Won't boot, black screen
I got an iMac 20" Intel duo Core of early 2008 (A1224).
It was running fine until 2 months ago : I got three times chime, white screen with the wheel running and fourth time black screen, no wheel, no chime.
With the help of Ifixit tutorials, I opened the iMac and observed the diagnostic LED's while pressing the On button :
LED 1 is on, LED's 2 and some time 3 go on and then the iMac shut itself Off and recycle.
LED 4 never goes On.
I disconnected the data display cable of the internal LCD and then I could hear the iMac doing the full boot sequence, chime and three first Led went On in sequence and stayed On.
Then I removed the internal LCD and plugged in an external Screen.
I recovered all the functions : iMac works perfectly.
Only LED 4 is off.
This would mean that Video Board is OK and that Logic board versus external display is OK. But the Logic board does not "see" the internal LCD display (Led4 off).
Yet I dismounted the Video Board and passed an air soldering gun on the video chip. Remounted the whole thing. Same result : iMac does not see internal LCD.
I tested the Power Supply voltages according to iFixit values. I got differences :
First on my power supply pins 2, 7 and 8 are on the same track as per Ifixit photos of the PSU, so they read all the time the same value, which is different than the table of Ifixit. By the way, this table should be corrected as per the photo.
Secund, Pin 5 on Standby reads 2.7 instead of 3.8V, but goes to zero in the On mode. Yet the standby mode is working properly. Is this value critical ?
Without the internal LCD, I tested adding car front light bulb between pins 1 & 2 (12V) of the PSU (55 watts) in the On mode to verify that the power supply could handle extra power. The PSU handled the extra load and Mac stayed on.
Is that enough to stop suspecting the PSU board ?
I tested the data display cable plugged to the logic board without the LCD on the other end. The iMac works fine.
I tested a different LCD internal display. Mac will not boot as with the old LCD display.
I replugged all cables of the LCD, but disconnected the Inverter from the power supply. Mac will not boot.
Tested a new inverter, Mac will not boot.
I then dismounted and cleaned the Logic Board and looked for damaged caps : all good.
Remounted the iMac : always the same thing. It will not boot on the internal LCD, but does on an external LCD.
Now, I am a bit out of ideas.
Changing PSU or Logic board is a lot of money.
That iMac machine is a wonderfull computer when working. Before buying a new one, I shall be very intrested to find the schematic of the Logic Board and its connections to the internal LCD.
Could someone help me ?
Has someone any other idea to test ?
Is this a good question?