820-01598 liquid damage no screen backlight

[wgaca]

Hello,
This originally had liquid damage on and around the lvds backlight capacitors (C8469..)
I first cleaned the board, it was dead prior, after cleaning it was turning on but no backlight.
I replaced the bad looking capacitors and it showed backlight. I cleaned the board again to remove the flux but after cleaning it was back to no backlight.

There is no short on PPVOUT and i couldn't find broken traces.

Q8400
Pin3 - 12.59V
Pin4 - 12.49V
Rest - 0V

BKLT_EN_R - 3.3V

Am I understanding correctly that U8400 has to pull down LCDBKLT_EN_L in order to turn on the backlight? And if so why it doesn't?

There was no damage on U8400 originally.

 

[2informaticos]

U8400 must pull down BKLT_SD, to enable Q8400.
It receives BKLT_EN_R, but backlight level is adjusted through I2C_BKLT bus.
Check R8450-53 and their traces.
Also post voltage and diode mode at SCL/SDA lines.
Do you get image now, only missing backlight?

 

[wgaca]

Edit: Added R8451-2 diode mode measurements
Edit2: I managed to log in and increase the brightness, it was at max and there is no difference when being moved around

R8450 - 0L
R8453 - 0L
R8451 - 0L
R8452 - 0L

R8450 - 4.5V
R8451 - 4.55V
R8452 - 4.5V
R8453 - 4.55V

BKLT_EN_R - 3.3V

The traces are not broken to the lvds connector.
I have image with no backlight only.

I was thinking to try and type the password and increase the brightness to see what will happen but after 10 minutes i gave up, the screen is way too dark for that.

 

[2informaticos]

You cannot get OL reading on diode mode.
Recheck with red probe at ground!!!

Check all traces, solders and components in backlight area.
Something is still affected by corrosion.

 

[wgaca]

the reading is indeed 0L on all 4 resistors measured with red probe to ground.
The lines are uninterrupted to the lvds connector

Can this be U8400's fault? If so can I use one fro 820-00875 to replace it?

 

[2informaticos]

U8400 doesn't have board dependant firmware, you can replace it.

 

[wgaca]

I removed U8400 and measured with the chip off, still 0L. Replaced it and no difference.
I checked SMC_LID_LEFT and RIGHT both at 1.8V
LID_OPEN_LEFT is 1.8 and right sits at 0.5 when not connected to the audio flex and 1.8V when connected.

BKLT_EN_R is still at 3.3V
LCDBKLT_EN_L is at 12.05 (0.4V lower than what it was before)
PPVIN_S0SW_LCDBKLT_R - 12.59V
PPVIN_S0SW_LCDBKLT - measures around 3V and 0L to ground (in resistance mode measurement it will start a 1M and increase infinitely)

Tested with another screen, no backlight.

Looks like it's trying to activate PPVIN_S0SW_LCDBKLT but something fails?


EDIT: Fixed

I should have spent more time checking on what you told me. It turned out that the BKLT_SD trace was partly broken. I have no idea how it measured good when I first checked for continuity but the probe point on R8402 turned out to be corroded. Honestly, it was nearly invisible and when I scraped the coating it was still connected..
I've reconnected the probe point to the trace leading to R8402 and it works.

Thanks for the help!