820-00875

[iFix]

Hi guys.

I have this A1708 with a backlight issue. There was corrosion damage on U8400 and surrounding it. I replaced U8400, C8471(shorted), and C8441 and all resistors are fine. I was getting on PPVOUT_S0_LCDBKLT 12v, but then I changed also D8410 and Q8410, just in case. I measured of course the values of the circuit and no short found and still rechecked the resistors and all of them are measuring well. Any ideas?

 

[iFix]

Also, I rewired the LCD backlight flex some time ago and it's good checked, and I get voltages to it. No other corrosion on the board. Value on PPVOUT_S0_LCDBKLT is 0.700, unlike other boards which are higher

 

[2informaticos]

Did you test with known good LCD assembly?

 

[iFix]

Yes I tested with another one also, but as I said it's not boosting with any up to 50v

 

[2informaticos]

Do not expect 50V there, that will mean open LEDs strip connection.
Normal voltage is less than 40V.

Do you get 5V_S0SW_LCD?

 

[iFix]

Yes, I do get 5V_S0SW_LCD on the connector, measured on C8512, of course with LCD attached

 

[2informaticos]

Check R8431/2 and their traces to adjacent components.
Also check R8450-3 and traces.

 

[iFix]

All rechecked and they go to the places where they needed to and I measured the resistors themselves and measured the values by schematics , all good

 

[2informaticos]

Check voltage at BKLT_SCL/SDA and BKLT_EN_R.

 

[iFix]

BKLT_SCL/SDA - both 4.5v
BKLT_EN_R - 3.3v

 

[2informaticos]

Something else affected by corrosion.
Possibly bad solder, broken traces...

 

[iFix]

Nothing obvious and the big caps the one blown I replaced. I will recheck everything and revert

 

[iFix]

I replaced R8431/2 , even though they were measuring good and I got backlight. Also I replaced Q8400 to future-proof since F8400 was also burned