Dell 5750 laptop boot screen does not light repair

Machine model: Dell 5450

Board No.: dajw8cmb8e1

Fault phenomenon: the laptop does not display when it is turned on

Maintenance process: after startup, it is found that the notebook does not display, occasionally it can display, but the screen is still dark. It is judged that the screen line is broken. After changing the screen line,splash screen(alternate dark and bright screen).
Press the interface splash screen on the main board with finger. When testing the dark screen, the led-bl 19v is normal, and the backlight EN LCD_PWM_In 0V, the fault point should be here,splash screen LCD_PWM_IN 0-1.7V current of change。

Dell 5750 laptop boot screen does not light repair Figure 1.

Find LCD_PWM_In to composite diode.

Dell 5750 laptop boot screen does not light repair Figure 2.

There is no broken line in the test, and the brightness and darkness of LCD PWM EC are 0V.

Dell 5750 laptop boot screen does not light repair Figure 3.

EDP at alternate flashing_PWM_Out VCC 0-1.7 changing, find EDP_PWM_Out is sent out by 12 pin of ps8620 / 23, and there is no broken wire in the test.

Dell 5750 laptop boot screen does not light repair Figure 4.

At this point, we can basically confirm the fault of ps8620 / 23 or EC or bridge. Combined with the traces of falling pot on the machine shell, it is estimated that the bridge may be relatively large. Finally, we decided to do Jump wire and find the screen VCC 3.3v.

Dell 5750 laptop boot screen does not light repair Figure 5.

Connect 10K resistance Jump wire to 3 pin of d17 diode in series. After starting the machine, there will be a PWM EN. The above figure shows that the device is easy to operate.