Night attack Konka black screen new example - appliance repair

LED light-emitting diode

Yesterday, I got my hands on a Konka LCD TV model LED32F2000. The screen was completely black, but I could hear sound and saw a faint image on the side when using a flashlight. That immediately made me think the backlight was faulty. I decided to check the LED cable first and used my homemade LED tester. It didn't work—some LEDs lit up, some didn't, which was strange. But I was lucky I didn’t open the panel right away, because that would have taken a lot more time and effort. Maybe it was just a protection issue in the LED driver circuit.

I went to the store and found an original Konka LED32F3300 light bar. When I turned it on, the light bar flickered very weakly but didn’t light up at all. This confirmed my suspicion that the problem was with the LED driver circuit. Since I was busy during the day, I decided to fix it later that night.

At night, I started working on the power board and the LED driver board. The driver IC was OZ9902CG. I checked pin 3 (the light switch control pin) and found it was at 3.2V, which indicated that the mainboard was sending a drive signal. Then I checked pin 13, the over/under-voltage protection detection pin, and found only 1V instead of the normal 2.5V. I tested the sampling resistor R154, and it was fine. Next, I checked the LED power boost diode VD751 and found it had failed, causing the boost circuit to malfunction. As a result, the OZ9902 was triggering under-voltage protection.

After replacing the diode, the voltage at pin 9 of the OZ9902 returned to 2.5V, and the boot LED lit up properly. I connected the TV to a signal source, and everything worked perfectly—no more black screen, and the image was clear and stable.

The homemade LED tester I used is now available in my career version—go make your own! It's a great tool for quick diagnostics and saves a lot of time in repairs like this one.