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820-00165 very dim LCD

Outofwhack

Outofwhack

Member
I have a known good screen connected to the board.
On boot I am getting such a dim display I need to view it perpendicular to the screen to see the faintest image.

pp5v_s0 5v
pp3v3_s0 3.3v

pphv_s0sw_lcdbklt is approx 16v

Current sense starts around 3v and ramps down after a few seconds but with an imbalance between bkl_isen[1,2,3] all about 1.3volts and bkl_isen[4,5,6] all about 0.6volts.
bkl_pwm is 0volts
Ive checked that resistor for value and the capacitor for shorts, seems Ok.

Ive done a PRAM reset.
I have replaced U7701 without effect.
 
2informaticos

2informaticos

Administrator
Staff member
"bkl_pwm is 0volts"
Remove R7704 and run a wire from R7715/31 to C7704.
If you get backlight, do PRAM reset 2-3 times and rebuild original circuit; test again the backlight then.
 
Outofwhack

Outofwhack

Member
Backlight came on bright after jumping.
I replaced resistor and cap but I am unable to verify continuity of track to cpu as there is no accessible test point at cpu end.
bkl_pwm remains at 0v and image very dim = same as before.

I have to admit also I am currently not getting a light on the charger lead although I get a chime and its starts to boot.
 
Outofwhack

Outofwhack

Member
I did see a .5v DM measurement on the track to the cpu whilst R7704 was removed indicated an intact ESD diode and thus ok track.
Ive tried a different charger and Im getting a green light now.

So Im left wondering what else can cause bklt_pwm to remain at 0v.
 
Last edited:
Outofwhack

Outofwhack

Member
No backlight after disconnection of LIO cable.
smc_lid is 0.5v
I removed the pull up resistor R5171 and found the pin of the SMC chip for smc_lid is 18ohms to ground. 0.006v DM

Im guessing this means I need to replace the SMC? Am I correct?

Things are beginning to make sense as this fault was caused by an owner plug and playing LIO boards between a 2017 and early 2015 MacBook Air. Im suspecting they are not compatible.
 
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