[SOLVED]820-4924 backlight headache

[laptopcity]

Hi ive got an 820-4924 with no backlight, board has no liquid damge, 5v on screen enable and have pic , 12v on backlight so its not boosting, its 100% working screen, at u7700 pins 17 and 18 i get 5v,, r7732 & r7731 feedback resistors measure fine also checked connection to the lcd connector and fine, changed u7700 three times just incase i had bad chips but still get no boosting, backlight fet driver resistor also measured fine and changed q7701 just out of desperation now im going round in circles and must be missing the obvious so if anyone can shed some light i would be more than greatfull

 

[aprendiz]

BKLT_EN_R present??

 

[laptopcity]

yes got 5v there

 

[aprendiz]

Diode mode on I2C_BKLT_SCL and I2C_BKLT_SDA???

 

[aprendiz]

Also try another display assambly, remember should 2015!!

 

[laptopcity]

0.527 i2c_bklt_scl , 0.549 i2c_bklt_sda

 

[laptopcity]

checked display assy with another board and alls fine

 

[laptopcity]

diode readings are with the screen connected

 

[aprendiz]

Remove display and measure again...

 

[laptopcity]

i2c_bklt_sda 1.821 , i2c_bklt_scl 0.575

 

[aprendiz]

i2c_bklt_sda is to high!! R7751 value??? this could be a bad R7751, broken trace or dead U7700...

 

[dukefawks]

i2c_bklt_sda 1.821, I would say there is a bad trace or dead backlight IC.

 

[laptopcity]

ok so yes the trace is bad , ive run a wire from pin 1 r7751 to pin 2 r7753 then up to pin 2 c8358 and test point next to it, then had to go across to pin 4 of screen connector now at each point i get the same reading of 0.454 but still no boosting on the backlight, done pram reset just incase

 

[dukefawks]

No boost is something else than no backlight. What is the voltage on backlight output pin?

 

[laptopcity]

Hi Duke im getting 12.2v pin 1 of j8300

 

[dukefawks]

That is good then there is communication with the LCD/driver chip. EDP_BKLT_PSR_EN present?

 

[laptopcity]

arh did have ealier now reading 0v

 

[laptopcity]

ok ive run a wire from r8309 which has EDP_BKLT_PSR_EN down to r7742 , r7742 is reading fine but still no backlight :-(

 

[dukefawks]

Check again for EDP_BKLT_PSR_EN at U770 to make sure. Was there corrosion around U7700? Did you do a PRAM reset? Known good LCD for 2015 model?

 

[laptopcity]

yep EDP_BKLT_PSR_EN at U770 present , as far as i can see no corrosion anywhere , did a pram reset and yes lcd works on another board