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速修a3机电源(Quick fix A3 power supply)

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速修a3机电源(Quick fix A3 power supply)速修a3机电源(Quick fix A3 power supply) 速修a3机电源(Quick fix A3 power supply) Short circuit, quick repair, A3 movement, power supply voltage is too high, fault For the fault that the output voltage of the switching power supply is too high, the usual maintenance me...

速修a3机电源(Quick fix A3 power supply)
速修a3机电源(Quick fix A3 power supply) 速修a3机电源(Quick fix A3 power supply) Short circuit, quick repair, A3 movement, power supply voltage is too high, fault For the fault that the output voltage of the switching power supply is too high, the usual maintenance methods are to find the fault components by the circuit resistance method and the open circuit resistance method. Because of the limitation of the resistance method, the components have to be removed and measured, which not only affects the speed of inspection and repair, but also makes the circuit board welded beyond recognition, and even further expands the fault. Here, a rapid and effective method of overhaul is summarized in many years of overhaul practice: short circuit method. This method requires only a short route without damaging a large number of components to minimize the damage to the circuit board. In addition, this method is to step by step to make the switch stop vibration or to make the main output voltage drop, so it is more safe to overhaul. Combined with the principle of this power supply, we can sum up a set of fast, accurate, very practical inspection methods to overhaul the main voltage output of this kind of model, and we can find out the damaged components in a few steps. First, the side of the photosensitive tube of the shorted optocoupler (i.e., the 3 and 4 pins of the optocoupler) is the most effective method of determining whether the fault is on the primary or secondary side of the switch transformer. After short circuit, if the main voltage output drops very low or the power supply fails to oscillate, it indicates that the primary side is normal, the problem is on the secondary side, otherwise the primary side has a fault, and the key point is to check the B pole circuit of the switch tube. Second: the error amplifier tube V553 C direct grounding, can judge the VD515 and R555 is normal, if the main power supply output voltage is low or non vibrating, indicating that the two element is normal, otherwise there must be a component damage. Second: short-circuit VD561, to determine whether V553 and R552 are damaged. If the main voltage is very low or the power supply is not vibrating, it indicates that the V553 and R552 are normal, otherwise one of the components will be damaged. Although rarely meet the primary side element damage caused by fault of the main voltage increasing, but from the power supply circuit working principle is not difficult to see, V511 and V512 CE short circuit, power supply will stop vibrating, the main voltage will have no output. Through the above steps, the secondary damage components can be found, eliminating the trouble of disassembling the components, and no damage to the circuit board. A3 power supply overhaul experience The common faults of A3 power supply are as follows: 1. Low output voltage. 2 、 high output voltage. 3 、 no output voltage. 4 、 burning switch tube. First, the low output voltage overhaul There are four cases of low output voltage: 1) the sampling circuit is faulty; 2) the load is short circuited; 3) the V511 of the error amplifying tube and the base of the switch tube are connected with the shunt tube V512, and the quiescent current is large; 4) the mains voltage is low. For the low output voltage fault, the key test points are: the voltage regulator and the photoelectric coupler contain the voltage drop at the two ends of the led. 1. When measuring the voltage drop across the diode, the result is 1V, or even greater than 1V. Indicates that the sampling circuit is faulty. If the base bias resistor of the sampling triode is open, the emitter is connected to a zener diode to short-circuit. When the output voltage is low, the sampling circuit is wrongly adjusted. 2. When measuring the voltage drop across the diode, the result is 0V. This shows that the sampling circuit reflects the low output voltage to the photoelectric coupler. Check the V512 base voltage, such as positive, indicating that the V512 base voltage does not meet the detection of low output voltage, when normal, this voltage should be negative pressure. There are two possibilities for a V512 base to be a positive voltage: 1) the voltage regulator power failure itself; 2) the load has the short-circuit position. The V512 base generates negative voltage is not normal, the reason is the base of the VD516 open circuit, short circuit, or a resistor in series with the VD516 increase the resistance (some machines using small goods, 1/8W is very easy to change, I encountered the resistance value, the maintenance time into a mess. What a lesson! In addition, after the secondary rectifier, the output load is short circuited (after the secondary load is short circuited, the negative pulse of the switching tube will be shorted, so that the base of the V512 has no negative voltage) At this point, the +B voltage is about 70V. You can disconnect the load and confirm it. For VD516 and the series resistance value increases with it, Measure its resistance in the road, you can determine whether damage. 3, with poor load capacity. Show low brightness image of normal image, high brightness +B contraction, reduce the voltage, voltage and photoelectric coupler light emitting diode voltage drop 0V, 512 base 2V above, this shows that the V511 output of the negative voltage V512 as required. At this point the output voltage is low due to the excessive quiescent current of the V512 and the abnormal shunting of the switch. When the base of the error amplifying tube V511, the base resistance, the open circuit or the resistance increase, the capacity of the C515 becomes smaller, and the resistance of the R523 or R526 becomes smaller, which can cause such a fault. But note: do not arbitrarily reduce the base of the switch current limiting resistor, reducing the base of the switch current limiting resistor will cause switching tube over excitation, otherwise it will burn the switch tube irregularly. 4, if it is found that the voltage users low low for a long time, the voltage detection optocoupler led down 0V, and the base for V512 - 1V ~ - 2V, indicating the normal voltage control circuit, a failure to switch the beta value or low switch base limiting resistor increases. A base switch current limiting resistor that can be replaced by a switch tube or appropriately reduced. But you can't use small resistance. Otherwise, days after the switch of the hidden danger of burning. Summing up the above situation, when the output voltage is low, first of all, photoelectric coupler, both ends of the voltage drop. 1) if sampling circuit is more than 1V, check circuit. 2) such as optocoupler, both ends of the light-emitting diode voltage drop to 0V, V512 base voltage is positive, the +B voltage drops to about 70V, then the load is short circuited or VD516 open or short circuited. 3) with poor load capacity and abnormal shunting action of V512, the bias circuit that causes the increase of V512 base current is examined. 4) the low voltage of the city leads to the reduction of the +B voltage, and checks the beta value of the switch and the base current limiting resistor. Note: the V512 and C515 beta, R526 C515 and capacitance V512 is determined by the dynamic work point. When the beta value of V512 is large, the capacity requirement of C515 is more appropriate, otherwise it is smaller. Note: some models, the capacity of C515 by 0.015 F reduced to 0.012 F, while R526 decreased from 2.7K to 2.2K, the R517 value decreased from 2.7K to 560 ohm; a limiter circuit and some models do not install VD519, the switch excitation current limiting resistor is changed to 82. The existence of these two conditions makes the lower voltage range of the circuit very unsatisfactory and should be noticed. Two, the power supply does not shake the maintenance First, check whether the starting resistor is open or the resistance is increased, and the positive feedback capacitor C514 (0.1 F) is connected between the feedback winding and the base of the switch tube When the above factors are excluded, a 60W/220V bulb can be connected to the back of the 300V filter capacitor, and then V511 and V512 are switched off one by one. 1 、 after disconnecting the V511, it can start, check whether the C, E pole of the V511 is short circuited, or whether the capacity of C515 is reduced. 2 、 after disconnecting the V512, it can start, check whether the capacity of C515 is reduced and whether the peripheral components of V512 are short circuited. 3 、 after disconnecting the V512, it still doesn't start. It may be the reverse peak absorption circuit, the C516 short circuit, the small beta value of the switch tube and the short circuit of the switch transformer. This method is used to check the low value of the switch tube and the short-circuit of the switch transformer. It is difficult to judge the short-circuit of the switch transformer accurately, but it can be used to judge whether the transformer is short circuited. Three 、 high maintenance of output voltage The output voltage is high, the fault is obviously voltage stabilized, and the negative feedback circuit is faulty. Safe and quick way to check is: A 100W/220V light bulb is connected to the back of the 300V filter capacitor. At this time do not have to pay attention to the level of output voltage B. Then, use tweezers to short-circuit the C and E poles of the optocoupler, and observe whether the + B voltage is decreased. 1) B voltage no change or change very little. The V511 and V512 circuits are not controlled. Check whether the two circuits fail and the capacity of the C515 is too large. 2) B voltage change. Explain the fault in the V533 sampling circuit, check the photoelectric coupler, the sampling amplifier V533, the upper bias resistor, the zener diode is open. When checking this fault, don't make any confusion about RP551, Otherwise, it is easy to misjudge in the overhaul or the output voltage is abnormal after troubleshooting. Four 、 troubleshooting of fire switch tube When the output voltage is normal, the switch can be checked: 1) check whether the anti peak absorption circuit element is open; 2) check that the voltage stabilizing circuit is in bad contact; 3) is the limiting current resistance of the switch tube too small?; 4) check whether the beta value of V512 is too small, the capacity of C515 is too large, and whether the VD519 limiting circuit is normal or not; 5) measuring whether the user's mains voltage is out of order for a long time. In addition, R524, R512, C515, VD519 abnormal, will cause voltage range, the upper and lower limits narrowed, thus leading to a fever switch tube.
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