Consult the maintenance method of switching power supply without 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 of the c-pole of the switch tube is normal, measure the power switch at the moment of starting up;
(3) If the oscillating circuit is found to be normal, test the power supply + b1 again at the moment of power-on; 1) Pulse width or frequency control circuit (including relying on optocoupler conduction to force open; 2) Load short circuit (refers to parallel switching power supply, due to series connection Type switching power supply; 3) The switching power supply causes the protection circuit to operate due to output overvoltage or overcurrent.
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:
1) 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 of the switching power supply is only 1/9 of that when it is turned on, so that the color TV has no sound or light);
2) Load short circuit (referring to parallel switching power supply, because series switching power supply will not stop vibration due to load short circuit);
3) 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.
2. Troubleshooting steps for row output stage not working or working abnormally
(1) Measure whether there is a voltage of -0.1~-0.25v on the b pole of the row output tube. If not, measure the row oscillation level of the relevant IC (such as ta7698{33} pin, la7680{25} pin, tda8362{36} pin Pin, ta8659/8759{40} pin) Is there a normal 8~9v input voltage, if not, check the power supply circuit.
(2) If it is found that the power supply of the row oscillating circuit is normal, then check whether there is a row excitation signal waveform (or normal 0.45~0.55v DC voltage) at the b pole of the row push tube. If there is no excitation signal waveform or the DC voltage is 0v, it may be that the horizontal oscillation circuit is not oscillating or the pre-push stage inside the ic is damaged, resulting in no output, or there may be an open circuit or short circuit in the b-pole circuit of the horizontal push tube. If it is the former, there is generally no excitation signal output at the row excitation signal output terminal of IC; as for the latter, it can be confirmed by the resistance value of the detection circuit. If the voltage of the b-pole of the row push tube reaches 0.6v or more, it means that the row oscillator is not oscillating.
(3) If it is confirmed that the horizontal oscillating circuit is not working, first consider whether the X-ray protection circuit operates. Detect the voltage of the x-ray terminal (such as ta7698{30} pin, ta8659/8759{52} pin) of the relevant IC. Normally, it is 0v. If it is above 1.2v, it may be that the beam current is too large or the reverse pulse If it is too high, the protection circuit will operate. Check whether the +b1 power supply voltage is too high, whether the reverse capacitor fails, whether the brightness control circuit is abnormal, whether the high-voltage nozzle is seriously dirty, etc.
(4) If it is measured that the X-ray protection circuit does not operate, the rc timing element of the horizontal oscillation circuit or the 500khz crystal oscillator should be checked by replacement method. If it is invalid, it can basically be determined that the ic is damaged.
(5) If it is measured that the line excitation signal has reached the b pole of the line push tube, but there is no excitation signal at the b pole of the line output tube, it indicates that there is an open circuit or short circuit fault in the line drive stage or the b pole circuit of the line output tube. Check whether there is voltage at the c pole of the push tube. If there is no voltage, most of the current limiting resistors of c are open or the solder joints of the primary winding of the row push transformer are cracked. If it is measured that there is no voltage drop across the limiting current resistance of the row push tube c (that is, the resistor does not generate heat), it can basically be determined that the row push tube is open.
(6) If it is measured that the row push stage works normally, it is obvious that there is an open circuit or short circuit fault in the b-pole circuit of the row output tube. The resistance, capacitance, inductance and other components of the b-pole circuit (including its solder joints, copper foil wiring, etc.) should be carefully checked.
(7) If it is measured that the line excitation signal has been added to the line output tube, then measure whether there is +b1 (105~150v) voltage at the c pole, if not, check the +b1 power supply circuit. If the on/standby of the machine is realized by cutting off +b1, the on/standby circuit must be tested. However, it must be noted that if the measured resistance of the row tube C pole to ground is 0ω, the no voltage is caused by the breakdown of the row tube or the reverse travel capacitor.
