[SOLVED]820-2915 no Backlight

[Fizzl92]

Hello,
this 820-2915-B has no Backlight. No waterdamage or other visible damage on the board. I measured around and BLK_EN is only 3.0V. I chekhed the voltage divider (both resistors are fine) and came to PPBUS_G3H. It is only 12.22V. I can clearly see it working on the screen and i can use it with an external monitor. Everything else works fine. I tried a different screen and still no light. I already exchanged LP8550 and R9731 & R9715 to be sure but still the same measurements. Can this be a SMC problem if PPBUS_G3H is stuck at 12.22V even if everything else works?

 

[dukefawks]

3V on enable is fine. 12.3V on PPBUS would at least indicate a problem with SMBUS to battery but should not affect backlight.
Did you do a PRAM reset? BKL_PWM present?

 

[Fizzl92]

If i unplug the battery PPBUS_G3H comes back up to 12.6V. It runs from Battery and charges it. When booted up into guest user BKL_PWM is 0V. R9704 is <50ohm and diode mode reading of BKL_PWM is 0.43V

 

[Fizzl92]

I cant believe it. The PRAM reset did the trick. I thought i already tried that... Maybe the LP8550 was bad too.... Any idea what could cause this? Some wrong settings maybe?

 

[dukefawks]

Dunno, sometimes you need to do PRAM reset and then totally power off and on again.