Model: TCLAT34187 Led Monitor,Widescreen Monitor,Desktop Monitor,Wireless Monitor Guangzhou Panda Electronic Technology Co., LTD , https://www.panda-3c.com
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 generally 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 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 turn it on because a tube failure usually has other underlying causes.
After a careful inspection of the calibration, deflection, and reverse-capacitance components, no issues were found. The high-voltage package showed no obvious short-circuit or ground fault. When I started the power-up test, the current needle flickered and then triggered the power protection, indicating a large current condition.
Based on this, I concluded that the high-voltage package was faulty. I bought a replacement and reinstalled it. After restarting, the line current stabilized at 310mA, which was very stable. I then reinstalled the current-limiting resistor R409. The sound came back, and the image appeared. However, due to differences in the parameters between the new high-voltage package and the original one, there was insufficient high voltage, causing the screen's horizontal and vertical scanning to be too wide.
In the memory, I noticed that the TV had a line adjustment function. I opened the bus interface and confirmed that adjusting the line and field amplitude settings would solve the issue. Once adjusted, 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, the sound should be present. Therefore, during maintenance, care must be taken to avoid misdiagnosis. To enter the service menu, I opened the remote control board and pressed the SERVICE button (which didn’t have conductive rubber). Then, within 3 seconds, I pressed the MUTE button once. This activated the service mode. From there, I could navigate using CH+/- and adjust the parameters with VOL+/-. After making the necessary adjustments, I shut down and exited the menu.
Tip: For TCL TVs with a TDA93XX series CPU, the process to enter the service menu is generally similar.