At 9 o'clock in the morning, a ring of phone calls woke me up. I answered, and it was my old friend from the village, Laofei. He said that he would come to my place in the afternoon because his nephew's TV was broken and needed repair. I immediately agreed. The distance from our hometown to Shantou is about 37 kilometers, and Laofei has always been my closest friend back home.
At 1 PM, he and his little nephew arrived. I greeted them with tea and asked them to sit down. While we were having a chat, Laofei mentioned that the TV had already been repaired by two technicians in their village, but nothing worked. His sister-in-law is a big fan of football, and she hadn’t seen a match for a while, so she wanted to sell the TV but felt it was still worth keeping. That’s why he came to me. I smiled and said, “Let me take a look.â€
The TV model was Changhong CHD29300. The issue was that it could power on, but only a small cell appeared in the center of the screen, with the line and field width reduced significantly, the image was dim, and there was a cross-scan line, though the sound was mostly normal. The first technician said the high-voltage package was faulty, but after replacing it, the problem remained. The second one spent half a day without finding the cause.
I opened the TV. Since it could boot, I first checked the power supply +B. When it wasn’t running, +B was around 145V, which seemed normal. But once it started (second boot), the voltage dropped sharply to 96V. This indicated a serious issue in the power supply or load circuit.
I disconnected the line power resistor R301 (1 ohm, 1W) and measured the line current, which was less than 200mA—so no overcurrent. The fault was definitely in the power supply circuit.
I removed the 300V filter capacitor C810 (560UF/400V) and tested its capacitance, which was normal. Then I checked the +B filter capacitor C845 (330UF/200V), and it also tested fine. It looked like the problem was in the voltage regulator or excitation circuit.
I turned on the computer and downloaded the schematic. After analysis, I found that the primary starting capacitor C825 (100UF/50V) was critical to the stability of the power output. Testing showed it was fine, so the issue must be in the optocoupler voltage regulator circuit.
Looking at the circuit diagram, the voltage regulator circuit was made up of the optocoupler NQ838, the three-terminal regulator NQ833 (SE140N), and the sampling transistor VQ822. I checked all three components, but found no issues.
This was strange. All the relevant parts were good, yet the power supply was unstable. I carefully examined the control circuit and noticed something unusual: the reference voltage wasn’t coming from +B, but from an independent winding rectified by VD833, marked as 24V. However, the actual measurement was only 17V. I checked VD833, which was fine. Could the filter capacitor C835 be bad?
I removed C835. It looked okay, but when I tested it with the capacitance meter, the capacity was below 50UF (the original was 470UF/35V). I replaced it, booted the TV, and everything worked perfectly. The picture was clear, and the sound was great. What a relief! I couldn’t believe it was just a bad capacitor. My friend had traveled over 30 km just for this! (As shown below):
After the repair, Laofei said, "I told you, you can do it!" He was really grateful, saying it was worth the trip. I felt a wave of emotion. How many people in our hometown still think of me? At first, I was a bit indifferent and cynical, even though I was recognized as a craftsman. But maintenance isn't my final goal. No matter what others say, I'm the laziest person in the world. Among relatives and friends, only my old buddies truly understand me, support me, and give me the best help during my toughest times.
Even though we've moved to the city for development, and the past ten years have been full of challenges, we’ve kept in touch. No matter how far I go, I will never forget my friends. This case shows how important it is to pay attention to every part of the circuit. In this example, a poor 24V filter caused the +B voltage to drop too low. I hope this case helps everyone in their repair journey.
Thank you for reading. The next article will be about another repair story. Stay tuned!
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