Hisense LCD TV LED42K11P irregular shutdown maintenance examples - appliance repair - Huaqiang electronic network

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A user of a Hisense LCD TV model LED42K11P reported that the TV would not stay on for more than two hours during use. After turning off, it couldn't be turned on immediately and had to wait a while before it could be used again. Upon receiving the unit, I checked the boot-up process and found that the 5V standby voltage was always on, while all other voltages were zero. However, the motherboard's STB signal remained at 2.35V, which suggests that the motherboard wasn’t issuing a shutdown command. The power supply is a dual-board design: RSAG7.820.2264/ROH. I first measured the PFC voltage, which was only 320V. This low voltage might be causing the main power supply to shut down. I traced the circuit and confirmed that the PFC chip NCP33262’s pin 8 and the main power chip NCP1396’s pin 12 were receiving normal power. Using an oscilloscope, I saw that the PFC chip was working, but the NCP1396 was not. The PFC voltage is connected to pin 5 of the NCP1396, which has undervoltage protection. At that point, the voltage on pin 5 was only 0.8V, triggering the protection. I then checked the PFC feedback resistors R874, R875, R876, R877, and the regulator loop resistors R826, R827, R828, R829, R830, all of which were within normal ranges. After replacing R829 with a 47kΩ resistor, the PFC voltage increased to 408V. It seems the issue was caused by a high feedback voltage from the PFC chip NCP33262. I removed R826, R827, R828, R829, and R830 again and noticed that R805/563 was marked as 0Ω on the schematic, which was incorrect. There was also glue leakage under these resistors. After completely removing the glue, the PFC voltage rose to 390V, and when the backlight was turned on, it dropped slightly to 365V. Then, the voltage on pin 5 of the NCP1396 increased to 1.2V, and after testing six units, none of them experienced the same shutdown issue.




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