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Modelo A1419 / EMC 2806 / Finales de 2014 o mediados de 2015. 3.3 o 3.5 GHz Core i5 o 4.0 GHz Core i7 (ID iMac15,1); EMC 2834 finales de 2015 / 3.3 o 3.5 GHz Core i5 o 4.0 GHz Core i7 (iMac17,1) Todo con pantallas Retina 5K

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Power / Boot Issues

I have been trying to figure this out but can’t.

The Mac is being quite unpredictable when it is powered on. I have removed the screen to get to the diagnostic LEDs and to change the internal battery.

Basically, when power is plugged in LED 1 is lit up - that is the same no matter what. When the power button is pressed, some times LED 2 will flash on and off, sometimes it stays on for a few seconds and goes off and sometimes it stays on until I turn it off.

The fan will kick in at full speed when the computer stays on for more than a few seconds. There was one time after removing the on board battery when it came on and chimed, then LED 3 was on. It was very dim but it was lit. haven’t been able to get that to happen since.

I have tried removing and re seating the RAM and changed out the sticks without any change.

Any Suggestions?

Contestado! Ver respuesta Yo también tengo este problema

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Not sure I follow your reasoning on replacing the PRAM battery. The battery only holds the settings when the system has become unplugged.

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What happened at the very start. Did the system not turn on in the morning? Did you leave it running and in the morning found it had turned off? Where you in the middle of something and it went dead?

Do you have a surge suppressor or a UPS and are all of the peripherals plugged into the same suppressor or UPS?

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Hi, thanks for the reply. I was reading that the third diagnostic light off could mean there was an issue with the battery so I replaced it as a matter of course, being that it's easy to do I tried it just to get that troubleshooting step out of the way. I also noticed the old battery was reading just over 2 volts so I thought it was worth a try.

The computer has been a problem for a while. At first it wouldn't turn on in the morning without resetting the SMC. That started getting worse and I was resetting it up to 10 times before the system would boot in the end but now its not booting at all. The computer is plugged into a standard 4 way power cable without surge protection. Any help would be appreciated.

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So I currently have the dim 3rd led status. Power supply?? Thanks!

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Sadly I’m suspecting your power supply has failed. Even though you have a surge suppressor it might have failed you. I would also recommend replacing the surge suppressor with a good quality unit or better yet getting a good UPS.

For reference here’s a full breakdown on the diagnostic LED’s

  • 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 dedicated GPU are communicating. This LED will be ON when the computer is communicating properly with the dedicated GPU. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the logic board is in 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 cables from it might be installed incorrectly or need replacement.

Here’s the needed part iMac Intel 27" (Late 2012-2020) Power Supply and here’s the guide to put it in Reemplazo del fuente de alimentación de iMac Intel 27" Pantalla Retina 5K

Imagen de iMac Intel 27" (Late 2012-2020) Power Supply

Producto

iMac Intel 27" (Late 2012-2020) Power Supply

$99.99

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Thanks, is there any way to test the PSU to confirm this is the case? Are there any telltale signs to look for or testing point that I can test the voltage of? Thanks!

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As LED is not lit I don't think you have any other way without having a second test system.

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OK Thanks for your help Dan, I will explore a few options and see about replacing the PSU. Will let you know if it worked.

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I managed to borrow a PSU from a working iMac and it worked so that is definately the issue here. Thanks for your help!

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