820-01987: No backlight

Got this board that had heavy corrosion damage on and around the backlight circuit. Found blow cap C8467 and also had to replace R8446 as pin 2 of the resistor along with the pad was corroded. Due to R8446 solder pads being damaged, i soldered it to C8457 and ran a line from pin 2 of the resistor to J8401, pins 1 and 2. The laptop powers up and works except when powered down, the keyboard backlight goes off as expected, but then after about a second or two, the keyboard backlight comes back on and stays on.

When OS is loaded, the keyboard backlight doesn't respond when i attempt to increase or decrease, it remains on at full intensity.

Any suggestions
it doesn’t respond to bright light or dark room settings for the keyboard. The display however does respond to changes in light.
So after reinstalling the OS, this resolved the backlight staying on once the laptop is powered down. But i still don't have control of the keyboard backlight once powered up. it remains at full brightness. Took the measurement as requested and here's the values

pins 1 goes from 1.15 to 1.3 when i increase the backlight intensity from 0 to max. Pin 4 never changes, remains at 3.3v. Should the output of this chip modulate as the input or is it a fixed 0 or 3.3v gate output.


Staff member
U8472 is a level shifter.
T2 sends it PWM 0-1.8V and U8400 requires PWM 0-3.3V.
Check with o-scope input/output signals.