MacBook A1990 board 820-01041 - backlight not working

[Vogelek23]

Hi there, I have some 15" MacBook A1990 on my bench. Backlight works only when battery is discharged below 1% (screen shows flat battery). When battery is charged to the point that the computer starts up (above 1%) there is no backlight, however, the Apple logo and login screen appear when I shine the LCD panel with some external light.

What's been checked/replaced/measured (and did not do the trick):
- checked MLB inside a different chassis (known working) to exclude all peripherials,
- reballed GMUX (U9850), GMUX expander (U9801) and T2 chip - after some suggestions found on the internet,
- replaced screen with a known working one (genuine),
- replaced LVDS flex,
- replaced backlight controller (U8400),
- checked both sides lid sensors; they work correctly - when the lid is closed there is 0V on LID_OPEN_LEFT, LID_OPEN_RIGHT, IPD_LID_OPEN points, when lid is open 1.8V is present on all 3 points,
- measured EDP_BKLT_EN (3.3V present in both cases), PPVOUT_S0_LCDBKLT (55V present in both cases),
- for one moment I've got the correct backlight on login screen - I've checked both side lid sensors using an external magnets, both work correctly.

After computer restarting, the problem came back. I stuck on this now and have no idea what to do next.

 

[2informaticos]

First of all, welcome to the forum!

Did you try DFU revive?
Even better restore and reinstall macOS, if no important data on SSD.

 

[Vogelek23]

It seems I cannot put this machine into a DFU Eevery time I try (I do exactly as explained here: https://help.apple.com/configurator/mac/2.7.1/#/apd0020c3dc2) it constantly shows an Apple logo (of course with no backlight) and goes to the login screen shortly. Tried to connect it to the other machine (A1706) via TBT-to-TBT (USB-C) original Apple cable which came with a genuine charger, but Apple Configurator 2 does not even detect that faulty machine, saying that it "cannot use Thunderbolt accesory". Is there any known issue with AC2 and Big Sur regarding that? Should I try with any lower OS version?

EDIT (because the post above has been edited in the meantime): can I use an external display (after disconnecting an internal LCD panel) in order to installing the fresh OS? Asking because it will be a big pain to do so on a non-backlighted LCD... Do you mean a full restore via Recovery rather than just cloning the fresh OS from another drive? Data is not important, customer has given a go-ahead for data wipe.

EDIT 2: I now see under requirements: "Verify that (...) you have the latest versions of macOS High Sierra" - it seems I cannot do a T2 firmware restore under Big Sur then...

 

[2informaticos]

We have this recent thread as reference for your problem:

820-01814 No Backlight - 55v

I have an 820-01814 with no backlight. The board is very clean and has no sign of liquid or anything else that is obvious. There is 55v on PPVOUT_S0_LCDBKLT - I understand that means it's not detecting a load/LED strings from the LCD. I have changed the LCD cable with a known LCD cable, a...

boards.rossmanngroup.com

About DFU mode, check this tutorial:

How to Restore BridgeOS on a T2 Mac + Boot a Mac to DFU Mode

macOS Update brick your Mac? This article will show you how boot to DFU Mode and restore BridgeOS on a T2 MacBook Pro, Air, iMac Pro, Mac Mini & Mac Pro!

mrmacintosh.com

 

[Vogelek23]

Many thanks for the links, will check and come back with a result as soon as possible.

 

[2informaticos]

You're welcome, thank you too.
I always found good things on your forum!

 

[Vogelek23]

Finally worked it out!

Made a DFU revive and DFU full restore, which did not change the situation, could not get any picture on the external screen (after a while I found out that the unit is locked to boot to an external drive by default and therefore, it cannot give the picture to the external screen before starting up the GPU drivers).

Having no further idea, I started to watch the MLB under a microscope inch by inch and eventually found two problems - one of them was broken capacitor (pads were ripped off as well so I'm assuming someone was here before) and the other was that the PMU had some strange soldering marks (like some kind of flux visible on the edges of an IC). Because the capacitor was only a small power filter, I paid more attention to the PMU. Removed it and found THIS underneath (see attached photo). After cleaning the dirt and reballing the PMU chip, backlight comes up correctly.

The question is: how the heck is all of that tied together if the dirt under PMU can cause such weird backlight problem???

Anyway, many thanks @2informaticos for your prompt advice and all valuable tips