Measuring voltage boot moments of up to 30 v + B and then down to 0 v, preliminary judgment this machine load switch power supply circuit, over voltage, over current fault occurring cause a protection circuit action, the state of protection. Switching power supply output is measured with a "resistance" of various load has no obvious phenomenon, estimation is ac fault line output circuit. Adopt the method of remove protection maintenance, fault operation output circuit, the a + B output 100 w bulb false load, turned on the light bulb light, + B output voltage 140 v normal, switching power supply is normal, the fault in the load circuit: will remove Q805 base and the emitter short-circuit protection, boot observation fault phenomena, grating dim and R835, R834 smoke, measuring Q831 collector output high level, further communication line output circuit are short circuit fault. Pull the deflection coil, boot failure, judge line output transformer short-circuit damage. After changing one branch the output transformer, restore protection circuit, boot no longer protect, troubleshooting.HtUservice manual download site