Through the analysis, judge the machine automatic switching ac input circuit or doubler rectifier protection circuit fault, the circuit as shown in the appended drawings. Because the machine is equipped with doubler rectifier protection circuit, and the protection is done by burning fuse. It can be seen from the chart. If by mistake when ac voltage is 220 v switch to the doubler rectifier state, will cause the rectifier voltage up to 600 v, the sampling resistor R814, R815, R816 voltage rise and breakdown voltage of diode D805, make the SCR Q805 get trigger voltage and control of the ac input voltage direct short circuit to ground, burn out the fuse FS01, protect the main power supply. JPmservice manual download site
 JPmservice manual download site
 JPmservice manual download site
Known from the analysis of the above, should check the doubler rectifier protection circuit mainly automatic switching circuit and ac input. Examination, due to the inconvenience, electric resistance method measuring voltage doubling rectifying protection circuit of SCR Q805 breakdown, not D805, R814 all normal. Seems a high probability of ac input switch misoperation, surrounding the possible causes of the malfunction, checking each key element Q804, R804, D802, 0809, C804, D803, Q807, besides D803 positive and negative to the resistance value is too small, without exception, other measure D803 will be removed, found that the positive and negative to the resistance value is less than 211, after the switch to a same diode, troubleshooting. JPmservice manual download site
 JPmservice manual download site
Summary: because D803 breakdown, in the case of a 220 v power supply is normal by R807, R808 sampling the voltage through D803 Q807 conduction, bidirectional thyristor Q804 get trigger voltage and the control of conduction. Rectifier circuit transformation to the state of doubler rectifier, wrongly, that too large rectifier voltage doubler rectifier protection circuit. Protection circuit of SCR Q805 conduction. To burn out the fuse FS01.JPmservice manual download site