820-00850 2019 Pro A1989 with display backlight issue

[sekidata]

MacBook arrived with no screen backlight and liquid damage in the backlight circuitry around U8400. Replaced all corroded components, including U8400. Now the backlight works but only when I select a very low brightness setting. It disappears when the brightness setting is raised. PPVOUT_S0_LCDBKLT is at a constant 49V. I assume there is some issue with the pulse-width modulation?

 

[2informaticos]

Did you test with known good LCD assembly?

 

[sekidata]

Yes, of course. All 3 assemblies I tested show the same behavior.

 

[2informaticos]

Remove and check all resistors out of board (ohm scale).
Also check the traces between U8400, D8410 and feedback resistors divider.

 

[sekidata]

All resistors check out correctly (I replaced almost all since they were liquid damaged). What's your guess as to what causes the backlight to disappear when brightness is increased?

 

[2informaticos]

PWM signal should be good, once you note backlight reaction; even is not the expected.

Maybe some corroded capacitors...

 

[sekidata]

After carefully looking at the liquid damaged area, I found the culprit. There was some invisible corrosion on the via for the powers supply to U8400 (PP5V_S0_BKLT_D). It was only delivering 4V instead of 5.1V. Bridged with a jumper wire and backlight is back in full strength.