My computer won’t turn on

Well, I don’t think you have a power supply or power button issue at all! Your symptoms sound like you had a SMC latch condition (you needed to unplug the AC power cable to clear it). This happens when SMC encounters a major failure within the logic board.

In this case I’m suspecting your GPU board has failed which is a common occurrence in this series. This happens as people try running more modern apps which over stresses the limited graphics this series has. Things like heavy photo/video editing and heavy graphics gaming.

The best way to think this you bought a 1955 T-Bird, you put it in the garage and pulled it out of the garage after getting everything running after being put away back in 1955.

Running on todays roads will slowly kill the car as it was not designed to run at 75 MPH or higher for long periods. The fastest back in 1955 would have been at best 60 MPH on the few highways back then. Many roads were still single lane you could have often been stuck behind slower traffic.

So you’ll need to decide how far you want to go with this system. I still have one which I love and I’ve replaced the GPU board three times! As I was pressing it hard with photo editing, now I use it for writing business letters and manage my image catalog making sure not to over do it ;-}

OK, so what’s involved here?

The first thing I would do is check the onboard diagnostic LED’s on your logic board to be sure.

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.

You should see LED 1 lit and the rest will be dark, as the GPU has shorted out.

You’ll need to take the logic board fully out to get to the GPU board. But you’ll need to find a replacement or you’ll need to see if you can fix it after cooking it in an oven.