Changhong 51-inch plasma protection shutdown maintenance 1 case - android - Phones Developers

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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 whole system went into protection mode. The indicator light didn't turn on, and the TV couldn't power up again without unplugging and replugging the power cable. This process had to be repeated multiple times before it would start working again.

After analyzing the fault symptoms, I noticed that the TV displayed the logo and played the melody, which suggested that the Y board, Z board, TCON board, and motherboard were likely functioning properly. The most probable issue was with the power board not being able to handle the load. Upon disassembling the unit, I tested the 5Vstb voltage and found it disappeared when the protection kicked in.

By tracing the circuit, I discovered that the 5Vstb voltage was controlled by Q305, which is a D4144 transistor. When the TV was in protection mode, the input voltage at D5 (5V) of Q305 remained stable. Further investigation showed that Q305 was being controlled by U901, an ATTINY40-SU 8-bit AVR MCU located on the power board. Unfortunately, I couldn’t find any useful English documentation online, so I had to trace the circuit manually.

I began by disconnecting the high-voltage and high-current power sockets for the Y and Z boards (CN803). The TV still produced sound, but no image. Then I reconnected CN803 and disconnected CN802 (D15V plug). At this point, the image was normal, but there was no sound. I then tested Q301, a P55NF06a N-channel FET on the power board, and found that the gate-source (GS) junction was leaking. Replacing it with a P50NF06a plastic package FET fixed the issue.

The following is a partial schematic based on the actual circuit and my repair analysis. If there are any inaccuracies, feel free to point them out!

The power board’s output voltages are monitored by U901 through different resistors. After the second boot, the D15V output current increased when the startup melody played. Due to the GS leakage in Q301, the transistor wasn’t fully conducting, increasing its internal resistance and lowering the D15V output voltage. U901 detected this on its 7th pin and sent a low-level signal to its 4th pin, triggering protection. This caused Q308 to turn off and Q309 to turn on, cutting off the 5Vstb output.