Measurement of the main switch power supply output + 140 v voltage, only around 120 v +, but very stable, judge the fault is caused by a switch power supply output voltage on the low side. And so on have an effect on the output voltage in the switching power supply circuit D812, IC801 components such as measurement, check carefully, no abnormalities are found. Finally suspect voltage regulator block IC801 performance is bad, but change after the fault remains. Voltage of the switch power control loop and the protection of control loop are analyzed. After finding of switch power supply output voltage is not only related to voltage regulator control circuit, also related to the protection control circuit, feedback circuit. Wp9service manual download site
 Wp9service manual download site
 Wp9service manual download site
Check the circuit, found the switch tube Q801 emitter is only a nominal value of 2.7 Ω resistance, while drawing on R802, R803 two resistors in parallel, one is 1 Ω, another is the o. 82 Ω, equivalent resistance is about 0.45 Ω total resistance. Switch tube emitter resistance is the main function of over-current detection, when there is flow switch tube, the voltage on the emitter resistance will rise. Trigger protection circuit. In addition, the emitter resistance for switch tube itself is a negative feedback resistor, when its resistance value increases, but normal electric current flows through a emitter voltage will be higher, as long as the rise in voltage is not trigger protection circuit starts. Rise in voltage is negative feedback voltage of the switch tube, cause the switch tube in saturated conduction time shortened. The output voltage is reduced. You have found the cause of the problem, change 1 Ω and o. 8212 two resistance after the commissioning, troubleshooting.Wp9service manual download site