@telepati I'd definitely start with the power supply based on the fact that your LED turns off. It's easier to diagnose and you always want to start with the easy things first. If that does not yield any results you can most certainly move on to the more complicated things.
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.
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.
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. Note: this LED is not used on systems with the integrated graphics.
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.