Chosen Solution
If I turn on the iMac, all four leds are on, but I hear no boot chime. The screen flashes a single time white and that’s it. I have connected the iMac with an external display, but nothing appears on the external screen. I know for sure that the speakers work, because the iMac beeps if I remove the ram. Can somebody explain me what’s wrong with the iMac or does anybody have a suggestion that might fix my iMac?
Good! So we know the logic board is good then. So the focus needs to be within the full backlight logic path, from the inverter to the display, and then onto the LED’s within the display assembly its self. I would start at the display end making sure the LED strip is working correctly. Test the resistance of the LED backlight power cable as well as the vertical sync cable ends disconnected from the LED Driver Board. I would also check the voltages with the cables connected to the LED Driver Board for both.
