I'm really close to the paper, and I know that I have to do it (transferred from Liu Changjun).
Fault cases are a form of learning that many people like and hear about, but writing a good one isn't easy. The main reason is that everyone interprets things differently, so perspectives vary. In my opinion, a good fault case mainly depends on the idea and inspiration behind it, with a focus on the method used.
If a fault case can make readers feel like they've learned something after reading it, then it's very meaningful. Of course, the value of this learning may be big or small. Knowledge requires a process of accumulation—this involves repeated thinking and practice. It also requires connecting internal and external knowledge to form new understanding, which helps expand thinking and deepen knowledge.
Model: TLM46V86P
Movement: LCD-MST6M68FQC
Fault Phenomenon: Untimed black screen, no sound.
Diagnosis and Maintenance: The boot sound and image are normal, but the screen goes black irregularly during use. When the black screen appears, the backlight is bright, and the sound remains normal. All functions including the remote control work properly, indicating that the CPU and motherboard are functioning normally.
According to the working principle of the LCD screen, a black screen usually occurs when the power supply is lost. Since the backlight is still on, the logic board becomes the main focus of inspection. During the fault occurrence, the voltage of the DC/DC converter on the logic board was found to be normal, and the format converter circuit was also operating correctly.
So where is the problem? Re-analyzing the situation, since the fault doesn’t happen regularly, everything seems fine when there’s no issue, so we temporarily ruled out the possibility of a screen failure. This suggests that the black screen might be caused by the absence of a display drive signal from the logic board. Why would the logic board not output a drive signal? Based on the measured voltages, the logic board should be working properly. Therefore, the only possible cause is a sudden interruption of the LVDS signal input to the logic board.
Following this line of thought, we started checking the LVDS output interface on the motherboard. According to the characteristics of LVDS signals, each pair of differential lines should show a normal voltage of around 1.2V. We tested each pair one by one. When we checked the XP11 pin, we noticed that the voltage fluctuated between 1.2V and 1.6V—clearly abnormal. This fluctuation indicated poor contact at that pin. Since the voltage rises under no-load conditions, we examined the differential line closely and found that the outer layer of the 22nd pin was intact, but the inner wire was completely broken. The outer layer remained tight, preventing the wire from falling off. After reprocessing the joint, the device was left running for an extended period, and no further faults were observed.
Why does a broken line lead to a black screen? According to the schematic, the pair of differential lines in question are the pixel clock signal lines. The pixel clock is a critical clock signal in digital circuits, responsible for transmitting and demodulating digital video signals. When the pixel clock is lost, the LVDS signal cannot be properly transmitted to the logic board for reading and demodulation, causing the logic board to lose its display drive signal. This results in a black screen failure.
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