[SOLVED]820-3437 Air 13 inch no backlight

[arnonabuurs]

Hi I have an Air 820-3437 with no backlight computer is starting up though. R7715 and R7731 had a bit of corrosion on them so i replaced those. Now I am lacking Voltage on R7731 pin 1 -> PPVIN_S0SW_LCDBKLT. Where does this come from, is this derived from PPVIN_S0SW_LCDBKLT_FET Q7706? All looks good, R7788 and R7789 also OK. What voltage should I measure at EDP_BKLT_EN?

Thanks

Edit: some more voltages measured:

nothing on D7701
8.6v on Q7706 pin 3
Diode Mode on D7701 Pin 1 = 0.262v Pin 2 = 0.396
Nothing at R7704 BKL_PWM
R7731 Pin 2 = nothing (well, 25mv)
resistance to GND on BKL_PWM = 355v

 

[dukefawks]

Did you check with a flashlight if you even have an image?

 

[arnonabuurs]

Yes obviously thats the first thing I do if no backlight. As mentioned the computer is starting up, because I get a login screen.

 

[aprendiz]

EDP_BKLT_EN should be 3.3V, diode mode measure on this line?? Also if you had corrosion on on R7731-R7715 also could be corrosion under U7701 or even dead.... have you tried other lcd assembly????

 

[dukefawks]

There is no BKL enable and PWM. I'd say SMC_LID is low due to bad IO board or cable.

 

[arnonabuurs]

I moved the board over to a fully working Air, but still no backlight. Will replace the U7701 for a new one although my guess is that if there is no 3.3v on BKLT_EN it wont start anyway?

 

[dukefawks]

Don't touch U7701, diagnose the problem! Is there 3V on EDP_BKLT_EN ?
Also diagnose SMC_LID, moving to a working machine does not prove anything.

 

[arnonabuurs]

There is no 3.3v on edp_bklt_en

 

[dukefawks]

For the 3rd time, SMC_LID??

 

[arnonabuurs]

Sorry Duke I already ripped the driver off, since the only liquid was beside that it made sense. After replacing it U7701 it works again, so case closed thanks for the help guys appreciated!