iMac doesn't boot, only 1st LED lit, disks and fans spinning

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iMac Intel 27" EMC 2429

Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor, ID iMac12,2

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Aaron @skyegoose

声誉积分: 11

发帖于: 2021年2月12日

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iMac doesn't boot, only 1st LED lit, disks and fans spinning

The iMac powers up soon as it’s plugged in, but I can shut it down and restart it with the power button. So the button’s fine.

When it’s in standby, I read 0 V on pin 4, 12 V on pin 11 and 4V on pin 13 on the logic board. On the PSU it’s 12 V on pin 4 and 4 V on pin 5.

After it has powered up, all readings on PSU and Logic Board seem to be normal, except there is still 0 V on pin 4 of the Logic Board (I don’t know if that’s normal).

BTW: Circuit continuity testing for Pin 4 on Logic Board and PSU is positive.

Any suggestions is it a PSU or a Logic Board issue?

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I would still check the power button!

Carefully inspect the wire from the logic board to the button. Do you have any damage in the wires them selves? Also check continuity between the the contacts in the connector as well as between each contact and the case.

And lastly try reversing the connection of the switch.

由 Dan 完成的 2021年2月12日

Thanks for your immediate response, Dan. The power button is definitely fully functional. I have no removed the board (with utmost care) and there's no component that looks kind of blown or faulty in any way. Ofcourse, a bricked resistor may well look fine on the outside. I#m pretty irritated to read 12 V on pin 11 and 0 V on pin 4 in standby mode. At the moment, I'm checking the connector cable for continuity.

由 Aaron 完成的 2021年2月12日

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I would still check the power button!

Carefully inspect the wire from the logic board to the button. Do you have any damage in the wires them selves? Also check continuity between the the contacts in the connector as well as between each contact and the case.

And lastly try reversing the connection of the switch. — Dan
Thanks for your immediate response, Dan. The power button is definitely fully functional. I have no removed the board (with utmost care) and there's no component that looks kind of blown or faulty in any way. Ofcourse, a bricked resistor may well look fine on the outside. I#m pretty irritated to read 12 V on pin 11 and 0 V on pin 4 in standby mode. At the moment, I'm checking the connector cable for continuity. — Aaron

Dan @danj 声誉积分: 479.4千 Self employed contract worker 35 329 971 · Answer 1 · 2021-02-12T06:20:58-07:00

On your logic board is a set of diagnostics LED’s. After shutting the system down what do they tell you?

Here's a breakdown of the onboard 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 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.

Only LED # 1 (501) is lit, and I wonder where it gets the trickle voltage from since I read 0 V at pin 4. — Aaron

iMac Intel 27" EMC 2429

iMac doesn't boot, only 1st LED lit, disks and fans spinning

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