First, the fault characteristics and maintenance of industrial control circuit board capacitor damage The failure caused by the damage of the capacitor is the highest among the electronic devices, and in particular, the damage of the electrolytic capacitor is the most common. Capacitor damage is manifested as: 1. The capacity becomes smaller; 2. The capacity is completely lost; 3. The leakage; 4. The short circuit. The role of capacitors in the circuit is different, and the resulting faults have their own characteristics. In the industrial control circuit board, the digital circuit occupies the vast majority, and the capacitor is used for power supply filtering. The signal coupling and the oscillation circuit have less capacitance. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not start-up and no voltage output is provided; or the output voltage may not be well filtered, and the circuit may be logically chaotic due to voltage instability, which may manifest itself as good or bad operation of the machine. No machine, if the capacitor is in the digital circuit between the positive and negative power supply, the fault is the same as above. This is particularly evident on computer motherboards. Many computers use the phenomenon that sometimes they cannot be turned on for a few years, and sometimes they can be turned on. When the chassis is opened, the phenomenon of bulging of electrolytic capacitors can often be seen. If the capacitor is removed, the amount of capacity is reduced. , found to be much lower than the actual value. The life of a capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the life of the capacitor. Often see a lot of beginners in the repair of the circuit torsion in the resistance, but also demolition and welding, in fact, much more repair, as long as you understand the characteristics of the damage of the resistor, you do not have to spend weeks. Resistance is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. Resistor damage is most common with open circuits, where resistance becomes larger and rarer, and resistance becomes smaller. Common carbon film resistors, metal film resistors, wire wound resistors and insurance resistors several. The first two types of resistors are the most widely used, and their damage is characterized by low damage (100Ω or less) and high resistance (100kΩ or more), high damage rate, and intermediate resistance (such as several hundred ohms to several tens of kiloohms). Very little damage; Second, low-resistance resistors are often charred and black when damaged, and they are easy to find, but there are few traces of high-value resistors damaged. Wirewound resistors are generally used as high-current limiting resistors with little resistance. Some of the cylindrical wire wound around the resistor burns black or the surface bursts, cracks, and some without traces. Cement resistance is a type of wirewound resistor that can break when burned, otherwise there are no visible traces. Some parts of the surface of the insurance resistor will blow up a piece of skin, and some will have no trace, but they will never burn black. According to the above characteristics, it is possible to focus on the resistance when checking, and quickly find out the damaged resistance. According to the above-listed characteristics, we can first observe whether the low-resistance resistor on the circuit board has burned black traces, and then according to the characteristics of most open circuits or resistances when the resistance is damaged, and the high-value resistance is easily damaged, We can use the multimeter to directly measure the resistance of the high-resistance resistor on the circuit board. If the measured resistance is greater than the nominal resistance, the resistor must be damaged (pay attention to the stability of the resistance display only In conclusion, there is a charge and discharge process because there is a possibility that the capacitor is shunted in the circuit. If the measured resistance is smaller than the nominal resistance, it is generally ignored. In this way, each resistance on the circuit board is measured again. Even if one "wrongly kills" one thousand, it will not miss one. The discrimination of operational amplifiers is difficult for a large number of electronic maintainers. It is not only the relationship between education level (under the tutelage of many undergraduates, certainly not if you do not teach, but it takes a long time to teach, and there is a special The tutor who learns the frequency conversion control of graduate students is actually the same!), I would like to discuss with you here and hope to help everyone. Ideal operational amplifiers have "virtual short" and "virtual-off" characteristics. These two characteristics are useful for analyzing linearly-used op-amp circuits. To ensure linear operation, the op amp must work in a closed loop (negative feedback). If there is no negative feedback, the amplifier under open loop amplification becomes a comparator. If you want to determine the quality of the device, you should first distinguish whether the device is used as an amplifier or a comparator in the circuit. According to the principle of the amplifier short circuit, that is, if the op amp is working properly, the voltages of the same input and reverse input must be equal, even if the difference is also mv class, of course, in some high input impedance circuit, multimeter The internal resistance will have a little impact on the voltage test, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier will be absolutely bad! (I'm using FLUKE179 multimeter) If the device is used as a comparator, allowing the same direction input and inverting input range, the same direction voltage> reverse voltage, the output voltage is close to the positive maximum; the same voltage <the reverse voltage, the output voltage is close to 0V or negative maximum (depends on dual power supply or single power supply). If it is detected that the voltage does not comply with this rule, then the device must be no doubt! This way, you can use the substitution method instead of removing the chip on the circuit board to judge whether the operational amplifier is good or bad. Some patch components are very small, and it is inconvenient to test and repair them with an ordinary multimeter. The first is that they are likely to cause short circuits. The second is that the circuit board coated with an insulating coating is inconvenient to touch the metal parts of the component pins. Here to tell you an easy way, will bring a lot of convenience to the test. Take the two smallest sewing needles, (deep industrial maintenance and technical column) close it with the multimeter pen, and then take a thin strand of copper cable, use fine copper wire to attach the table pens and sewing needles. Together, soldering is used again. In this way, when the SMT components are tested with the tip of a small needle tip, there is no short circuit, and the tip can pierce the insulating coating and go straight to critical parts, so there is no need to bother to scrape the films. In the circuit board maintenance, if the common power supply is short-circuited, the failure is often a big problem because many devices share the same power supply. Each device that uses this power supply is suspected of being short-circuited. If there are not many components on the board, the “power supply†is used. After all, the short-circuit point can be found. If there are too many components, then it is up to luck to see whether or not the situation can be solved. In this case, a method of comparison is recommended. Using this method, you can find fault points quickly and often. To have a voltage and current are adjustable power supply, voltage 0-30V, current 0-3A, this power is not expensive, 300 yuan. The open circuit voltage is adjusted to the device power supply voltage level, the current is first adjusted to the minimum, this voltage is added to the circuit power supply voltage point, such as 74 series chip 5V and 0V side, depending on the degree of short circuit, slowly increase the current, with Hand touch the device, when touched a device obviously heat, this is often a damaged component, which can be removed for further measurement and confirmation. Of course, the voltage during operation must not exceed the operating voltage of the device and must not be reversed, otherwise it will burn other good devices. There are more and more boards used in industrial control. Many cards use gold fingers to insert slots. Due to the harsh industrial environment, dusty, humid, and multi-corrosive gas environments can easily cause poor contact failures on boards. Friends may have solved the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some imported equipment. In fact, you may wish to use an eraser to rub a few times on the golden finger, clean up the dirt on the gold finger, and then try the machine, which may solve the problem! The method is simple and practical. Various kinds of good and bad electrical faults may include the following situations in terms of probability: 1. Poor contact between the card and the slot, the internal disconnection of the cable, the disconnection of the line plug, the terminal plug and the terminal contact, and the component's imaginary welding. 2. The signal is disturbed For digital circuits, under certain conditions, the fault will be presented. It may be that the interference is too great to affect the control system to make mistakes, and there are circuit board individual component parameters or overall performance parameters. Change, so that anti-jamming capability tends to be a critical point, resulting in failure; 3. Thermal stability of components is not good From a lot of maintenance practices, the thermal stability of the first electrolytic capacitor is not good, followed by other capacitors, transistors, diodes, ICs, resistors, etc.; 4. The circuit board has moisture, dust and so on. Moisture and dust will conduct electricity, have resistance effect, and the resistance will change during the process of thermal expansion and contraction. This resistance value will have parallel effect with other components. When this effect is stronger, it will change the circuit parameters and cause failure. Occurrence; 5. Software is also one of the factors in the circuit to use many parameters to adjust the software, some parameters of the margin is adjusted too low, in the critical range, when the machine running conditions meet the software to determine the reasons for the failure, then the alarm Will appear. 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This rule applies not only to electrolytic capacitors but also to other capacitors. Therefore, when looking for a faulty capacitor, it is important to check the capacitance that is close to the heat source, such as the capacitance beside the heat sink and high-power components. The closer to this, the greater the possibility of damage. After repairing the power of an X-ray flaw detector, the user reported that smoke had emerged from the power supply. After disassembling the chassis, a 1000uF/350V large capacitor was found to have oily material flowing out. A volume was removed. Only a few tens of UF, it was found that only this capacitor and the heat sink of the rectifier bridge are the closest ones. Others are far away and the capacity is normal. In addition, there was a short circuit in the ceramic capacitor, and it was also found that the capacitor was relatively close to the heating component. Therefore, it should be emphasized in the inspection and search. Some capacitors have more serious leakage and may even be hot when touched by a finger. This capacitor must be replaced. In the case of faults that are good or bad during inspections, the possibility of poor contact is ruled out, and most of them are generally faults caused by damage to the capacitors. So in the event of such failure, you can focus on the capacitor check, replace the capacitor is often surprising (of course, we must pay attention to the quality of the capacitor, to choose a little better brand, such as ruby, black diamond and the like).
Second, the characteristics and discrimination of resistance damage 
Third, the merits and demerits of operational amplifiers
Four, multimeter test SMT components a small trick
Fifth, the circuit board public power short circuit fault repair method
Sixth, a small eraser to solve big problems
Seven, good and bad electrical fault analysis
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