When performing maintenance, we always hope for a simpler and more stable circuit, so that the board doesn’t feel overwhelmed during repairs. However, in most cases, things don’t go as expected. With so many different types of boards and circuit designs, it's essential to accumulate practical experience and continuously enhance your theoretical knowledge. Once you successfully fix a device, you often feel a sense of satisfaction and clarity.
Let’s take the CRT color TV circuit diagram as an example (based on a post from the "Longxing World ACE" community yesterday). The case involves a Konka P2902T high-definition color TV (models like P2908T, P3460T, P3409T, P2903T, P3618T, P2906T, etc.). The symptoms include no high voltage, +B voltage at 150V or 200V, but only 150V, acceleration stage voltage at 0V, bright filaments, and 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 lit, and the line should be oscillating, yet there's no high voltage or acceleration voltage, and the line isn’t working. When troubleshooting, the first thing to suspect is the power supply, not just jumping to conclusions. Let’s look at where the filament voltage comes from (see Figure 1). It’s derived from the output of the switching transformer at pin 1, then stepped down twice to produce a DC 6.3V that powers the picture tube. The winding at pin 8 for the filament is actually unused.
Understanding this issue is straightforward. Don’t start by checking the line oscillator (see Figure 2). If the line isn’t oscillating, the usual suspects are the power supply, the line driver, the horizontal deflection transistor, the reverse process capacitor, the S-correction capacitor, or a shorted damper diode. You can check by measuring the resistance. If nothing is wrong, move on to the transformer section, the push-pull transistor, and then the power supply. Check the waveform at the output pins of the oscillator and the main output. If all seems okay, the problem might be with the flyback itself. (The specific circuit is shown in the attached PDF based on the model.)
Now let’s analyze an example of this model’s “three-no†fault (see Figure 3 for related circuits): After inspection, the fuse and switch transistor were damaged, and R918 was open. After replacement, 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 the inductor was reconnected, the raster appeared, and the image looked good. However, after running the TV for about five minutes, only sound was heard, and the image disappeared completely. The fuse and switch transistor were damaged again.
In real repair scenarios, the switch transistor operates under high current and is mainly controlled by N901. If the TV’s IC keeps damaging the switch transistor, and the absorption circuits (V901, R904, R905, C911) are fine, check the resistor at pin 2 of R918 and the start resistor R920 (which is prone to failure). Poor performance of N901 is a major cause, so it must be replaced with an original part. During thunderstorm season, it’s best to replace the N902 optocoupler, as it’s easily damaged by lightning. Using a substandard optocoupler could lead to further damage to other components.
The above examples are just a few of the many special cases encountered during repairs. These situations come from my own hands-on experience and also refer to the repair processes shared by netizens online. I’d like to express my gratitude to those who contributed. I often see users asking for circuit diagrams on forums, but they don’t have the original version of the machine, making it hard to proceed. In reality, experienced technicians don’t get stuck on such issues. No matter how the circuit changes, there are always certain rules to follow. The most typical ones are the switching circuit and the scanning circuit. When you have time, you’ll find that at least 60%-80% of the related circuits resemble each other. There are differences, of course. Looking at the drawings I’ve uploaded, you’ll likely recognize similar patterns in at least 20 different boards. Therefore, if someone needs circuit diagrams, I sometimes provide similar ones for reference.
As a technician, it’s crucial to constantly strengthen your theoretical foundation and adapt to evolving technological demands. I’ve been in the field for 15 years, and over time, I've gained a deeper understanding of various boards and circuit diagrams. Any circuit, in the end, is just a combination of basic circuits—amplifier circuits, switching circuits, oscillator circuits, and IC control circuits. Nothing is too complicated. I believe that to truly master the craft, one must develop solid foundational skills and avoid a money-driven learning approach. Don’t dismiss older models just because they may not be profitable. Without studying them, you’re losing the best opportunity to practice and refine your skills. If everything is done solely for profit, you won’t achieve anything meaningful in life.
You can also check out the article titled "Take the Dragon" by the netizen [Kangjia HD color TV P2902T burning tube himself repair method]. It's a very useful post worth reading. Also attached is the Konka P2902T HD color TV schematic PDF (applicable models: P2908T, P3460T, P3409T, P2903T, P3618T, P2906T, etc.).
Konka 1.pdf
Konka 2.pdf
Konka 3.pdf
Konka 4.pdf
Konka 5.pdf
Konka 6.pdf
Konka 7.pdf
Konka 8.pdf
Konka 9.pdf
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