Dell 5547 (LA-B012P ) Screen Brightness Cannot Be Adjusted Repair

Laptop Model : Dell 5547

Motherboard Number: LA-B012P ZAVC0 R1.0

Fault: Only The Lowest Brightness Can Be Used. When The Brightness Is Increased, The Lamp Flickers And Becomes Darker.

 

Repair Process:

 

I got the laptop for a test run and found that the brightness could not be increased regardless of using keyboard shortcuts or the control panel, and the problem persisted when changing the screen.

 After disassembling the machine and flashing the BIOS, the problem is still the same. It seems to be a circuit problem. I found a drawing from the forum and first measured the screen interface voltage.

When I measured the signal of pins 39 and 40 + INV_PWR_SRC, I found that it was only 5V. Normally it should be 19V, which is low.

Continuing to trace this signal, we found that it came from the LCD backlight control circuit.

The EN_INVPWR signal is turned on QX1 to form a voltage dividing circuit, and the measured EN_INVPWR voltage is normal. 

From the figure, we know that the common point B+ 19v is sent to the QX2 S level, and at the same time, it is divided by two resistors and sent to the QX2 G level. If all voltages are normal, SD turns on and outputs 19V +INV_PWR_SRC. 

The measured G-level voltage is abnormal. The voltage dividing resistor RX2 is normal at 100K, but the infinity of RX3 is abnormal. After the replacement, the measurement turned out to be infinite again.

The measuring capacitor CX4 was short-circuited, and it remained the same even after the replacement. Replaced the small six-pin QX2, measured the resistance of the resistor and found it was normal.

After installing it, the system test was normal and the machine was repaired.

Happy Repairing !!