Imac 2011 A1311 power leakage/grounding problem.

[madvinegar]

Hi guys. I would like your kind assistance and ideas of what could be wrong with an iMac 2011 I bought second hand.
It is an A1311 21,5" mid 2011 model.

The ad said that the iMac had a problem to boot. Sure enough, when I got it and plugged it in, pressed the power button and nothing happened.
So I started to dissasemble it and see if I could find what was wrong. I am sure that there is something wrong with the power delivery, as I managed to get it working but only after I took out everything and removed the CMOS battery. It is like there is a "grounding" problem and I need to remove any leftover electrical current first. For starters I changed the power supply (with a 2nd hand one), and then I also changed the plug part (the part that the power cable connects at the back of the imac). I also tried with a new GPU just in case.

As said, I can now get it to power up. Sometimes it doesn't, and I need to unlpug it from the power outlet, press the power button for 10seconeds to discharge any leftover electrical current, plug it back in, and then it starts.

But even when it starts, I need to set the brightness to low, otherwise the screen flickers. Meaning that when it asks for more power, there is somekind of leakage, and it cannot deliver the correct amount of power to the screen. (please see the attached video in which I move the brightness slider from low to high and vice-versa).

Has someone had the same problem? Is it a problem with the motherboard? If yes, do you know which capacitor? (I have a spare motherboard and I could source a capacitor from there).

Any assistance and/or ideas, will be highly appreciated.

Many thanks.

 

[USB3foriMac]

This is not the power supply but the backlight power. It's a separate circuit board, next to the power supply. It could be as simple as a loose wire which could have been impacted by changing the power supply, or it could be the backlight circuitry. Maybe some experimenting can help you narrow down.

 

[madvinegar]

This is not the power supply but the backlight power. It's a separate circuit board, next to the power supply. It could be as simple as a loose wire which could have been impacted by changing the power supply, or it could be the backlight circuitry. Maybe some experimenting can help you narrow down.

Thanks for your reply. I have ordered from ebay a new (used) backlight board and I will replace mine to see if that makes any difference.
Otherwise it may be the psu cable...? (The one the provides power to the mobo and the backlight board.)?
Since the imac is booting up (as you see in my video) can I at least rule out the mobo?

 

[USB3foriMac]

If it was a power issue, the Mobo would reboot. It can't tolerate power fluctuations at all. So the Mobo is ok, and the power supply, too.
If you heard the Chime at boot but screen was dark, it's a clear sign of backlight (and less likely the screen). If screen just goes dark, you can setup VNC access / sharing of your imac, then connect through another machine. This should not show any issue, hence confirms backlight problem.

 

[madvinegar]

Thanks my friend. I will let you know how it goes when I replace the backlight board.