Philips TV: Automatic shutdown during viewing at irregular intervals
Fault Phenomenon: The TV automatically shuts down at irregular intervals while in use. After the shutdown, it can be restarted and functions normally for a period of time (without any specific pattern) before the automatic shutdown occurs again. When the TV automatically shuts down, the red light turns off.
Circuit Characteristics Analysis:
(1) This is a multi-system large-screen color TV;
(2) The switch power supply uses the new thick-film circuit SIR-S63C9, with the sampling comparator amplifier using the SE140 integrated block. The switch power supply is a self-excited parallel output type circuit;
(3) It has overcurrent and overvoltage protection circuits. The overcurrent protection circuit consists of the internal circuit of the thick-film block and the external overcurrent protection sampling resistor R714 connected to pins ② and ⑦; The overvoltage protection circuit is mainly achieved through D712, a one-time overvoltage protection Zener diode;
(4) The remote control power on/off function is controlled by the microprocessor IC1001 (600105), pin 36 (power on/off control), and the interface circuit (Q703, Q702). The control method uses optocoupler isolation control, controlling the conduction state of the switch tube (the switch tube remains weakly conductive in standby mode).
Repair Techniques:
(1) Determine the fault range roughly based on whether the red indicator light on the TV panel is lit: If the red light is on, it indicates that the switch power supply circuit is in standby mode, and the fault may lie in the remote control system's power on/off control circuit; If the red light is off, it suggests that the switch power supply is not working, and the fault may be in the switch power supply circuit or the load circuit;
(2) Use the dummy load method to determine whether the fault is in the load circuit or the switch power supply and protection circuit;
(3) Use the method of detecting the voltage of each pin of STR-S6309 to find the exact location of the fault. Given that the indicator light goes out when the automatic shutdown fault occurs, it shows that the switch power supply circuit stops oscillating and does not work. To find the reason, a 100W bulb was used as a dummy load for testing, which showed no voltage output, further indicating that the fault lies in the switch power supply circuit. Upon checking the operating point of STR-S6309, it was found that the voltage at pin 1 (internal switch transistor c terminal) was around 300V (normal), and the voltage at pin 3 (switch transistor b terminal) changed to 0.6V (the normal value should be around 0.2V, indicating that the start-up circuit is normal but the switch power supply circuit does not oscillate). The positive feedback components R713 and C710 were found to be normal. Finally, replacing the power block IC701 (STR-S6309) resolved the fault.
Fault Cause Analysis: The fault was caused by poor performance of the internal oscillator circuit of the power block STR-S6309, which resulted in the switch power supply circuit occasionally stopping oscillation and causing the automatic shutdown fault.
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