According to the phenomenon analysis, fault range should be in the power supply circuit. Check the power fuse intact. And then disconnect the output of the power supply board strip PG1001, under the power-on state measurement PG1001 all output voltage of 0 v, switching power supply has no job, and the fault has nothing to do with the load. Check the power supply circuit, measured switch tube Q1001 sk1198 (2) the drain voltage is 300 v, normal; Measured under-voltage protection circuit of controlled triode Q1003 sc2959 (2) the base voltage of 0.7 V, normal should be 0 V, the switch power is in a state of protection. 449service manual download site
 449service manual download site
 449service manual download site
In order to confirm the fault is in sampling circuit and control circuit, the photoelectricity coupler with a jumper wire IC002 (TLP643) {3}, {4} feet short circuit. Measured again after power up, PG1001 various voltage, results all normal; Keep the power supply work status, measured under-voltage protection sampling resistor in the circuit R1059 Ω (150) there are about 0.5 V voltage, showed that the luminous tube is in normal work condition, IC1002 problem may be IC1002 internal photoconductive tube failure. After replacement of a same model of photoelectric coupler, fault be ruled out.449service manual download site