A visual inspection of the power thick film circuit cracking, fuse fuse with F801 blackening, open circuit resistance R802. There are two breakdown diode rectification circuit of full bridge D806, photoelectric coupler take the d803 (1) and (2) foot open. After replacing the damaged element, the power supply and the load are not found obviously short circuit faults, which determines the power of the test machine.

In order to secure, off line output circuit, 60W is connected to the output end of the +B bulb false load test machine, and +B voltage monitoring. After the start, the brightness of the bulb is changed, the +B voltage is not stable. Analysis of voltage regulation or protection of circuit fault. First check the voltage loop, no abnormal, and then check the protection circuit (the machine's protection circuit is more perfect complex, with over-voltage, over current, short circuit protection function). The circuit consists of Q831 q805 over current detecting circuit, D837 d830 load short-circuit detection circuit, D831 overvoltage detection circuit and optocoupler D811, Q827, Q826, Q804 composition. Normal D811 16V power by R864, D825 power supply, the "B" is a high potential, D811 light-emitting light-emitting tube; triode saturation conduction, "a little" low potential, Q827, Q826, Q804 are closing, on the thick film circuit switch tube does not produce effect.

When the flow, over-voltage, short circuit detection circuit in action, "B" drop as low potential, cut-off D811. "A" variable is a high potential, Q827, Q826, Q804 turn-on, thick film circuit switch B very short stop vibrating. Disconnect the power supply output voltage Q804c, normal fault, and in the protection circuit. In the protection circuit and Q826 Q827 composed analog controllable silicon circuit, once enter the state of protection, under the constant electric cannot exit protection status, and the dummy load lighting bright, dark, that analog controllable silicon circuit failure. Check the analog circuit of controllable silicon, we D823 open, D811 3, 4 feet open. After the replacement of D823, D811, the brightness of the lamp is no longer flicker, the output voltage is stable at 110V. Remove the false load, then the output circuit, the TV repair.