Today, I'm sharing a repair case of a 46-inch Samsung LCD TV that had a black screen issue. After opening the unit, I checked the PFC voltage on the IP board BN44-00341A and found it 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 (INVERTER ON/OFF and PWM) were both high, indicating a problem in the backlight circuit and its load.
I tested the backlight driver chip ICLX6503AID and found that pin 1 (12V supply) was normal, but pin 16 (VREF) had no voltage. Further testing showed that pin 7 (EN enable) was actually at 0V. After turning off the power, I checked for any short circuits between pin 7 and ground, and there were none. I then 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. The motherboard’s lighting control signal was connected to pin 4 of the same optocoupler. It became clear that the fault was due to an open connection between pins 3 and 4 of the PC803S.
After cleaning the solder mask around pin 2 of the optocoupler, I found that it was connected to the K-terminal of the UB801 (a 431-type regulator). I checked whether the PFC voltage reached 380V and confirmed that the optocoupler's pin 1 was receiving power through two resistors: RI875 (silk screen 30B, 2K) and RI873 (silk screen 30B, 2K). However, upon testing, RI875 had increased to over 200K, which was causing the failure. Replacing RI875 restored normal operation, and the TV worked perfectly after the repair.
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