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iMac won't start but power supply is working


My iMac won't start up. Led 1 is on when it is plugged in. When I press the power button, no change in diagnostic led, no fan, no display, no Harddrive.

Could it be as simple as a busted power button?

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I tested the logic board test points. I'm getting 12v for ground and pin 4, standby.

I tested the power button connection to the logic board. I'm getting 3.3v. Should it be zero if I didn't turn it on?

I'm in the process of taking everything apart to look a the actual power button.


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#1 LED will come on if there is any power going to it. This does not mean it is working.

  1. 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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Ah, thanks.

I got a replacement used power supply from China but the problem is still the same. I would have gotten it from ifixit but they're out of stock.


You have to start with a known good power supply, here's one that might work:


Yes, that's exact seller I got it from.


Was it new or used?


Used. Is there a way to check if the power supply works with my volt meter? Is there a pin voltage chart somewhere?


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professorminh sera éternellement reconnaissant.
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