I took over a Changhong LCD TV model LT32710 with three no-power issues. After opening the unit, there was no 5V output, but the internal 300V was present. I tested the power supply output from the Schottky half-bridge and found that the 24V power supply capacitor (20200) had failed. After replacing it, all voltages returned to normal, but the backlight wasn't bright, and there was an image shadow on the screen. This indicated a problem in the backlight circuit. The backlight start signal from the motherboard and the brightness pulse width modulation signal were both normal. I checked the OZ964 backlight chip's 5V power supply and found it working properly. However, the third pin of the chip showed zero voltage, which is controlled by two channels. The ENA signal from the mainboard is regulated by a comparator circuit using LM358. Since the ENA from the mainboard was normal, I suspected the LM358 might be faulty. Q206 is the FET responsible for controlling ENA, and it was turned on. The turn-on voltage comes from a thyristor circuit formed by Q1 and Q13, which is triggered by a low level. That low level is generated by Q14, which is controlled by the LM358. Since the LM358 was at a high level, I believed it was damaged. After replacing it, the backlight briefly lit up, but then the protection circuit kicked in again. This confirmed that the LM358 was indeed faulty, but there could be other underlying issues causing the protection. I shorted the first pin of the OZ964 to ground to bypass the protection, connected an external lamp to the board, and turned it on. The lamp lit up, but only partially. Using an oscilloscope, I measured the waveforms at pins 11 and 20 of the OZ964. According to theory, they should be similar, but in reality, pin 11 had no output while pin 20 showed a 65kHz, 5Vp-p signal. This suggested a fault in the pin 11 circuit. A multimeter test revealed a short at pin 11. Further inspection found that the gate-source junction of the drive transistor Q28 was broken down. The model is K712. I replaced it with another FET from the overvoltage protection sampling circuit, but the issue remained after booting. Rechecking the waveform, I noticed that the OZ964 output was now normal, but the waveform on the coupling transformer looked different. The rising edge of the square wave on the T102 transformer was clearly attenuated and jagged, indicating a load short circuit. I tested the gate-source resistance of the FETs Q201 and Q202 inside the power supply and found them to be shorted. Replacing them with two 5N50 FETs restored full illumination to the screen. After restoring the protection circuit and testing the TV, everything worked normally—sound, image, and display. The repair was complete.
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