Dell E1909W - 4.Power (include IF +5 V and +3.3 V) supply normal,key function OK, but backlight cant be turned on

Service Manual
33
Is the frequency 14.318MHz at
#127 of IF Board U105?
NO
4.Power (include IF +5V and +3.3V) supply normal ,key function OK, but
backlight can’t be turned on
Backlight cannot be turned on, is the
on-off control signal come from IF ok?
Is the CN853
inserted CN103 OK?
NO
Yes
Is the on-off control signal #109 of U105
high-low convertible when DC on/off
NO
Yes
Is the low active reset signal at #105 of
IF Board U105 normal?
NO
Change
C164
Yes
Is the PWM signal at #125 of IF Board
U105 normal when power AC on?
Change
U105
NO
Update Firmware to Serial Flash U108
by VGA port with ISP_Tool
V4.100.exe,after this process, no the
control signal yet, change U108
Yes
Change
X101
Yes
Is the voltage
24V at Q6/Q7
pin3?
Check
T850 or
F850
NO
Yes
Yes
Check U1
pin11
Voltage is 5V
Yes
Check U1 pin2
Voltage is
lower
Check
Q7 or
Q6
NO
YES
Check T1/T2 pin7 and pin10 Open
or Short
Change T1/T2
Yes