AC voltage generated by the rectifier D818, c822 filtered DC voltage obtained by R825 and R826 is applied to the voltage divider D817. When the AC voltage is higher than 160V, D817 breakdown conduction, so Q854 saturated conduction, D815 deadline, Q853 off triac Q801 deadline. At this point and the filter capacitor C819 c818 series circuit form a bridge rectifier circuit. When the AC input voltage is less than 160v, D817 deadline, Q854 deadline. At a time when the rectified voltage by D814 and R824 through R823 and divider, c824 filter, the partial pressure is greater than the voltage value of the Zener diode D815 7.5V, so that breakdown conduction D815, D816 is turned on, bidirectional Q801 is triggered thyristor conduction, so that the filter capacitor c818 positive electrode is formed on the double voltage rectifier, the rectifier circuit into a voltage doubler rectifier.m6mservice manual download site