When performing maintenance, we always hope for a simpler and more standard circuit to make the repair process smoother and less stressful. However, in reality, things rarely go as expected. With so many different types of boards and circuit designs, it's essential to accumulate practical experience and continuously improve theoretical knowledge. Once you solve a problem, you often feel a sense of accomplishment and clarity.
Let’s take the CRT color TV circuit diagram as an example. (This is based on a post from the "Longxing World ACE" forum yesterday.) The model in question is a Konka P2902T high-definition color TV, which includes models like P2908T, P3460T, P3409T, P2903T, P3618T, P2906T, etc. The issue reported was: no high voltage, +B voltage at 150V or 200V, but only 150V, acceleration stage voltage at 0V, filament bright, no raster. The relevant circuits are shown in Figure 1 and Figure 2.
Many people might wonder, “This doesn’t make sense!†The +B voltage is normal (nominal value is 150V), the filament is bright, so the line should be oscillating, but there is no high voltage, no acceleration voltage, and the line isn’t working. In such cases, the first suspect is usually the power supply. Start by checking the switch tube and then move on.
Looking at the filament voltage source (see Figure 1), it comes from the output of the switching transformer at pin 1. After two stages of voltage reduction, it becomes DC 6.3V, which is sent to the picture tube. The winding at pin 8 for the filament is not used here.
To resolve this, start by checking the line oscillator (see Figure 2). If the line isn't oscillating, the problem could be with the line power supply, the line driver, the horizontal deflection transistor, the reverse process capacitor, the S-correction capacitor, or the damper diode. You can check by measuring the resistance values. If everything checks out, move on to the transformer, push-pull transistors, and then the power supply. Check the waveform at the output pins. If all is good, the problem may lie within the main board itself. (The related circuits are shown in the attached PDF according to the specific model.)
Now let's look at an example of a "three-no" fault (see Figure 3 for related circuits): After inspection, the fuse and switch tube were damaged, and R918 was open. After replacing them, for safety, I disconnected inductor L905 and used a dummy load test. The bulb lit up, and the +B voltage was 142V, which was normal. When I reconnected the inductor, the raster appeared and the image was normal. However, after running the TV for about five minutes, the sound was still present, but the image disappeared completely. The fuse and switch tube were damaged again.
In real repairs, the switch tube operates under high current and is mainly controlled by N901. If the color TV IC keeps damaging the switch tube, and the absorption circuits (V901, R904, R905, C911) are fine, check the resistor R918 at pin 2 and the start resistor R920 (which is often faulty). Poor performance of N901 is one of the key reasons. It's important to use genuine parts. During thunderstorm season, it's best to replace the N902 optocoupler, as it's prone to damage due to lightning strikes. Using a non-genuine optocoupler can cause voltage spikes that damage other components.
These special cases are just a few examples of what I've encountered. They come from my own repair experiences and also from the maintenance processes shared by other netizens. I want to express my gratitude to those who have contributed their knowledge. I often see people asking for circuit diagrams in forums, but they don't have the original version, making it hard to proceed. However, experienced technicians know that regardless of the circuit design, there are always underlying principles to follow. The most common ones are switching circuits and scanning circuits. When you have time, you'll find that these circuits are at least 60%-80% similar across different models. There are differences, but the core concepts remain consistent.
I believe that if you want to become skilled in repair, you must build solid foundational knowledge and avoid a money-driven approach. Don’t dismiss old boards just because they seem outdated or unprofitable. Without studying them, you’re missing out on the best opportunities to practice and improve your skills. If your goal is only profit, you’ll never achieve meaningful success in your career.
You can also refer to the article titled "Take the Dragon" by the netizen "Kangjia," which details the self-repair method for a Konka HD color TV P2902T with a burned tube. This is a very informative post worth learning from.
Attached is the PDF of the Konka P2902T HD color TV circuit diagram (applicable models: P2908T, P3460T, P3409T, P2903T, P3618T, P2906T, etc.).
![Filament supply]()
![Line output circuit]()
![Power circuit]()
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Konka 1.pdf
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Konka 2.pdf
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Konka 3.pdf
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Konka 4.pdf
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Konka 5.pdf
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Konka 6.pdf
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Konka 7.pdf
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Konka 8.pdf
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Konka 9.pdf
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