This LCD TV was purchased in 2013. The user mentioned that the shutdown works fine, but after turning off, the backlight doesn't light up anymore. However, the sound is still normal. When I tested the unit, it really behaved this way—there was no reaction on the boot screen at all. The model number is shown below.
[attachimg]478411[/attachim]
When I opened the back cover, I noticed that two screws were missing, which made me think it might be a reworked unit. The user confirmed this. After the sale, I realized that the board needed to be replaced, specifically the orange, black, and brown ones. But the user didn’t want to replace the board, so I just pulled the TV out. The power board is shown below:
The main circuit on the back of the power board is shown below:
I did some initial tests: 12V, 24V, and 5V outputs were normal. The standby voltage was 3V, and the brightness control voltage was 1.3V, which was also normal. However, the backlight voltage was only around 24V, indicating an issue with the backlight circuit. The secondary backlight chip model is OZ9967GN. I checked the power supply for the backlight chip, which was 12V, and the switching voltage and brightness control voltage were also present. However, the boost transistor driving voltage was 0V. After checking the boost transistor and diode, they both seemed fine. Next, I checked the backlight protection and voltage detection circuits. The 10-pin low-voltage protection terminal of the chip measured 1.7V, which was normal. The over-voltage protection terminal (pin 4) was 0.1V, which also looked good. The open-circuit protection terminal (pin 3) was 0.7V, which was normal as well. At this point, I suspected the chip itself was faulty.
Although the chances were high, I wasn’t 100% sure because the part was out of stock. If I made a mistake, it would cost me time and effort. So I decided to look for more clues. Suddenly, I remembered that many modern chips have a reference voltage pin. This is a common thing for technicians to check. I quickly looked up the data sheet for the OZ9967GN and found that pin 14 is the reference voltage output pin, which should provide 5V internally and externally. I immediately tested the voltage on pin 14 and found it to be only 1.5V, which was abnormal. My heart jumped a bit—this was the key clue. I then measured the resistance from pin 14 to ground and found it to be around 200 ohms, which was not normal. After removing the surrounding RC components and measuring again, the resistance returned to normal, confirming that the chip had an internal short on pin 14. The image is shown below:
I ordered the replacement chip, installed it, and the TV turned on with a blue screen. I plugged in the signal and everything worked normally. The repair was successful.
Repairing LCD TVs can be tricky, especially when parts are hard to find. Most components are usually bought from Taobao or similar platforms. If you make a wrong judgment, it's a waste of time and effort. Therefore, when repairing an LCD TV, it's crucial to be cautious when determining if a chip is faulty. If the chip isn't commonly used and you buy it without being certain, you might end up waiting a long time before getting a replacement. This experience taught me to double-check everything before making a decision.
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