Hisense LCD TV TLM46V86P untimely black screen soundless map failure analysis and maintenance - Home Appliances

Electronic scale crystal oscillator 3.2*2.5mm 3225 16M (16.000MHZ) 12PF 10PPM 20PPM 30PPM

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 enjoy and find interesting, but writing a good one isn't easy. The main reason is that everyone understands things differently, so perspectives vary. In my opinion, a good fault case mainly relies on ideas and inspiration, with a focus on the method used.
If a fault case can make people feel like they've learned something after reading it, then it's very meaningful. Of course, this learning might be small or big. Knowledge requires a process of accumulation, which involves repeated thinking and practice. This process connects internal and external knowledge to create new understanding, promoting broader thinking and deepening knowledge.
Model: TLM46V86P
Board: LCD-MST6M68FQC
Fault phenomenon: Random black screen with no sound. Diagnosis and repair: The boot sound and image are normal, but a black screen appears irregularly during viewing. When the screen goes black, the backlight is still on, and the sound remains normal. All functions, including remote control, work properly. This suggests that the CPU and motherboard are functioning normally.
According to the working principle of an LCD screen, a black screen usually occurs when the power supply is lost. Since the backlight is bright, the logic board becomes the key area to check. During the fault, the voltage of the DC/DC converter on the logic board was found to be normal, and the main conditions of the format converter circuit were also normal.
So where is the problem? Re-analyzing the situation, since the fault doesn’t occur regularly, everything seems fine when there’s no issue. Therefore, the possibility of a screen failure was temporarily ruled out. The black screen may be caused by the absence of a display drive signal from the logic board. Why would there be no drive signal output? Based on the measured voltages, the logic board should be working normally. The only possible cause for the lack of drive signal is a sudden interruption of the LVDS signal at the input of the logic board.
Following this thought, the first step was to check the LVDS output interface on the motherboard. According to the transmission characteristics of the LVDS signal, each pair of differential lines should show a normal voltage of 1.2V. Based on this, a pair-by-pair check was performed. When measuring the pin XP11, it was found that the voltage fluctuated between 1.2V and 1.6V, which is clearly abnormal. This fluctuating voltage indicates poor contact at the pin. Only under no-load conditions can the voltage rise, so further inspection of the differential line revealed that the outer layer of the 22nd pin was intact, but the inner wire was completely broken. Due to the tight outer sheath, the wire didn’t fall off. After reprocessing the connection, the device was tested for a long time, and no further faults occurred.
Why does a broken line lead to a black screen failure? According to the schematic diagram, the pair of differential lines are pixel clock signal lines. The pixel clock is a crucial 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. This results in the logic board being unable to output the display drive signal, ultimately causing the black screen failure.