[SOLVED]820-2915-A no backlight

[WeLoveApple]

No liquid damage.
SMC_LID is 3.4V
LCD_BKLT_PWM is 0V
LCD_BKLT_EN is 0V
PPBUS_SW_LCDBKLT_PWR is 0V
Tried with another LCD too...

 

[larossmann]

Do you have an image?

 

[WeLoveApple]

Yes. Tried with flashlight from back.

 

[larossmann]

Three possibilities

1) SMC_LID is less than 3.4v so the machine thinks it is closed, and not turning on the backlight.
2) Q9707 is bad and shorting it to ground, which never happens
3) U9600 is toast.

 

[dukefawks]

I bet the measurement in the first post were done without an LCD connected. Enable and PWM being low with an image present just doesn't happen.

 

[larossmann]

ok, #4

you aren't plugging in a screen and expecting backlight.

I am giving you the credit that you actually are measuring with a screen plugged in. Please tell me you are.

 

[WeLoveApple]

LCD_BKLT_EN 3.32V
PPBUS_SW_LCDBKLT_PWR 12.58v
LCD_BKLT_PWM is 0V
SMC_LID is 3.4V

with lcd :I

 

[dukefawks]

No PWM . I would change the LP8550 and pray it comes back.

 

[larossmann]

Either resetting PRAM will fix or U9600 is fried. Quick fix is sending PP3V3_S0 directly to LCD_BKLT_PWM, real fix is replacing U9600. Obviously, no brightness control with quick fix.

 

[dukefawks]

Louis you are forgetting that the LP8550 could pull low PWM too. Also without PWM signal I would expect at least a little boost. My first guess is still the 8550.

 

[WeLoveApple]

I changed LP8550. (not working the backlight)
PP3V3_S0 directly to LCD_BKLT_PWM(working)
Thank you! Maybe the smc.

 

[dukefawks]

Bad MUX probably. Working now so fine.