820-00165 Backlight Again

[iambehr]

Hello,

Feeling stumped on this one. No backlight.
Fuse at F7700 OK. U7701 backlight driver was shorted to ground.
No voltage getting past Q7706 at all. Voltage at pin 4 of Q7706 is 9.83 volts. Pin 3 is at 9.55 volts.
Resistance at R7788 is 300K. C7782 is not shorted open. Pins 1,2,5,6 are at 0 volts.

I replaced the backlight driver IC U7701. Figured that Q7706 was bad and replaced, along with R7788 and C7782.
Removed R7789 temporarily to isolate the MOSFET and still 0 volts on output.

I am stumped. Voltage on pin 3 should be much lower than pin 4 for the gate to function I believe, but I cant seem to find what I am missing.

Any ideas?

Thanks,

Stefan

 

[aprendiz]

Do you see any image on display????
Voltage on pin 2 and 5 from Q7707???

 

[iambehr]

Oh, right. Display is working.
Q7707:
Pin 2 - 3.7 volts
pin 5 - 0 volts

 

[aprendiz]

Diode mode on pin 5.....

 

[iambehr]

Well, shoot. Pin # 5 EDP_BKLT_EN is shorted to ground.
Removed Q7707 and retested to be sure.

Pretty sure this is done then.

Cleaning in ultrasonic just in case I am extra lucky, but I doubt it.

Thanks,

Stefan

 

[dukefawks]

If it is really shorted you can disconnect the line and tie pin 5 to LCD_EN