Please advise how to repair the power supply with no output
1. The maintenance method and steps of switching power supply without output; (1) Check whether there is a 300v DC working voltage at the c pole of the switch tube, if not; (2) If the voltage at the c pole of the switch tube is normal, measure the power switch at the moment of power-on; (3) If Check that the oscillating circuit is normal, and then test the power supply + b1 at the moment of starting up; 1) Pulse width or frequency control circuit (including relying on optocoupler conduction to force open; 2) Load short circuit (referring to parallel switching power supply, because of series switching power supply; 3) The switching power supply causes the protection circuit to act due to output overvoltage or overcurrent (; and the switching power supply + b1 terminal is grounded
1. The maintenance method and steps of switching power supply without output
(1) Check whether there is a 300v DC working voltage at the c pole of the switch tube. If not, check the AC input circuit and the mains rectification and filtering circuit. The on/off circuit is of the type that cuts off the AC input voltage (as shown in Figure 1), so check whether the on/off control circuit is normal.
(2) If the voltage of the c pole of the switch tube is normal, check whether the voltage of the b pole of the power switch tube is normal 0.4~0.6v at the moment of starting up. If it is 0v, it means that the starting circuit of the switching power supply is open or the related components of the switch tube b and e are broken down; Pole-related components are normal, and the fault is in the oscillating circuit (including positive feedback resistors, capacitors, discharge diodes, positive feedback windings of switching transformers and their connecting circuits).
(3) If the oscillating circuit is found to be normal, measure the output voltage of the power supply + b1 at the moment of turning on the power. If the voltmeter has a small reading for a moment and then quickly drops to zero, the fault may be in:
Pulse width or frequency control circuit (including the control circuit that relies on the conduction of the optocoupler to force the switching power supply to stop vibrating or to realize standby by weakening the oscillation. For example, the Konka "06" series color TV switching power supply belongs to this type. The output voltage is only 1/9 of that when it is turned on, so that the color TV has no sound or light);
Load short circuit (referring to parallel switching power supply, because series switching power supply will not stop vibration due to load short circuit);
The switching power supply causes the protection circuit to operate due to output overvoltage or overcurrent (including the malfunction caused by the damage of the protection circuit itself). The identification skills and steps of this fault are: Connect a 500w AC voltage regulator to the mains, connect the TV power input terminal to the output terminal of the voltage regulator, and adjust the output voltage of the voltage regulator from 100v ( monitor with a table),
And connect a 60-100w incandescent lamp (or 51ω/50w resistor) and a voltmeter in parallel to the ground at the +b1 terminal of the switching power supply. After confirming that the +b1 filter capacitor is normal, disconnect the power supply circuit of the line tube c-pole, and then try machine. If the light bulb is on (or the resistance is heating), it indicates that the power supply has output. You can measure the output voltage (referring to +b1) every time the input voltage increases by 10v. If the input voltage rises to a certain value, +b1 has exceeded the specified value. It shows that the failure of the switching power supply is caused by the action of the overvoltage protection circuit. At this time, the sampling, error amplification and pulse width (frequency) control circuits should be checked. If the bulb does not light up or the resistor does not heat up during the above debugging and monitoring process (the voltmeter has no indication), it may be that the on/standby control circuit is faulty, making the machine in the shutdown (standby) state; or the voltage stabilization system of the switching power supply is broken Abnormal, so that the machine is in a state of no output; or the protection circuit components are damaged. If it is confirmed in the above inspection that the switching power supply can output normally and the voltage stabilization performance is good, it means that the switching power supply has no output originally, which is caused by the action of the protection circuit caused by the load short circuit or overcurrent. At this time, a milliampere meter test machine can be connected in series to the c-pole circuit of the line output tube that was originally disconnected. If the current is greater than 500ma (for machines with over-current protection function, the over-current protection circuit will act at this time, that is, the ammeter will have no indication immediately), it means that the horizontal output circuit (including horizontal deflection coil, horizontal output transformer and its secondary connection) load circuit) has a short circuit. If it is found that the three faults are caused by the failure of the line scanning circuit, the line scanning circuit should be overhauled. There are two types of faults in the line scanning circuit: one is that the line output stage does not work because there is no line excitation signal (such as no signal output from the line oscillation stage or damage to the line push stage); the other is the line load (such as line deflection Coil, horizontal pillow school circuit, line output transformer and its load) or line output stage (such as line output tube, line return capacitor, etc.) caused by breakdown and short circuit.
