Under normal circumstances, the fuse tube fuse tube fuse tube fuse tube fuse tube is the charger internal circuit short circuit or over current fault. This is because the charger for a long time in high voltage, high current state, high failure rate of internal devices. In addition, when the grid voltage fluctuates, the surge causes an instantaneous increase in the current in the charger, resulting in a fuse. The solution: Look at the parts on the board first, see if they’re burned out, or if there’s an electrolyte overflow, and it doesn’t seem to make a difference. Then measure the resistance of the power input. If it is less than 20ok Ω, there is a local short in the back end. Then the forward resistance, reverse resistance and two current limiting resistors of the four rectifiers were measured. Finally, the normal charging and discharging of the power filter capacitor and the damage of the switching power supply tube are tested. FOUND UC3842 and its surrounding components damaged and burned out. It should be noted that, since the measurement is made on the road, it may cause errors or misjudgments in the measurement results. If necessary, these parts can be welded together for measurement. If not, measure if the input and output power lines are internally short-circuited. In general, in fuses, rectifiers, power filter capacitors, switching power supply tubes, UC3842 and other vulnerable components, the damage probability can reach more than 95% , the focus of inspection of these components, it is easy to eliminate the faults without DC voltage output or voltage output instability, if the fuse is intact, in the no-load situation, the main causes of these faults are: over-voltage over-current Protection Circuit Open and short circuit; Power Overload; High Frequency rectifier Filter Circuit rectifier diode fault, filter capacitor leakage, etc. . Maintenance method: First, the multimeter is used to measure whether the components of the high-frequency pulse transformer are damaged.After the high frequency rectifier diode breakdown and load short circuit are eliminated, the DC voltage at each output terminal is measured. Finally, the damage of rectifier diode and low-voltage filter capacitor in high-frequency filter circuit is measured by multimeter. If the above parts are damaged, new parts should be replaced. General failure can be eliminated, but attention should be paid to: Output Line fracture or open welding, virtual welding may also cause such a failure, maintenance should pay attention to this situation. No DC output, but the fuse is intact, indicating that the charger is not working or is in working protection. Maintenance methods: first of all to confirm the charger UC3842 variable control chip in working state or damage. The specific judgment method is: The electricity is added to UC38427 legs voltage, if 7 legs voltage is normal, 8 legs voltage + 5 legs voltage, 1,2,4,6 legs voltage is also different, then UC3842 basic normal. If 7 feet of low voltage, the rest of the pins have no voltage, UC3842 has been damaged. The most common type of damage is 7 feet off the ground, 6 feet off the ground, 7 feet off the ground, and 1.7 feet off the ground. If these feet have not been punctured, the charger still can not start properly UC3842 has been damaged and should be replaced. If the chip is not bad, check whether the gate limiting current resistance of the switch is open, welded or variable, and whether the switching power supply tube itself is poor performance. In addition, the power output line broken or bad contact will also cause this failure, so in the maintenance should also be noted.