1. Maintenance methods and steps of switching power supply without output
(1) Check whether there is 300v DC working voltage at the C pole of the switch tube; Otherwise, check the AC input circuit and the power rectifier filter circuit. The on/off circuit is a 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 at the C pole of the switch tube is normal, measure whether the voltage at the B pole of the power switch tube is normal at the start-up moment by 0.4 ~ 0.6V. If it is 0v, it means that the starting circuit of the switching power supply is open or the related elements of the B and E poles of the switching tube are broken down; If the voltage at the B-pole of the switch tube is normal at the moment of starting, but then drops to 0v, it means that the starting circuits and related components at the B-pole and E-pole of the switch tube are normal, and the fault lies in the oscillation circuit (including positive feedback resistor, capacitor, discharge diode, positive feedback winding of the switch transformer and its connection circuit).
(3) If the oscillating circuit is normal, measure the output voltage of the power supply +B 1 at the starting moment. If the voltmeter reads very little at this time and then drops to zero quickly, the fault may be:
1) pulse width or frequency control circuit (including the control circuit that relies on optocoupler conduction to force the switching power supply to stop oscillating or relies on weakening oscillation to realize standby). For example, Konka's "06" series color TV switching power supply belongs to this type. When the switching power supply is turned on, the output voltage is only 1/9, which makes the color TV sound and light completely absent.
2) Load short circuit (refers to parallel switching power supply, because series switching power supply will not stop vibration due to load short circuit);
3) Switching power supply protection circuit action caused by output overvoltage or overcurrent (including misoperation caused by component damage of the protection circuit itself). The skills and steps to identify this fault are as follows: connect a 500w AC voltage regulator to the mains, connect the input end of the TV power supply to the output end of the voltage regulator, and adjust the output voltage of the voltage regulator from 100v (monitored by an electric meter) to 100v.
A 60 ~ 100 W incandescent lamp (or a 51Ω/50w resistor) and a voltmeter are connected in parallel to the +B 1 terminal of the switching power supply. After confirming that the filter capacitor of +B 1 is normal, disconnect the power circuit of the exhaust pipe C, and then start debugging. If the light bulb is on (or the resistor is hot), it means that the power supply has output, and the output voltage can be measured every time the input voltage increases by 10v (meaning +B 1). If the input voltage rises to a certain value, +B 1 has exceeded the specified value, indicating that the switching power supply does not work because of the overvoltage protection circuit. At this time, the sampling, error amplification and pulse width (frequency) control circuits should be checked. If during the above debugging and monitoring, the light bulb does not light up or the resistor does not heat up (the voltmeter does not indicate), it may be that the on/standby control circuit is out of order, which makes the machine in the off (standby) state; Or the switching power supply voltage stabilizing system is abnormal, which makes the machine in a state of no output; Or to protect circuit elements from damage. If it is confirmed in the above tests that the switching power supply can output normally and the voltage stability 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 short circuit or overcurrent of the load. At this time, a milliammeter can be connected in series with the C-pole circuit of the original disconnected row output tube. If the current is greater than 500ma (for the machine with overcurrent protection function, the overcurrent protection circuit will act at this time, that is, the ammeter will not indicate immediately), it means that the row output circuit (including the row deflection coil, the row output transformer and the load circuit connected to its secondary stage) is short-circuited. If it is found that all three faults are caused by the faults of the line scanning circuit, the line scanning circuit should be overhauled. There are two kinds of faults in the line scanning circuit: first, the line output stage does not work because there is no line excitation signal (such as the line oscillation stage has no signal output or the line push stage is damaged); 2. Line load (such as line deflection coil, horizontal pincushion circuit, line output transformer and its load) or line output stage (such as line output tube and line flyback capacitor). ) is caused by breakdown short circuit.
2. Maintenance steps for the line output stage not working or working abnormally.
(1) Measure whether there is a voltage of -0. 1 ~-0.25 8~9v at the B pole of the line output tube. If there is no voltage, measure the line oscillation level of related ic (such as ta7698{33} pin, la7680{25} pin, tda8362{36} pin and TA8659/).
(2) If it is found that the power supply of the line oscillating circuit is normal, check whether there is a line excitation signal waveform (or a normal DC voltage of 0.45 ~ 0.55 V) at the B pole of the line pusher. If there is no excitation signal waveform or DC voltage is 0v, it may be that the line oscillation circuit does not oscillate or the pre-push stage inside the ic is damaged, resulting in no output, or it may be that the B-pole loop of the line push tube is open or short-circuited. If it is the former, the row excitation signal output terminal of ic generally has no excitation signal output; As for the latter, it can be confirmed by detecting the resistance of the circuit. If the voltage at the B pole of the row push tube is above 0.6v, it means that the row oscillator does not oscillate.
(3) If it is confirmed that the line oscillation circuit does not work, first consider whether the X-ray protection circuit works. The voltage of X-ray terminals (such as ta7698{30} pin and TA8659/8759 {52} pin) of ic is usually 0v. If it is above 1.2v, it may be that the beam current is too large or the line retrace pulse is too large, which leads to the protection circuit action. Check whether the power supply voltage of +B 1 is too high, whether the flyback capacitor fails, whether the brightness control circuit is abnormal, and whether the high-pressure nozzle is seriously dirty.
(4) If it is detected that the X-ray protection circuit does not work, the replacement method should be adopted to check the rc timing element of the linear oscillation circuit or the 500khz crystal oscillator. If it is invalid, it can basically be considered as ic damage.
(5) If the measured row excitation signal has reached the B pole of the row push tube, but there is no excitation signal at the B pole of the row output tube, it means that the B pole loop of the row push stage or the row output tube is open or short-circuited.
Road failure. Test whether there is voltage at the C pole of the push tube. If there is no voltage, most of them are open circuit of C current-limiting resistor or cracking of primary winding solder joint of row-push transformer, and a few are breakdown of C and E poles of push tube or short circuit of damping capacitor connected to primary of row-push transformer. If there is no voltage drop across the current limiting resistor of the push-pull tube C (that is, the resistor is not heated), it can basically be considered as an open circuit of the push-pull tube. (6) If it is measured that the row driver stage works normally, it is obvious that the B-pole loop of the row output tube has an open circuit or a short circuit fault. Resistance, capacitance, inductance and other components of B-pole circuit (including its solder joints, copper foil wiring, etc. ) should be carefully checked.
(7) If the excitation signal of the tested line has been added to the line output tube, test whether there is a voltage of+b1(105 ~150 V) at the C pole. Otherwise, check the +B 1 power supply circuit. If the machine is turned on/off by turning off +B 1, the on/off circuit must be tested. However, it must be noted that if the resistance of the C electrode of the row tube to the ground is 0 Ω, the breakdown of the row tube or the row flyback capacitor will not generate voltage.
(8) If the line output stage is working, but it is abnormal (such as "squeaking" sound, serious overheating of the line output tube, and obvious drop of the voltage at the C pole of the line output tube, indicating that the line load is seriously short-circuited), the following methods can be used to determine:
1) connect a 500ma ammeter in series in the power circuit of the row output tube C. Unplug the plug of the row deflection coil. If there is a pincushion correction circuit, disconnect the pincushion correction circuit (vd408 can be disconnected, as shown in Figure 3) and disconnect all the row output transformers except the two pins of the power circuit of the row output tube C. ..
2) Observe the reading of ammeter when starting. The normal value is about 40 ~ 70ma. If it is greater than 70 mA and less than 120ma, it means that the output transformer has serious loss and radiation, so it is not suitable for use. If it is greater than 120ma, it means that the line output transformer has been short-circuited and must be replaced.
3) If the reading of the ammeter is within the normal range after starting, the deflection coil and the pincushion calibration circuit can be connected in turn. If the current value increases to more than 500ma immediately after switching on a load, it means that the load has a short circuit.
4) If the ammeter reading does not exceed 200ma after the above load is connected, it means that the short circuit fault is in the secondary load loop of the line output transformer. Loads can be recovered one by one. If the ammeter reading obviously increases to more than 500ma after a certain load is restored, it means that the load (or rectifier and filter system) is short-circuited and needs to be checked and eliminated. Third, troubleshooting examples? [Example 1]? A Konka t2587h color TV has three faults, and the machine has a weak "squeak" sound. From the analysis of fault phenomena, the possibility of short circuit of line load is greater. A 500ma ammeter was connected in series with the C-pole loop of the line output tube for debugging. It was found that the pointer of the ammeter "hit the meter", and then the plug of the line deflection coil was unplugged, and the current dropped to 450ma. Obviously, the fault point was not in the line deflection coil. Then, after disconnecting the isolation diode vd408 (see Figure 3), the current drops to 125ma. Obviously, the fault lies in the pincushion circuit. First, vd408 was detected and found to be faulty. After replacement, the fault is eliminated. ? [example 2]? A Jiali EC-2583m color TV is trouble-free. According to the investigation, the terminal voltage (130v) of +B 1 has dropped to 120v, and the temperature of the output tube of this row rises rapidly, so it is very possible to analyze the short circuit of the discharge load. An ammeter is connected in series on the C-pole loop of the line output tube, and it is found that the current reaches1.2a. After the line deflection coil is pulled out, the current is still1a.. Disconnect the line again.