Laptop motherboard repair (Winbond WPCE775LA0DG replace?)

[kahlenberg]

Hello,

I have a Toshiba satellite U405D, the backlight of LCD screen does not work. Inverter works, I checked it. VGA output also works if I connect an external display. I have found out that LCD inverter doesn't get "display on" and "pwm" signals at all. I also checked the cable, it is also healthy. I have found schematic of mainboard and I suspect EC(KBC) (Winbond WPCE775LA0DG) chip on motherboard. The schematic is below:
https://www.s-manuals.com/pdf/motherboard/quanta/quanta_bu2_r1a_schematics.pdf

The chip is shown an on page 28. How can I be sure that this chip is damaged? I am asking this forum before buying a chip, because maybe the reason is different that I have thought.
What else can cause this problem?

Thanks.

 

[D.A.(Tony)Stewart]

Does external display work? Did you disable onboard?
Does backlight CFL work? was it ever dim or pinkish?

 

[kahlenberg]

Yes, everything is working, external display, backlight CFL, inverter. It is dim. I know that inverter works, it just does not get any PWM signal.