To observe the machine boot instantaneous high pressure build voice, measure the secondary boot instantaneous output voltage switch power supply, and then down to 0 V, measurement of Q7991 control voltage of 0.7 V, judgment is thyristor Q7991 into protection state. Because Q7991 external eight-way protection circuit, for narrowing the scope of the fault, in the boot of the instantaneous measured isolation diodes D710, D604 the anode voltage of 0 v, that is associated with the isolation diode protection circuit has not entered the state protection; All other protection circuit with no isolation diodes, disconnect each protection circuit and silicon controlled G connection method, one by one to remove protection, observe the failure phenomenon, when disconnect zD7991 lift + B overvoltage protection circuit, boot no longer protection, and the sound, light, all normal, normal voltage is 120 v + B, judgment is a + B caused overvoltage protection circuit protection by mistake. + B overvoltage protection circuit components for the road test, no exception. Doubt zener diode ZD7991 voltage value change, remove the ZD7991, measuring the voltage value, the smaller and unstable, when + B voltage normal, cause breakdown, the error protection. After replacing ZD7991, restore protection circuit, boot no longer protect, troubleshooting.G36service manual download site