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

A user of a Hisense LCD TV model LED42K11P reported that the TV would not stay on for more than 2 hours during use. After shutting down, it couldn’t be turned on immediately and had to wait for a while before restarting. Upon picking up the unit, I checked the boot-up sequence and found that the 5V standby voltage was continuously on, but all other voltages were zero. However, the motherboard’s STB signal (start command) remained at 2.35V, which suggested that the motherboard wasn't sending a proper shutdown command. The power board in this unit is a two-in-one design: RSAG7.820.2264/ROH.

I first measured the PFC voltage and found it only at 320V, which was significantly low. This could indicate that the PFC circuit was not functioning properly, leading to the main power supply shutting 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 an under-voltage protection threshold of around 0.8V. At the time of testing, the voltage at this pin was only 0.8V, triggering the protection mechanism.

I then checked the PFC feedback resistors R874, R875, R876, and R877, as well as the regulator loop resistors R826, R827, R828, R829, and R830. All of them were within normal limits. When I replaced R829 with a 47kΩ resistor, the PFC voltage increased to 408V. It appeared that the original resistor might have been faulty, causing an over-voltage feedback from the PFC chip NCP33262.

After removing the resistors R826, R827, R828, R829, and R830 again, I noticed that the resistor marked as R805/563 on the schematic was actually 0Ω, which was a mistake. There was also some glue underneath these resistors that might have caused a leakage issue. After completely removing the glue, the PFC voltage rose to 390V. Once the backlight was turned on, the PFC voltage dropped slightly to 365V. Then, I measured the voltage at pin 5 of the NCP1396, which increased to 1.2V. After this repair, I tested six units, and none of them experienced the same shutdown issue again.

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