820-3437-B no backlight/has image on screen

[galvestonmac]

I have another MacBook Air 13” Mid-2013/Early-2014 820-3437-B that I'm fighting with no backlight. Any help would be appreciated. I seemed to be getting quite a few of these lately.
No backlight on screen. Flashing question mark can be seen when illuminated from back of panel. All measurements are with known good LCD panel and taken when connected.

U7701
Pin 3=0v Diode mode measurement .579ohms
Pin 4=1.46v
Pin 5=0v
Pin 6=0v
Pin 16=5.13v
Pin 11=5.13v
Pin 14=3.32v


L7701 Pin 1=0v Pin 2=0v
D7701 Pin 1=0v Pin 2=0v
C7704 Pin 1=1.46v
R7701 Pin 1=1.46v
R7714 Pin 1=0v
R7715 Pin 1=0v
R7731 Pin 1=0v
C7711 Pin 1=3.32v
R7716 Pin 1=0v
C7710 Pin 1=5.13v
C7714 Pin 1=5.13v
R7714 Pin 1=0v
R7755 Pin 1=0v

Q7706 Pin 4=8.59v Pin 3=6.61v Pin 1,2,5,6=.34v Diode measurement .503 Ohms
R7788 Pin 1=8.59v Pin 2=6.61v
C7782 Pin 1=8.59v Pin 2=6.61v
R7789 Pin 1=6.61v Pin 2=5.91v

Q7707 Pin 3=5.91v Pin 4=0v Pin 5=3.31v Pin 6=0v Pin 2=3.31v


XW7720 is not on board but is show in schematic

 

[aprendiz]

Bad Q7707... Pins 2 and 5 have propper voltaje so both mosfets should be colse and you should have 0V on pin 3....

 

[galvestonmac]

Thanks! I replaced Q7707 and now I have some different readings:

U7701
*After changing Q7707 Pin 3= .97v

L7701 *After changing Q7707 8.53v both pins
D7701 *After changing Q7707 8.53v both pins

Q7706
after Q7707 change. Pin 1,2,5,6=8.58v Pin 4=8.58v Pin 3=2.62v

Q7707
Changed out Q7707 Pin 3=0v

Further down the rabbit hole! Ugh!

 

[aprendiz]

so it is not boosting now, check feedback trace, voltaje on BKL_EN??

 

[galvestonmac]

ok will do. thanks!

 

[galvestonmac]

Looks like I have a short on pin 5 PPVOUT_SW_LCDBKLT_FB of U7701 as it leaves and goes to D7701 in one of the layers of the board. Should I try to dig out the connection from under pin 5 of U7701 and then run a wire from there to D7701? There's a componet labeled XW7720 on the schematic but it's not on the board so I can't jump it from there. Ideas?

 

[dukefawks]

A short on PPVOUT_SW_LCDBKLT_FB would blow the fuse unless the trace from D7701 to ball A5 is also corroded.

 

[galvestonmac]

I have a short to ground on both LCDBKLT_BOOST & PPVOUT_SW_LCDBKLT_FB. I can find a place to make a jumper for BOOST but there's nowhere that I can see to make a jumper connection to the LP8550 for the FB. There's a TP shown on the Boardview but on the logic board it's not there. Is it worth it to try to get a wire to A5 on the LP8550 or should this go to the scrap heap? ugh!

 

[dukefawks]

A5 ball you normally run a fine wire under the LP8550 if the pad is totally gone, no other way. You can run a small piece of fine wire under it and then solder something larger to that and run that to the other side of the board.