I recently repaired a Changhong 3D51C5588 plasma TV. After the second boot, the "CHANGHONG" logo appeared on the screen, and the speaker started playing the startup melody. However, shortly after that, the TV went into protection mode. The indicator light was off, and the TV couldn’t power on again unless I unplugged and reconnected the power cord.
At first, I analyzed the fault symptoms. Since the TV displayed the logo and played the melody, it suggested that the Y board, Z board, TCON board, and motherboard were functioning properly. The most likely issue was with the power board not being able to handle the load.
After disassembling the TV, I tested the 5Vstb voltage and found that it disappeared when the TV entered protection mode. Tracing the circuit, I discovered that the 5Vstb voltage was controlled by Q305 (D4144). When the TV was in protection mode, the D5 input voltage for Q305 remained stable.
Continuing my investigation, I found that Q305 was controlled by U901, an 8-bit AVR MCU (ATTINY40-SU) located on the power board. Unfortunately, the available information online was in English and not very helpful.
Faced with no other options, I decided to manually trace the circuit. I first removed the high-voltage and high-current power connectors CN803 from the Y and Z boards. The TV still made a sound, so I restored CN803 and tested CN802 (D15V). The image was normal, but there was no sound.
Next, I tested Q301 (P55NF06a), a power MOSFET on the power board, and found that the gate-source (GS) junction was leaking. I replaced it with a P50NF06a, and after testing, the TV worked perfectly.
Below is a partial schematic based on the actual board and my analysis. If you find any inaccuracies, feel free to point them out.
The output voltages from the power board are sent to U901 through various resistors. After the second boot, when the startup melody plays, the D15V output current increases. Due to the leakage in the GS junction of Q301, the transistor didn't fully conduct, increasing its internal resistance and lowering the D15V output voltage.
U901 detected this drop on its 7th pin and sent a low-level signal to its 4th pin, which turned off Q308 and turned on Q309, ultimately shutting off Q305 and cutting the 5Vstb output.
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