Today, I received a 25-inch assembly machine. The main ICs are: lc863528b, LA76810A, and LA78041. The owner mentioned that two technicians have already looked at it. It started with high-voltage ignition and shorted the line. The first technician replaced the high-voltage package and the line tube, but the issue remained. Then they replaced the large 76810 and the field block 78041, but the problem still wasn't resolved. After picking up the unit, I opened the cover and found that the high-voltage package and line tube had already been replaced with faulty components. Testing revealed that the line was shorted to ground. I tried replacing the down tube C5296, and the ground resistance was normal. Then I swapped it with D1557, and there was no obvious short circuit in the line. However, when I turned on the power, the high-voltage line emitted a "beep" sound, indicating a discharge to the core. After replacing the BSC26-3606A, the field width was only about 10cm, with a bright white line in the center. The +120V supply was stable, and the large block provided 9V, 5V, and 25V for the field power supply. Since both the 76810 and 78041 were newly replaced, I checked the voltages on the field output pins. The 3rd pin showed 5V (normal should be around 2–2.5V), the 5th pin was at 18V (should be half of the supply voltage), and the 7th pin was 2.7V (normal is around 2V). I also checked the related pins 23 and 24—pin 23 (excitation output) was at 2.2V, and pin 24 (sawtooth wave forming end) was at 2V, which seemed normal. I suspected the field block might be faulty, so I replaced it, but the issue remained. By analyzing the circuit diagram, I noticed that the 5th pin's voltage is closely related to the 7th pin. Following the 7th pin’s trace all the way, I found it connected to the 5V regulator (IC7805) through a 1.8k resistor (R453). I tried changing R453 to 2.7k, and the boot improved slightly. The voltage on the 5th pin dropped to 13V, and after adjusting the bus, the screen was fully visible.
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