Appeared the fault, measuring C906 + 300 v voltage is normal, measuring + B no + 115 v voltage on both ends of the filter capacitor C909, boot, but cooler + B has a voltage output, and then to Ov. Start doubt protection circuit, measuring Q902 G voltage control, indeed as expected for 0.7 V, the state of protection, adopt the method of remove protection overhaul, with Q902 disconnected individually control connected zener diodes, and boot test, when disconnect ZD752 boot no longer protect, judgment is the result of the line output overvoltage protection circuit protection. But in road line measurement output overvoltage protection circuit components did not see abnormalities, remove the zener diode ZD752 measuring the voltage value, lower than normal value, the output voltage in normal, ZD752 breakdown, caused no protection. Change after a 27 v zener diode, restore the protection circuit, troubleshooting.2UTservice manual download site