TCL color TV AT34187 sound and shadowless maintenance and access to the bus method - android - Phones Developers

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Model: TCLAT34187
Fault phenomenon: There is sound, but no image.
Maintenance ideas and steps: Since the sound is normal, it can be assumed that the power circuit is basically working fine. When the TV is turned on, the filament does not light up, so I checked the line scan and line output circuits. During the shutdown check, I found that the line tube Q402 (BU4508) was damaged, and the high-voltage package power supply current-limiting resistor R409 (1 Ω / 1W) was open. I replaced the line tube and removed R409 before powering on again. I didn't rush to power it up because a tube failure usually has other underlying causes. After carefully inspecting the calibration, deflection, and reverse-capacitance components, I found no issues. The high-voltage package showed no obvious short or ground problems. When I tested the startup line current, the needle flickered and the power supply immediately protected itself, indicating a large current state. Based on this, I concluded that the high-voltage package was faulty. I bought a replacement and reinstalled it. After restarting, the line current measured 310mA and was stable. I then installed the current-limiting resistor R409. At this point, both sound and image appeared. However, due to differences in the parameters of the new high-voltage package compared to the original one, the high voltage was insufficient, resulting in a larger screen line and field range. I checked the memory and found that the unit had a line adjustment function. I opened the bus interface and confirmed that I could adjust the line and field amplitude. After making the necessary adjustments, everything worked properly. It's important to note that the power for the high-frequency head, middle-position, and sound amplifier comes from the switching power supply. So when the power supply is normal, there should be sound along with the image. During the maintenance process, care must be taken to avoid misjudgment. To enter the service mode, I opened the remote control, located the SERVICE button on the remote board, pressed the conductive rubber button (which wasn’t originally equipped), and then pressed the mute button within 3 seconds to enter the service menu. Once inside, I used the CH+/- and VOL+/– buttons to navigate and adjust the settings. After completing the adjustments, I shut down and exited the service mode. Tip: For TCL TVs using a TDA93XX series CPU, the method to enter the service mode is generally similar.