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Hello, I have an iMac A1311 EMC 2389 helping a friend fix it. Suddenly it wont power ON. Checked on AC power the cpu fan runs continuously but no chine is heard when power button is pressed. If I hold power button for long the fan eventually turns off. Also a subsequent press of the power button will then prompt the fan to run once again but no chime. I have taken off the LCD and inspected LEDs Only LED 1 keeps steady on with AC power supply the rest keep OFF when power button is pressed. The PSU gets input 240v Voltage psu pin 1 & 4 is 12v voltage psu pin 1 & 12 is 12v voltage psu pin 1 & 11 is 12 v when power button pressed. Any directions from here or help with schematics and board viewer will be highly appreciated Thanks and Regards!
So it sounds like your power supply needs replacing. Even though you are measuring the 12 volt lines you don’t appear to be measuring the 5 volt lines. Hopefully the logo board its self is OK.
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 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. Make sure a good surge suppressor is used or better yet a UPS. Often times power supplies get killed by power line surges or low/high voltage conditions as well as lightning strikes.
