify power button functionality

• Verify cable connection between power supply and logic board

• Verify power supply

• Verify logic board

LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found.

If third LED is not visible after power on:

• Verify that the MXM video card is seated properly

• Verify if boot chime is present and fans are running when powered ON (reset SMC

and PRAM, check backup battery if necessary for proper boot up):

• If POST boot chime is not heard, go to Won’t Start Up symptom ow,

• • If POST boot chime is heard, go to No Video symptom ow.

LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card found, and internal LCD found.

If fourth LED is not visible after power on:

• Verify cable connections between LCD panel and logic board

• Inspect LCD display cables for cable damage

• Verify external video functionality, and according to result check the following items:

-If external display works then verify/replace the LED backlight board -If external display works then verify/replace the LCD panel

-If external display does not work verify/replace the logic board

If you need to play with your multimeter, here is where to use it:

Lower right corner of logic board

Logic Board Test Point


Pin 1


Pin 3

5V power for Hard Drive(s). Note: If present, the 12V power will come directly from the Power Supply to the Hard Drive thru the power harness cable.

Pin 4

Standby 12V power (permanent power coming from power supply and present as long as AC cable is connected, even if computer is o ); corresponds to LED #1

Pin 6

Backlight Control pulse width modulated signal (from logic board to LED backlight board, to adjust the backlight level setting according to user setup)

Pin 11

12V Run-Mode power to logic board (coming from power supply, present as long as system is on or asleep); corresponds to LED #2


Power On Request signal (from logic board to power supply when power button is pressed)

Pin 13

Backlight Enable (signal from logic board to Backlight Controller board, to enable backlight

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I took it in at Apple store

No, I was not aware of such recall - my serial number qualifies

I started measuring the voltage because I suspect variable voltage in my house - is not bad, but, since NOTING else made sense (like I said, Apple cannot reproduce the issue - I assumed they must have devices that are stabilizing the power fluctuations) I started measuring the voltage to see if there is any connection

- Unfortunately, my device cannot measure the frequency, which could be interesting too.

- If GPU was at fault, will I be able to use an external monitor?

- Also, what baffles me the most is the fact that the blackout occurs very randomly. Sometimes I cannot keep it up for 2 seconds, sometimes image does not even come back, and other times ( like most of today and yesterday) it acts like nothing is wrong.

- Would apple still honor the recall at this point (this is a 2011 after all)?


I am willing to take it back to Apple, BUT - they don't seem to be able to get the blackout - so they will try to guess again, unless, if the voltage really has anything to do with it, I may have another clue.

That is why I put the question about the voltage regulator - I suppose there is one somewhere that is probably damaged but not completely "out" (somehow...) - and that is the reason it evades detection and even suspicion.

- my external monitor continues working when iMac monitor blacksout - doesn't even senses the problem


I tried several times when the blackout occurred - I could not see anything. Maybe is not dark enough here, I have no idea. The screen is just as black as it is when it goes in normal screen sleep

Update - I do see the image "beyond" the black screen


I see. It did not work for some. Can LED board be actually tested before replaced? My iMac was in Apple Store twice, last time for a week. It passed ALL the tests. They replaced the video card after they erased the iMac and could not find anything. It turned out WAS NOT it. It seems to react to power button sometimes too. Again, it NEVER happens at Apple Store, but home. - It just went down again.


Thanks. I'll update when /if I get it fixed.


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I've seen your exact problem! We solved it using UPS's.

It appears Apple's power supply has a narrow voltage window at the 110/120 point. If you go higher than about 120 it will wink out. For some reason 210/220 does not have as narrow a voltage window (our London site has not encountered this).

As to your basic question does the iMac have a voltage regulator. Its a more advanced switch mode power supply. It is not a fill-in or handle over voltage beyond what is expected.

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Thanks. That will explain why the voltage fluctuations didn't seem to matter because they were always over 120V, never under, but Apple store didn't see the issue. But I'm still baffled why it is doing much better with the original memory vs the upgraded memory. Maybe different power usage with upgraded memory? Or the memory itself?

Thanks for the info on switch mode power supply. I didn't know exactly how to call it or what it is exactly but I figured what kind of role it has to have.


I suspect your houses feed is off a failing transformer. Contact your service provider to have them place a monitoring device on your power-line so they can see how much load is in your line run. depending on how good they are they may replace the upstream transformer or re-align the feeds from it or better yet break the feed to a new transformer. Thats why Apple had a problem in seeing it.

As to why its more pronounced with one memory set than another it gets into the specs of the different modules. Remember the DC side is now running high as well here so some modules handle it better than others. If you can give me your modules specs I might be able to find chip specs. Take a set of clear images of the labels on the modules as well as a set of clear shots showing to top of the chips (both sides if populated).


About 3-4 weeks ago I had contacted my power supply company and they said: "we can only guarantee that your voltage is 120V plus minus 5V". He checked it and it was slightly under 125V with .3V fluctuations. But over time, as I started to check it myself, during the day I saw it at over 126V . I never saw it under 120V. I don't really know how to make them change anything. I can't really prove that something is really wrong for them to take action in this sense.

I always had the feeling, still have, that the voltage was/is( one?) of the reasons Apple didn't see it, like you mentioned it.

I hope the images go through. Yes, they are 8 GB, 4 of them to a total of 32 GB while the specs for this iMac were up to 16GB. They were listed as compatible with this iMac and Apple store didn't make a fuss about the capacity, but they were concerned about the quality of these modules, that's why they recommended to be replaced with memory "provided and installed by a reputable company like someone apple certified".

See the pic as "update"


Well you could reach out again and ask for a monitoring device Vs some guy coming by with a Volt meter which will not necessarily catch the event/s. Or, just breakdown and get a good UPS.

We use APC Back-UPS 700 & Back-UPS 1500 units. Given the Kick-A aspect of your system I strongly recommend you do get one if you don't already.

As to your memory you have the right modules here (Crucial 8 GB DDR3L - 1333 SODIMM 1.35 volts). Thats what we use in the older 2011 models. I was expecting some other off brand, so we're good here! Are all of them the same? If not lets get a snap on the others.

But, you are pushing the system to 32 GB! Are you sure you need that much? What are you doing? Image processing, Video editing, or CAD. I can see these needing gobs of RAM but do you have the needed internal/external storage which is often needed here as well? I'm wondering if you are getting close to the systems power supply load limits. That would be the only other thing I can see here. What do you have inside for storage (make & models of the drives) asd what are you also plugging in (USB, FireWire & Thunderbolt) which is sapping the power from your system.


Thanks for the advice regarding my power provider. As for UPS, I will look into those.

Now, regarding the memory: I decided to upgrade the memory when 4GB were obviously not enough anymore in Jan of last year. The problems started to appear in May this year. I went all the way to 32GB (all modules the same) because why not, not because some super rendering or video making programs. . That was the max available at very little price difference from 16GB. Internally is as they come - no other upgrades, and externally I only have an external back-up on usb WD 2TB, and occasional phone/keyboard/trackpad charging,

But, I wonder, what took it so long - about 14 months of working perfectly with the new memory - until the blackout appeared.


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Anna will be eternally grateful.
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