Visual inspection, found that the power supply section, crack, there are more than the preliminary judgment fault may be associated with the crack, after all the crack repair welding, and between a + B outputs and to answer the multimeter to observe dynamic voltage changes. During the normal watch, + 113 v voltage is normal and stable, for about a minute about voltage suddenly dropped to 52 v + B and volatility, "chirp" sound immediately appear, near the machine plate listen, "chirp" sound from the switch transformer T801. Break operation load circuit, and a 60 w bulb of commissioning, after electrify + B and output voltage of 108.5 V, due to disconnect the load circuit, power line loss inverse synchronization pulse, the judge should belong to the normal 108.5 V voltage. Based on past experience, pay special attention to observe boot instantaneous voltage changes, the results found a quick boot instantaneous voltage + B on impact phenomenon, downtime instantaneous voltage also has a + B in phenomenon, analyzes believed that rush on this kind of phenomenon indicates + B voltage is too high. So in + B a R2M protection diode output and fault operation after R805 end flyback pulse input circuit, disconnect the overvoltage protection tube Q834 (TFD312M), in the case that does not leave the power switch power of commissioning, figure appeared, try access line flyback pulse, + B voltage immediately appear in phenomenon, immediately shut down, suggests that existence overvoltage switching power supply. Looked through IC801 peripheral elements are no exception, judge STR50213 may performance is bad, will be a long time after the replacement of commissioning troubleshooting.YUqservice manual download site