Last night, an old customer called and said, "My LCD TV makes a loud bang sound when I turn it on." He didn’t say anything else, just asked me to come over and check it out. It's been more than a year since I've been in the repair business, but some of my old customers still act like they don't trust me — it's hard to get rid of those guys.
The TV model was Hisense TLM4628. After opening the back cover, my first thought was that the capacitor might be the issue. I carefully checked and found that CE027 had cracked and turned black, making it impossible to read. There were also two resistors that were burnt and completely charred — it looked really bad. (As shown in the image below):
Since I didn’t know the original specs of the damaged components, I tried searching online. I was lucky enough to find the circuit diagram. Here are the parameters for the burned parts:
All the damaged components were replaced. Then I checked the power supply voltage regulator IC (NE003). The resistance between pin 1 and 2 was around 20Ω, which seemed normal. I tested other components and then powered it up again.
I saw the "Hisense" logo on the screen, then turned it off. It looked like there were still issues, so I shut it down and took another look. I measured the low-voltage outputs of the unit. This group of burnt components was responsible for the 24V regulated output, which powers the high-voltage lamp circuit. When I turned it on again and checked the output voltage, +5V and +12V were fine, but the 24V was low.
To determine if the problem was with the power supply itself or due to over-current protection in the high-voltage section, I disconnected the two high-voltage boards from the power supply and turned it on again. The 24V voltage dropped to 0V within 20 seconds, so I concluded that the 24V regulator circuit still had an issue.
I started checking the power switch regulator IC (NE003). Pin 1 had 300V, and the start voltage at pin 4 was normal. The boot capacitor also showed no abnormalities. I suspected the switching regulator power supply NE003 (STR-X6769) was faulty. I asked my apprentice to bring me a new one. Comparing the resistance between pins 1 and 2, the values were significantly different. I installed the new part and ran the TV for nearly an hour. Everything worked fine after that.
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