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Repaired a Changhong 3D51C5588 plasma TV. After the second boot, the “CHANGHONG†logo was just displayed on the screen; when the speaker started the melody, the whole machine was protected. The indicator light is not lit, and it cannot be turned on twice. It is necessary to unplug the power cord and re-plug it to re-boot it twice.
First of all, the fault phenomenon is analyzed, because there is light after the second boot, there is a Changhong logo and there is a boot melody; basically, the Y board, the Z board, the TCON board and the motherboard are good. The fault is most likely that the power board can not afford the load! After the teardown, the 5Vstb voltage was tested and disappeared when it was protected! After running the line, I found that 5Vstb was controlled by Q305 D4144! The D5 5V input of the Q305 is quite stable when the TV is protected! Continue to run the line, found that the Q305 is controlled by the U901 ATTINY40-SU 8-bit AVR MCU controller 4 pin on the power board! So I searched the U901 for information online. After the search is very helpless, all in English instructions!
No way, only I run around U901. After clearing the circuit; hands-on repair. First remove the high voltage and high current Y board and Z board power supply socket CN803, the test machine sounds still; restore CN803, pick up the CN802 D15V plug, test machine, the raster image is normal, only silent. Remove the Q301N channel FET P55NF06a test of the power board, and it will damage the GS pole leakage! After replacing a plastic P50NF06a, test the machine to repair!
The following is a partial circuit diagram drawn according to the real thing and my maintenance analysis. If there is anything wrong, welcome to criticize and correct!
The voltage output terminals of the power board are sent to the U901 through different resistors, and the U901 determines whether the output voltages are normal after the second power-on. After the second boot, the D15V output current increases when the starter melody sounds. Because the GS pole of Q301 leaks, the Q301 conduction is not complete, that is, the internal resistance of Q301 becomes larger; the voltage of the D15V output terminal becomes lower. After detecting this information on the 7th pin of U901, it outputs a protection low level from its 4 pin, Q308 is turned off, Q309 is turned on, and Q305 turns off the 5Vstb output.
Physical map of the above circuit diagram
TV model
Bad piece Q301P55NF06a test chart
Replace the picture after Q301
Internal picture
Repaired image