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Model A1311 / Mid 2011 / 2.5 & 2.7 GHz Core i5 or 2.8 GHz Core i7 Processor

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iMac ne démare plus - iMac does not turn on anymore

Suite à la question iMac ne s’allume plus

Bonjour pour faire suite à ce tutoriel, j'ai quelques questions avant d'investir dans du matériel.

J'ais suivi la marche à suivre décrite.


Enlever la câble secteur et appuyer20 secondes.

Allumer le Imac et tenir le power on 10 seconde (pas de réactions des ventilateurs différentes de celle décrite plus bas)

Contrôler le voltage sur l'alimentations 12V OK 5V Ok.

Led N°1 allumée seul à la mise en marche.

Symptôme les ventilateurs tournent deux secondes et s'arrêtent et recommence ce cycle.

Pas moyen d'arrêter la machine sans débrancher le câble secteur.

Précision Imac A311 EMC2428

Alors Carte mère ou alimentation ???

merci de votre aide

iMac does not turn on anymore

Hello, I followed this tutorial, I have some questions before investing in the parts.

I followed the procedure described:

  • Remove the mains cable and press 20 seconds.
  • Turn on the iMac and hold the power on 10 seconds (no fan reactions different from the one described below)
  • Checked the voltage on the 12V OK 5V connections on power supply.
  • LED # 1 lit only when switched on.
  • Symptom the fans turn for two seconds and stop and start again this cycle.
  • No way to stop the machine without disconnecting the power cable.

iMac A311 EMC2428

So is logic board or power supply ???

Thank you for your help

Mise à jour (10/09/2018)

Motherboard changed, Imac OK. I just would not want to buy a power supply that was OK.

Thanks for your help.

Answered! View the answer I have this problem too

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Replace the power supply first. Here’s how:

iMac Intel 21.5" EMC 2428 Power Supply Replacement

  • LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.
  • LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.
  • LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.
  • LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

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Isn’t replacing the power supply rather dangerous? I heard that the iMac’s power supply is rather unprotected, or is that just an issue on the iMac Pro


Aaron, unplug the machine first. The are no large capacitors on the board. I have never had a hit from working on an iMac power supply. The old CRT iMacs did carry a charge in the tube. Of course I keep my grubby fingers off the components when handling.


Okay, because on the iMac Pro there are large capacitors that hold a charge for days up to years after being used and can be deadly if touched, so good to know


@ajcooke01 Can you tell me where you got this idea? iMac Pro Teardown


I watch Linus Tech Tips from time to time and saw this video. Jump to 10:56 in this video. I also find it ironic that iFixit sponsors this video.


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I went mad with the same problem for days and tried everything I saw on forums but wouldn’t work with me. One day after many tries I plugged the external sound card onto the computer and switched it on, the light on the sound card went green which meant it was taking the power from the computer, I pressed the start button on the computer and it worked immediately. I wonder if it would work with other peripherals which take the power supply from the computer. The only “problem” is that I have to start the computer with the external sound card plugged in but after the computer starts I can disconnect it if I am not using it. My imac is a late 2009 so I won’t care to have it repaired when I can start it this way.

I hope it will work with you.

Cheers from Spain

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guygindroz will be eternally grateful.
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