no video, external monitor works if main screen unplugged

With the display removed locate the onboard diagnostic LED’s located at the bottom left edge near the RAM bay. There is a piece of black tape covering it.

Carefully re-connect the display making sure its not shorting anything out. Re-connect the power and turn on the system what do the LED’s tell you? Now disconnect the internal display and plug in your external what does the LED’s tell you?

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with the display plugged in all 4 are on, with it unplugged only 3 light up

craig price さんによる2019年7月28日

Which ones? Each has a number

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.

Dan さんによる2019年7月28日

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iMac Intel 27インチ EMC 2390

no video, external monitor works if main screen unplugged

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