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820-01958 board with keyboard backlight issue

S

sekidata

Member
This board from an Air A2179 arrived with liquid damage and PPVOUT_S0_LCDBKLT shorted to ground. After replacing several corroded capacitors, the short was removed, and the machine started up with the LCD backlight fully functional. Initially, the keyboard backlight appeared functional as well, but after a few days, it started turning off on its own, never staying on for more than a couple of seconds. There was corrosion in the area around U6500, so I replaced it and several resistors, capacitors, and inductors.
Now the OS fails to recognize the keyboard backlight at all: in System Settings, the Keyboard preferences do not show the option "Adjust keyboard brightness in low light" and no backlight comes on.
I've tried NVRAM reset, SMC reset, and a different topcase, but no change. How does the system detect the presence/absence of the keyboard backlight?
 
richilg1

richilg1

New member
Hi, I'm sure you've already considered this, but is the screen original or a tester? Every time I've had this keyboard backlight issue, it's the screen being from an incorrect revision of the 2179 (or 1932). KB backlight shows only prior to login (first boot at lock screen), but goes away after logging in, and all KB lighting settings disappear in preferences. If it's the original screen, inspect logic board for LCD assembly. Camera connection being corroded/damaged can cause this, since ambient sensor is part of that connection.
 
2informaticos

2informaticos

Administrator
Staff member
As first step, check screen backlight reaction directing a flashlight to ALS.

Inspect U6550 and adjacent components and traces/solders for corrosion.
 
S

sekidata

Member
Correct about my tester screen: it was either the wrong revision or malfunctioning. Connected a known good 2019 display and the keyboard brightness option is back in Settings. However, still no backlight is produced. The output (PPVOUT_S0_KBDBKLT_R) starts at 5.1V, then briefly jumps to 29V during startup, then settles around 16V. Not the 40V required to run the backlight. I replaced U6550 with no change.
 
richilg1

richilg1

New member
Have you tried a new top case/keyboard assembly since finding known good (and correct version) LCD assembly? Also, when you see KBDBKLT voltage jump, does the keyboard show any brief sign of illumination? I had a 1932 with this behavior, turned out the keyboard itself had corrosion on ICs behind keyboard lighting diffuser.
 
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