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iMac 27. Led 1 is on, but no power on Logic board


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My iMac 27 (late 2009) died after a power shortcut in the house. I replaced the power supply, but still no result. I Can see the green light of one of the diagnostic leds (i assume it is led 1), but if I measure the power on pin 4 on the Logic board there is no power. I dont get this? Can somebody please help me?

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this is from a 2008 it may be correct??  1 is near a heat sync and 4 is near the blue tooth/wifi card

 

LED #1

Indicates that the trickle voltage from the power supply has been detected by the main
logic board. This LED will remain ON whenever the iMac is connected to a working AC power source. 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 source 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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Hi Lou.

 

I kmow the meaning of the leds. The weird ting here is that, when led 1 is on, there should be power on pin 4 on the Logic board (according to Apples service manual), but there is no power on pin 4. Normally i would think the power supply was the problem, but i have already put in a new power supply.

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I know if you buy power supplies from DVwarehouse they will work with you. I know when I had to get a replacement LCD screen for an iMac the numbers were not the exactly the same but through an email we were able to get the right part. I haven't replaced an iMac power supply as of yet. Possible the power supply could be bad?  I am going to send the link of this forum and see if one of my friend can give some advice.  I will get back to you one way or another.

Lou Cioccio

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This is from an Apple Certified repair person:

*****************************************

If you have a known good supply, then that either leaves a faulty power switch or the main board. Do make sure the power connection from power supply to main logic board is fully seated.

If you can find where the switch plugs into the board, with the unit disconnected from power, check the switch with an ohm meter. If that checks out then you have a logic board problem.

Something else you can try, with the cord unplugged, push and hold the power button for a few seconds. While still holding the power button down, plug in the power then let go the power button. Wait a sec and try powering on again. If nothing happens, then you have to start pricing logic board.

Power isn't either engaging because the power supply may still be bad, the switch is faulty, the logic board SMC is corrupted or the main board is just defective.

*************************************

Lou Cioccio

 

Did you check your home owners insurance sometimes it may be covered???

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Finally. I got it fixed. I had tried the SMC-fix with the powerbutton before but without luck.

I tried it again, but still no reaction. I kept the power connected, and Then i dismantled the plug between the powersupply and the Logic-board - to measure if there was power coming from the powersupply. This time there was power (couldn't measure anything the day before), then i connected the plug BUT did it, with the iMac connected to power - and got a clicksound. And after that i could turn on the computer.

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