Recently, I worked on a Haier TV that had a power supply issue. The problem started with the power voltage circuit. Initially, I used a light bulb as a load to test the system, but I made a mistake when handling the high-voltage package. After discussing with the customer (who was quite frustrated), we decided to replace the high-voltage module.
When the TV was brought in, it wouldn’t turn on at all. The power indicator didn’t light up, and after checking the voltage, I noticed that the +B voltage dropped from 125V to 0V. At first, I suspected the +B filter capacitor was faulty. However, when I connected a 100W bulb as a load, the boot-up process worked, and the +B voltage stabilized at 125V. I tested the B pole and found a small voltage of about 0.5V, which led me to conclude that the high-voltage module was defective.
After replacing the high-voltage package, the same issue occurred again. I realized I hadn’t thoroughly checked everything. The power chip used here is STR G9656, which can output voltage, so the power block wasn’t the problem. I moved my focus to the voltage regulation circuit.
I immediately tested the KA431 regulator and found a short between pins 1 and 2. After removing it, I confirmed that pins 1 and 2 were shorted, while pins 3, 1, and 2 were open. I replaced the KA431. I also suspected the optocoupler might be faulty, so I tested it and found a short between pins 3 and 4, which measured around 20K. I replaced it as well.
After checking other components online, everything seemed fine. I then used the bulb again as a load and measured the +B voltage, which came out to 140V—this turned out to be the normal operating voltage for this model.
Once the bulb was disconnected and the original circuit was restored, I powered on the TV. A picture appeared, the AV signal was working, and both video and audio functions were normal. It was a good feeling to see the TV back in working condition after such a detailed troubleshooting process.
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