Today, I'm going to share a repair case of a 46-inch Samsung LCD TV that had a black screen issue. After disassembling the unit, I checked the voltage on the IP board (model BN44-00341A) and found the PFC voltage to be 389V. The IP board was providing 12V to the main board, and the 5V output was normal. However, the main board's signals for the backlight control, specifically INVERTER ON/OFF and PWM, were both high. This indicated a potential fault in the backlight circuit or its load.
I tested the backlight driver IC, LXC6503AID. Pin 1 (12V supply) was normal, but pin 16 (VREF) had no voltage. Then I checked pin 7 (EN enable), which was actually at 0V. After turning off the power, I confirmed that there was no short to ground on pin 7. I cleaned the PCB around pin 7 and discovered that it was connected via resistor RI814 (silk screen 01D) to pin 3 of the optocoupler PC803S. Meanwhile, the main board’s lighting control signal was connected to pin 4 of the same optocoupler. It became clear that there was an open circuit between pins 3 and 4 of the optocoupler.
I then inspected pin 1 of the PC803S and found it connected to the K-terminal of the UB801 shunt regulator (431). I checked the R-terminal of UB801 and confirmed that the PFC voltage was indeed reaching 380V. Pin 1 of the optocoupler was connected through two resistors—RI875 (silk screen 30B) and RI873 (silk screen 30B)—to the +14V PFC drive power. Upon testing, RI875 was found to have increased from 2K to over 200K, which was clearly faulty.
After replacing RI875 with a new 2K resistor, the TV was tested and successfully repaired. The backlight functioned normally, and the screen turned on without any issues. This case highlights the importance of checking resistor values and ensuring proper connectivity in sensitive circuits like the backlight control system.
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