Fault Phenomenon: Irregular automatic shutdown, sometimes self-recovery, sometimes not. After the power is turned off, it can work again. Sometimes it is difficult to start, etc.
Beijing Philips TV Repair Center organized on August 10, 2011.
Analysis of Circuit Characteristics:
(1) The switch power supply circuit adopts a self-excited parallel output type, and the circuit is composed of fully discrete components, with a relatively complex structure;
(2) There are over-voltage, over-current, and under-voltage protection circuits, which are respectively composed of Q805, R823, R822, Q805, R825, and R823, Q807, etc.;
(3) The remote control power supply circuit also adopts a switch power supply circuit, composed of Q809, T804, etc. The remote control on/off machine control circuit is composed of the microprocessor TMP47C433AN and the interface circuit, using an optocoupler (D830) control method.
Inspection Techniques: Due to the wide range of this fault and the complexity of the original circuit, the key-point voltage monitoring method is used to narrow down the fault range. When the fault occurs, check the voltage at the following key points:
(1) The ⑧ pin of the microprocessor TMP47C433AN (power on/off control end) has high voltage, normal, and changes with the remote control on/off machine control;
(2) The main power supply voltage of 135V becomes around 30V, which is too low, possibly related to excessive load or low output voltage from the power supply itself;
(3) Use the dummy load method to determine whether the fault lies in the switch power supply circuit or the load circuit. Use a 60W bulb as a dummy load for testing, and the fault persists, indicating that the fault is within the internal circuit of the switch power supply. Beijing Philips TV repair telephone.
(4) Since there is output from the power supply (too low), the fault is unrelated to the start-up circuit, protection circuit, etc. Therefore, focus on checking the voltage stabilization control circuit. It was found that when using the DC working point method to detect the emitter of the sample comparison error amplification circuit Q806, it suddenly returned to normal operation, indicating that the fault is related to this circuit. Carefully inspecting with a magnifying glass revealed fine cracks at the weld point between the emitter stabilizing diode D819 of Q806 and resistor R831. After re-soldering, the fault was eliminated.
Cause Analysis: The fault is due to the virtual soldering at the weld point between the emitter sampling comparison error amplifier Q806 D819 and R831, causing Q806 to fail to work normally, resulting in abnormal control loops from the control tube Q803 to the switching tube Q801, reducing the output voltage, and stopping the operation of the line scanning circuit.
Philips TV repair case: TV irregular automatic shutdown