820-4924 no backlight

[arnonabuurs]

Happy Easter fellows!

CLient came with this 820-4924, backlight didnt work, sometimes it turned on and worked for a while and then off again. After checking the board found liquid U8330 and U5510. Cleaned it and replaced the IC, and I thought all be OK, but not! I still have no backlight. I do have 49v on the pin of the connector (that looks very clean). Tried a known working display from our own office computer (2015 too) and same issue happened. Tried his display in our office computer, same...... Replaced 2 different cables on his display as I know these always fail, same! This is kinda weird. Is there any other thing beside display - display cable and U8330 that can cause issues? The area around U7700 looks very clean, and it works as its sending 49v to the connector. Maybe change the display connector? Or change U7700? ALso no shorts on pin 1 PPVOUT.

 

[JohnB8812]

If you have 49 volts with a known good screen, I would say fucked LCD connector. Replace it and see if any change.

 

[2informaticos]

"Tried his display in our office computer, same"
So his LCD assembly has a problem for sure.
If is not the cable, then LCD screen.
High backlight voltage menas open LED strips...

Do you get image, only missing backlight for sure?
Try few PRAMs and see what happens.
Using original 85W Magsafe 2 charger?
This board gets weird issues with 1-->2 adapters; especially for backlight.

 

[arnonabuurs]

I can see apple logo with a flashlight. At the same time I have 49v on the pin of the LCD connector.
Using an original 60 watt charger, will try the PRAM and update tomorrow as its getting late here thanks!

 

[JohnB8812]

Just replace the connector...

 

[2informaticos]

If video cable and LCD display are good for sure, then bad connector...

 

[arnonabuurs]

Ok here we go:

- Replaced the display connector
- Orignal 60W charger
- 49v on LCDBKLT
- Known good display

Never seen this before. Is there anything else beside LCDBKLT thats needed to turn on the backlight?
I can only image this board destroyed my good display....... so I will put that back in the original machine and see if it still works.

 

[arnonabuurs]

OK so I put our own dispay back in our own machine, all works fine. One thing we noticed. When we use the clients display in our Mac, it is working, however when we attach a USB device the backlight does not turn on while booting. Backlight fixes are usually my favourite repair, but this one has me freaked out....

 

[2informaticos]

Sometime is very dificil to note backlight presence on this machines.
This happens when a problem with ide data lines exist.
I found machines where backlight detected with broken screen, but on good one; very dificil to detect backlight presence.
But you get correct image and there is nothing else bad, apart of backlight.
49V is also too high, as 40V is the correct voltage.
Looks like open circuit for LED strips.
There are no return resistors, like in other boards; the backlight circuit is closed through ground directly.

Check if get any activity on BKLT_SCL/SDA lines when pressing backlight adjust keys.

 

[arnonabuurs]

I figured out it has to do with the OSX Mojave. Our own laptop also boots with backlight dimmed, can see the logo but no backlight, once its half way boot process it turns on and finishes boot. The issues with backlight after login in are fixed, so this one is closed . Thanks again!