Recently, I encountered an issue with a Hisense LED LCD TV. On the same day, I took over three units, all of which were less than two years old. Each had different fault symptoms. For a while, I didn't share the results with others, but now I have time to organize and release them one by one.
This particular unit starts with a blue screen and no response from the remote control. The buttons don’t work either. I opened the back cover and checked the voltage on the power board (HLE-4046WE). The readings were: STB 3.6V, SW 0V, V5 5.2V, 12V (6–15V), and 16V (8–24V) — two sets of main voltages. During testing, the board was jumping, and I heard a faint "Tara" sound, suggesting some instability. My initial thought was that the power output was abnormal, causing the system to fail. This type of failure is similar to what other users have experienced, and some have already fixed it.
I decided to follow my own approach. I removed the board and started by checking the sampling circuit. The machine samples from 12V. I tested the optocoupler N840, specifically pins 3–4. Usually, this helps distinguish between hot and cold side faults. There was no voltage output, so I suspected a problem on the cold side. I checked the control pin of the sampling IC N842 and found it shorted to ground. However, after replacing the optocoupler, the issue remained.
Next, I removed the original optocouplers N833 and N840 and replaced them with a good one, following the principle of fast and reliable repairs. I made sure not to install the wrong pins. After replacement, the board still didn’t produce any output. That confirmed the failure of N840. I replaced it with a new optocoupler and restored the 12V and 16V outputs. I then removed the forced boot point, reset the board, and it booted up successfully. The music played, the picture appeared, and the input signal was normal. The repair was complete.
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