A2141 01700 no backlight

[adrko]

Hello

I've got A2141 2019 16 inch macbook pro touch bar, 01700.
Liquid damaged. backlight was shorted on the connector, I replaced following:
- display connectors and flex
- burned components and a connector around touchbar area
- mux chip
- display tcon board(corrosion)
Laptop boots normally, I have image but no backlight. 0V on the backlight line when laptop is powered on and booting normally, theres image but no backlight.

I assumed that the screen itself has backlight working fine because when I connect it to the external display and start recovery the laptop screen has good and even backlight, but no image.

Question is - how do I diagnose the backlight controller chip? Its very difficult to measure anything backlight related when the mainboard is in the chasis, otherwise it won't start.
So I cannot check the backlight enable pins for signal because its located on the other side of the board.

I'm trying to check chips U8400 and U9801.

Or if you can point me in the right direction if I'm completely wrong about it, that would be amazing.

 

[2informaticos]

I recommend to test with known good working LCD assembly.

 

[adrko]

I'm afraid that's out of the question. I can't really spend 600-700$ on a screen then have it lying around without any use.
What I'd like to know - is there any way I can check these two IC's if theyre damaged?
U9801 has connection to the mux chip, so if the mux got burned so could the U9801?

 

[2informaticos]

You've said to get backlight at some moment, so I doubt to be a problem with backlight circuits.

U9801 has internal firmware, may need donor board, same model (not sure however).

 

[adrko]

Yes that’s correct, but that backlight was switching on when using external display.
The way I understand this:
- even without good or properly soldered mux chip I had display on external
- mux chip has connections to the backlight controller and U9801
- I have assumed it might be one of those chips or something else that is bad

I would love to be able to go through all this myself but I’m not able to power on the board outside the chassis to have a look at the side where the chips I mentioned are located

 

[2informaticos]

The board should start automatically, when the charger is connected.
You just need to connect an USB port and LCD video cable; not easy however.
Board will not work properly (missing trackpad thermal sensors), but should serve for test.

 

[adrko]

Well I have only managed to see a proper boot then it has all connectors plugged in, but it still required some acrobatics to even try to measure anything.

I assume working board doesn’t turn on the backlight when it’s not booting properly, am I correct?

 

[2informaticos]

Backlight should appear, even only LCD and charger connected.
Possibly flexgate problem and need good LCD assembly to discard this problem.

 

[adrko]

Alright, I understand now. Thank for explaining this to me. Thank you for the the help and your time.