[SOLVED]820-3437 backlight enable missing.

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jgraffis

New member
working on a 820-3437 macbook air with no backlight(why does it seem i suck at this.) so what I've found is that i have no backlight enable coming from q7707.

pin 1 has 0.0v
pin 2 has 3.33v
pin 4 has 0.0v
pin 5 has 3.33v
pin 6 has 0.0v

i chased the backlight reset to plt_reset_L coming from the pch. kinda lost on possibilities.
 

dukefawks

Administrator
Pin 2 and 5 are both high, what is the issue? What is the backlight output on D7701? Is there an image on the LCD for sure?
 

jgraffis

New member
The issue is no backlight on screen. there is video on the lcd, i put a light on the back of the screen and ran ASD on it. the voltage on d7701 pin 1 is 8.06v and 8.04 v on pin2.
 

dukefawks

Administrator
So the enable is not missing or you would have no voltage on D7701. Also 8.06V would be on battery, on AC it should be 8.4V.
Was this liquid damaged?
 

jgraffis

New member
yes i did test on battery. i have no idea if it had water damage before i got to it. i did take it out of the case didn't see water damage and water indicators are still white. i am missing the voltage to pin 3 which is lcdbklt_en_l still. but no backlight to screen i did test with a known working screen also.
 

larossmann

Administrator
Staff member
something makes no sense here.

What voltage is on BKL_PWM at R7704 pin 2?
What do you get on R7731 pin 2 for BKL_EN?
d U7701, do you have continuity between ball A5(PPVOUT_SW_LCDBKLT_FB) and pin 2 of D7701?
 

jgraffis

New member
yes this is a strange one.

pin 2 on R7704 is 0.13v BLK_PWM
pin 2 on r7731 is 2.84v BLK_EN
yes continuity between ball 5 to pin 2 of d7701
 

jgraffis

New member
172kh resistance to ground at r7704. it seems like the Q7707 is getting what it needs to send the LCDBKLT_EN_L signal but just isn't sending it out. gonna try to change it and see what happens.
 
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dukefawks

Administrator
You really do not understand how this works. Q7707 is fine, there is 8.4V on D7701. Are you measuring without an LCD again?
BKL_PWM is missing, this is due to backlight being set all the way down in OS or a bad PCH or a bad LP8550.
 

larossmann

Administrator
Staff member
Let's go back.

Q7706 allows PPBUS_G3H to pass through to the system. Q7706 is a P channel so it will open when voltage on the gate is lower than the source.

There is a voltage divider in place that allows the voltage on the gate of Q7706 to be lower than the source. It only works when Q7707 is allowing that voltage divider a path to ground.

If you have voltage passing through Q7706, stop looking at Q7707. That part works fine.
 

jgraffis

New member
sorry guys some times i get a little ahead of myself. All measurements have been with lcd connected. i have done a PRAM reset. I've even booted it into ASD os and EFI with no failures. Don't know the exact history of this machine didn't see water damage or any signs of an issue.

I'm hoping its not a bad PCH, when i search on open board viewer for lp8550 nothing come back. Im guessing this one is gonna go to the " look at this another time" stack.

thanks for all the help.
 

SMMRepair

Member
LP8550 is the chip model identifier. On boardview and schematic, it's U7701. If you replace U7701 and still have no backlight, you're either missing something obvious (i.e. bad fuse) or it's the PCH. Be done with this one.
 
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