Maintenance and trouble shooting of photoelectric switch

When repairing the photoelectric switching power supply, the step-down overhaul method can be used. The method is: connect the power plug of the display to an AC voltage regulator, and then adjust the output voltage of the voltage regulator to about 100V, then power on for maintenance, and gradually increase the power supply voltage for maintenance.

Fault example 1: The fuse is burned out when the machine is turned on, and the output voltage is zero. This situation is generally caused by the breakdown of the switch tube and the short circuit between the emitter and the collector. At this time, the switch tube can be removed first, and the resistance of the emitter and collector to the ground can be measured. If it is zero or very small, just replace it. But it is necessary to check whether there are any problems with other components before booting.

Fault example 2: "S"-shaped distortion of the grating appears. This kind of problem should focus on checking the filter circuit and the voltage stabilizing circuit. Generally, it is because a diode is open, which changes from full-wave rectification to half-wave rectification. This may also be caused by the reduction in the capacity of the filter capacitor.

Fault example 3: AC 220V rectifier filter circuit has a short-circuit fault, and the fuse is burned when the machine is turned on. First check whether the rectifier diode is short-circuited and the filter capacitor is leaking seriously. You can also unplug the degaussing coil to check whether the degaussing thermistor has short-circuit faults, and replace it if necessary. Failure example 4: No grating, no display, power indicator does not light up, but the insurance is not burned. At this time, check whether the AC mutual inductance transformer is open, whether the current-limiting resistance of the rectifier circuit is open (broken), or whether the rectifier diode is open.

Fault example 5: No grating, no display, and abnormal noises in the machine. If a "squeak" sound is made, it indicates that the oscillation frequency is low, and components related to the oscillation should be checked. If a "click" sound is made, it indicates that the power supply is over-current protection, and the over-current protection circuit should be checked.