After the machine can hear the power on-off relay action sound, it should be a comprehensive protection circuit control this factor. By the standby indicator light red emitting diode constant light to infer, the fault is certainly the main switching power supply oscillation circuit. When the first test of the D7, the, the foot DC voltage is 290V, the normal, the rectifier mode switching control circuit components. Then the IC801 thick film detection foot start voltage is 13.2V, that started the normal work of the circuit oscillation. Then the measured voltage is 2.6V IC801 feet (icon = 1.2V), the pin voltage is too high to make IC801 internal Overcurrent protection circuit control. To cause IC801 4 feet high voltage factors are: IC801 effect of infield of switch tube leakage current is too large; IC801 II feet connected over current detection resistance larger; T801 secondary rectification filter circuit of leakage or short circuit of load; rectifier filter output terminal over-voltage protection monitoring electrical components bad error protection Q803 saturated conduction or Q803 set emitter junction leakage damage and resulted. To narrow the scope of the problem, try desoldering pin D822 and lift the control voltage of 4 feet, and short start-up detection + B output voltage 140V, normal, this time to observe the whole, all control functions were restored to normal. According to the judgment of the fault of R816, D820, R817, Q803, the protection and control circuit of over-voltage caused by wrong action. At this point, the B C, e,,, 13.1V, 3.6V, 4.1V,, B, 290V, e,, 0.3V, Q803 and 0.2V, respectively, were detected. Thus, it is concluded that the Q803 has been in the state, and the R816 - R818 or 10V - D820 - voltage regulator tube is damaged. Test retest D820 cathode voltage to ground 9.4v, clearly below the D820 Zener turn-on threshold voltage of 10V, indicating that the D820 conduction is certainly the poor performance of leakage or damage caused by. A trial of 1W/10V regulator for replacing the original D820, and then try to restore the original circuit boot machine, normal, troubleshooting.