820-00281 no power

[Kyle93]

Caps lock light comes on along with external video.

There was minor liquid damage on LCD connector. PP5V_SOSW_LCD was 0v, after replacing U9850 PP5V_SOSW_LCD now 5v but still no backlight

 

[Kyle93]

EDP_BKLT_EN 0.31v should U9800 be changed???

 

[2informaticos]

U9800 should be good, if you get external video.
You've said to have 12.5V at backlight output before.
That means EDP_BKLT_EN was high then.

DP_INT_HPD goes high now?

 

[Kyle93]

DP_INT_HPD 3.2v

 

[2informaticos]

0V at backlight output now?

 

[Kyle93]

PPVOUT_SO_LCDBKLT still 12.3V

 

[2informaticos]

Really strange...
To get 12V on backlight output, Q8400 must be on.
That is possible if BKLT_SD is low (0V), which requires EDP_BKLT_EN to be present.
"EDP_BKLT_EN 0.31v"
Can you re-check?

 

[Kyle93]

Just checked again EDP_BKLT_EN still 0.31 and PPVOUT_SO_LCDBKLT 12.3V

 

[2informaticos]

And BKLT_SD?

 

[Kyle93]

12.44V

 

[2informaticos]

Then Q8400 is bad, shorted D-S; change it.

 

[Kyle93]

Replaced Q8400

PPVOUT_SO_LCDBKLT 0v

EDP_BKLT_EN 0.30v

BKLT_SD 12.33v

 

[2informaticos]

Something else was affected by corrosion.

Backlight circuit requires EDP_BKLT_EN.
But also need correct communication with T-CONN via I2C bus (SCL/SDA).

You should test with known good LCD assembly.
Better to change small video cable too.
Be sure J8500 is now good.

 

[Kyle93]

Solved after ultrasonic cleaning