Questions about switching power supplies

1. The reason why the switching power supply always has no voltage output
The switching power adaper supply always has no voltage output, which means that after pressing the power switch to turn on, the output terminal of the switching power supply is always 0V. This is because switching power supplies do not oscillate. Further confirmation is to measure the voltage of the switching power supply after the 100UF/400V capacitor is turned off. The switching power supply will not oscillate if the voltage drops slowly after 300 volts. Reasons why a switching power supply does not oscillate include:
(1) The collector of the switch tube does not have enough working voltage
(2) The base of the switch tube has no startup voltage and leakage of related circuits
(3) Failure of the positive feedback element of the switch tube

2. Methods and steps for judging faults
The main task in troubleshooting this type of failure is to determine which of the above three parts of the drum baffle is. The specific method is to measure the collector and base voltage of the switch tube. The following situations may exist:
(1) The collector voltage of the switch tube is 0V, which is 1.4 times lower than the mains. The switch tube does not have normal working voltage. If the voltage is 1.4 times, the collector voltage of the switch tube is normal, and the AC 220 volts and the rectifier filter circuit work normally.
(2) The base voltage of the switch tube is 0V (including the start-up time), which indicates that the start-up circuit does not provide the start-up (turn-on) voltage to the base of the switch tube, or the related components between the base and the emitter are faulty. The start-up circuit, switch tube transmitter and related components should be checked. If the voltage is 0.6~0.7 (including the start-up time), it means that the start-up circuit and emitter element of the switch tube are normal. If the voltage is higher than 0.7V, it means that the startup circuit is normal, but the emitter junction of the switch tube or its components is open or the resistance becomes large.
(3) The switch tube has conduction conditions: the voltage of the switch tube base is 0.6~0.7V, and the collector voltage is greater than 250V, indicating that the switch tube has working conditions. The fault is in the positive feedback circuit, including the positive feedback resistor, capacitor, freewheeling diode, and positive feedback winding of the switching transformer, and the connection between them should be made into a board.