Imac 27 mid 2011 backlight problem

[veiu0107]

Hello,
I have bought myself a broken iMac which i tryed to fix. It had fryed psu and led inverter board. I have revived the psu and replaced the led inverter board. I couldn't get the picture so I have decided prematurely that the display itself is defective. Now when i bought another display and that has also failed to post a picture i jhave started to search for another possible problem and i saw that eaven lvds conector was a little bit damaged. I have replaced the lvds conector and cable also and now i have picture but there is no led backlight on the screen. My old screen and the new screen are acting the same. 4 leds light up on the motherboard. I eaven tested the gtx 680m (with mac bios) instead of hd 6970m but the result is the same.

My question is now, can anyone tell me some main diagnostic for led backlight? I am complete noob when it comes to iMac. The things that i find out are:
- led inverter board is powered from psu (12v and 3.3v on 6pin connector)

- led inverter board has output to the screen (12v output)

- dp power conector also has 12v

- lvds connector is questionable but since it is giving picture i thought that everything is ok with it

 

[Nguyen Duc Hieu]

All I know is that the 3.3V line of the 6-pin connector is supposed to be PWM signal.
You can purchase a PWM module and disconnect that 3.3V line from the logic board, control the PWM signal manually.
Another option is to purchase the replacement LCD backlight board to see whether the existing one is defective or not.

More detailed explanation and how-to guideline is in this amazing post of wlagarde

2011 iMac Graphics Card Upgrade

Backlight Control Background: The 2011 iMac display backlight system uses pulse-width-modulation (PWM) to control backlight intensity. A PWM control signal is generated within the logic board-video card circuit and routed to the PWM input on the backlight board. The PWM frequency is 13KHz and...

forums.macrumors.com

 

[veiu0107]

All I know is that the 3.3V line of the 6-pin connector is supposed to be PWM signal.
You can purchase a PWM module and disconnect that 3.3V line from the logic board, control the PWM signal manually.
Another option is to purchase the replacement LCD backlight board to see whether the existing one is defective or not.

More detailed explanation and how-to guideline is in this amazing post of wlagarde

2011 iMac Graphics Card Upgrade

Backlight Control Background: The 2011 iMac display backlight system uses pulse-width-modulation (PWM) to control backlight intensity. A PWM control signal is generated within the logic board-video card circuit and routed to the PWM input on the backlight board. The PWM frequency is 13KHz and...

forums.macrumors.com

Thank you for the info. Is it possible that the main psu is maybe deffective? Or complete backlight is going threw led inverter?

 

[Nguyen Duc Hieu]

Thank you for the info. Is it possible that the main psu is maybe deffective? Or complete backlight is going threw led inverter?

You have already fixed the PSU and the LED inverter board.
So I guess if your fixed PSU could power the whole logicboard, then there is no problem with it.
Now the LED inverter board need signal to power on from the logicboard, then PWM signal to control brightness. Check these 2 lines, you may find your issue.