820-00164 no backlight

[peterfixit]

macbook air came in with corrosion over the j8300 and surrounds as well as u7701, I replaced J8300 and u7701 and now have no backlight
history was an entire glass of milo was spilled on it
with a torch shined through the apple logo we get folder icon
PPHV_S0SW_LCDBKLT is 10.92V
SMC_LID is 3.422V

 

[dukefawks]

Did you check the feedback trace to ball A5?

 

[peterfixit]

the test point for feedback is 3.338
XW7720 is nostuff so i measured continuity between the test point and c7799 and get nothing
do I need to take off u7701 to test from pin 5?

 

[larossmann]

3.338 what?

You're testing for continuity between ball A5 of U7701 and pin 2 of D7701.

 

[peterfixit]

volts,
I took off U7701 and there is continuity ball A5 of U7701 and pin 2 of D7701.

 

[dukefawks]

OK if you say the trace is good then put back a LP8550. Then check BKL_EN and PWM. Check traces from B1 and B2 also for continuity before putting chip back.

 

[peterfixit]

ok i found a problem, traced it back to EDP_BKLT_EN i get 0V on pin 2 of r1430, there is continuity between q7707 and r1430

 

[dukefawks]

You said there was 10.92V on backlight output. This is not possible without EDP_BKLT_EN.......

 

[peterfixit]

ive just checked it again, it starts at 8v and goes down to 3.3v and then back up to 8v

 

[dukefawks]

You measuring this without an LCD connected?

 

[peterfixit]

no with, without is 0V

 

[dukefawks]

Do a PRAM reset. Then if there is an image on the LCD and SMC_LID is high, EDP_BKLT_EN should be high too. Could be a bad LCD assembly too...