I was tasked with repairing a Hisense LCD TV model TLM42V68PA that had a flower screen issue. When the TV first turned on, the English icon appeared clearly, but the image was distorted and out of frame. My initial thought was that either the screen or the logic board might be faulty. I started by testing the voltages on the logic board. The 12V supply was stable, and VGL at 5.6V, VDD at 3.3V, VDA at 14V, and VCOM at 5.6V were all within normal ranges. However, the VGH voltage was fluctuating heavily, which made me suspect the logic board. I removed the two flexible cables connected to the screen and tested the VGH voltage again, but it still fluctuated between 12V and 18V. I checked the power control chip TPS65161 on the logic board and found that pin 10, which should have around 10V, was only measuring about 2V. This discrepancy made me consider replacing the chip, but I was worried about causing further issues. Since the board wasn't expensive, I ordered a replacement logic board with the part number V420H1-C12 (for a Chi Mei screen) online. After receiving the new board, I installed it in the store and tested it. The VGH voltage was still fluctuating around 18V, sometimes even dropping to 12V. I asked the seller what the expected VGH voltage should be, but they weren’t sure. Based on my experience, I believed it should be around 24V. Testing the board again confirmed the same issue—fluctuating VGH and inconsistent display output. The user mentioned that the screen might be damaged, which would make returning the board complicated. They agreed to return the board and test it using a universal board instead. To my surprise, when I connected the universal board, the screen displayed a video in the upper right corner against a blue background. That indicated the problem was not with the motherboard, but likely with the original logic board. However, I remained skeptical because the VGH voltage should have been stable if the logic board was the issue. I decided to investigate the motherboard itself. All the main voltages like 1.2V, 2.5V, and 3.3V were normal, and the main chip wasn’t overheating. I also checked the 27M crystal signal with an oscilloscope and found it to be stable. The flash memory was also functioning correctly. Eventually, I noticed that the LVDS output voltage was abnormal—should be around 1.2V, but it was only about 0.6V. After checking the LVDS cable, I found that the resistance was much lower than normal (around 1KΩ instead of 4KΩ). There were no components between the main chip and the LVDS port, so I suspected the main chip MT8222ARSU might be faulty. I ordered two replacement chips and prepared to replace them. While removing the motherboard, I measured the resistance from the power supply to ground and found it was shorted. After desoldering the main chip, I tested the problematic LVDS output again and still got low resistance. Finally, I removed the output strip and discovered that there was a lot of dirt under the plastic housing. After cleaning everything thoroughly, I reinstalled the original chip and tested the TV again. The image returned to normal! Even after replacing the chip, the picture still didn’t display properly, and I was confused about why the image was being compressed. It took some time to figure out the root cause, but eventually, everything worked as expected.
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