Today, I really wanted to take a break and go out shopping. But my friend's dad called and offered to bring me a TV for help, so I had to give up on my plans. He eventually brought the TV over. It was a Sony LCD model KBL-32BX320 that had been used for about seven years. I made a cup of tea, gave him a drink, and we started talking about the TV’s issues. The problem is that after turning it on, it only works for a short while—sometimes just half an hour—before it automatically shuts down. The power light stays on, but the remote control doesn't work. You have to unplug the power cord to restart it, and even then, it only lasts a few minutes before shutting off again. There's no clear pattern to the shutdowns.
Based on this, I thought there might be a thermal instability issue with some components or possibly a solder joint that’s not holding up under heat. I decided to start by checking the power board since that seemed like the most likely culprit. I turned the TV on and checked the voltages: the standby 3.3V was fine, but the other two supplies—12V and 24V—were both at zero. The machine was in standby mode, and the voltage at the power-on pin (from the remote) fluctuated between 3.0V and 0V, which suggested the motherboard was working normally. So the issue must be in the power supply.
I took a closer look at the power board. The main filter capacitor, C6330, was around 320V, which meant the PFC circuit wasn’t functioning properly. When I pressed the remote, the voltage would rise to 400V briefly, and the screen would light up for a moment. Then it would drop back to 320V, and the TV would go into standby. I measured the voltage at the power-on terminal, which was 1.6V—normal, but still not enough to keep the system running. I concluded that the power board had either a poor solder joint or a faulty protection circuit.
I removed the power board and tested it separately. The 3.3V standby output was okay, but the 12V and 24V outputs were both zero. I connected a dummy load to the 12V output and monitored the voltages over time. After a while, the system went into standby again, and the 12V and 24V outputs dropped to zero. The PFC capacitor remained at 320V, and the switch control terminal stayed at 1.6V. I couldn’t find any obvious damage or burned components, so I used a magnifying glass to check all the solder joints, which looked good.
I took a moment to smoke a cigarette and calm down, trying to reorganize my thoughts. As I reviewed the data from the normal startup and the previous test, I noticed something strange: the voltage at the P3 control pin was 3.0V when the TV worked normally, but it was only 1.6V during the fault. I tried connecting a 4.7K resistor between the 3.3V standby line and the P3 pin, but the voltage didn’t change. That ruled out a simple wiring issue.
I traced the path from the P3 pin through Q6203 and Q6204, then to the FET Q6201, and finally to the optocoupler PH6104 and the IC6301-CXA3810. Since there were no schematics available, I had to check each component manually. I suspected the power management IC might be faulty, but without parts, I expanded the search to the protection circuit. I found Zener diodes D6024 and D6025 connected to Q6023B. Testing D6025 showed some abnormal reverse leakage. The diode model was 2J300M, which should be a 30V/2W regulator, but it wasn’t performing as expected.
I replaced the diode and retested the TV. After powering it on for an hour and then testing it for three hours, everything worked fine. It turned out that the faulty 24V protection diode was causing the system to shut down due to internal leakage. Looking back, if I had checked that diode earlier, I could have saved some time and avoided unnecessary troubleshooting steps.
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