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I. Fault characteristics and repair of damaged capacitors on industrial control circuit boards. Faults caused by damaged capacitors are the most common in electronic devices, with electrolytic capacitors being particularly prone to damage. Damage to the capacitor is manifested as a decrease in capacitance ； Complete loss of capacity ； Leakage ； Short circuit. Capacitors serve different functions in circuits, and the faults they cause also have their own characteristics. In industrial control circuit boards, digital circuits make up the vast majority; capacitors are primarily used for power filtering, while those used for signal coupling and in oscillation circuits are fewer in number. If the electrolytic capacitor used in a switching power supply is damaged, the power supply may fail to operate properly and no voltage will be output; or the output voltage may not be filtered properly, resulting in logical errors in the circuit due to unstable voltage, which manifests as the device working intermittently or failing to start up. If the capacitor is located between the positive and negative poles of the power supply in a digital circuit, the symptoms will be the same. This is particularly evident on computer motherboards; many computers develop the problem of sometimes not starting up and sometimes starting up after being used for a few years. When the chassis is opened, it is often possible to see that the electrolytic capacitors are bulging. If these capacitors are removed and their capacitance is measured, it turns out to be much lower than the actual value. The lifespan of a capacitor is directly related to the ambient temperature; the higher the ambient temperature, the shorter the capacitor’s lifespan. This rule applies not only to electrolytic capacitors but also to other capacitors. Therefore, when looking for faulty capacitors, priority should be given to those located near heat sources, such as those next to heat sinks and high-power components; the closer they are to these heat sources, the greater the likelihood that they are damaged. I once repaired the power supply of an X-ray flaw detector; the user reported that smoke was coming from the power supply. After opening the chassis, it was found that a large capacitor with a capacity of 1000uF/350V was leaking a substance similar to oil. Upon removing it, its actual capacity was only in the range of a few dozen uF. It was also observed that this particular capacitor was the one closest to the heat sinks of the rectifier bridge, while the other capacitors, which were further away, were intact and had normal capacities. Additionally, there were cases of short circuits in the ceramic capacitor chips, and it was also found that these capacitors were located quite close to the heat-generating components. Therefore, focus should be placed on certain aspects during inspection and troubleshooting. Some capacitors have severe leakage; they can even be hot to the touch, and such capacitors must be replaced. In cases of intermittent faults during maintenance, aside from the possibility of poor contact, such faults are usually caused by damaged capacitors. Therefore, when encountering such faults, the capacitor should be checked carefully; replacing it often yields satisfactory results (of course, attention should also be paid to the quality of the capacitor – it’s best to choose a reputable brand such as Ruby or Black Diamond). II. Characteristics and identification of resistor damage: It is common to see many beginners struggling with resistors when repairing circuits, disassembling them and soldering them over and over. In fact, with enough experience, once you understand the characteristics of resistor damage, there is no need to go through such extra trouble. Resistors are the most common components in electrical devices, but they are not the ones with the highest failure rate. The most common form of resistance damage is an open circuit; an increase in resistance value is less common, while a decrease in resistance value is extremely rare. Common types include carbon film resistors, metal film resistors, wire-wound resistors, and fuse resistors. The first two types of resistors are the most widely used. Their failure patterns are such that resistors with low values (below 100Ω) and high values (above 100kΩ) have a higher failure rate, while those with intermediate values (such as a few hundred ohms to several tens of kiloohms) rarely fail ； Secondly, when low-resistance resistors are damaged, they often burn and turn black, making them easy to detect, whereas high-resistance resistors show few signs when damaged. Wire-wound resistors are generally used for high-current current limiting, with relatively low resistance values. When cylindrical wire-wound resistors burn out, some may turn black or develop peeling or cracks on their surface, while others show no signs of damage. Cement resistor is a type of wire-wound resistor; it may break when damaged, otherwise there are no visible signs. When a fusing resistor burns out, in some cases a piece of its surface may blow off, while in others there are no visible signs; however, it will never burn and turn black. Based on these characteristics, attention can be focused when checking resistors to quickly identify the damaged ones. Based on the characteristics listed above, we can first check whether there are any signs of burning on the low-resistance resistors on the circuit board. Given that in most cases, damaged resistors either result in an open circuit or an increased resistance value, and that high-resistance resistors are prone to damage, we can use a multimeter to directly measure the resistance across the high-resistance resistors on the circuit board. If the measured resistance value is higher than the nominal value, then the resistor is definitely damaged (it’s important to wait until the resistance value stabilizes before drawing a conclusion, as there may be capacitive components in parallel in the circuit, resulting in a charging/discharging process). If the measured resistance value is lower than the nominal value, then it is generally not necessary to concern oneself with it. In this way, every resistor on the circuit board is checked; even if a thousand are mistakenly identified as faulty, not a single one will be missed. III. Methods for Determining the Quality of Operational Amplifiers Determining whether an operational amplifier is of good quality or not presents a challenge for many electronics repair technicians. This is not solely due to educational levels – there are many undergraduates among them; without proper guidance they certainly won’t be able to understand it, and even with guidance it takes time before they grasp it. There’s even a graduate student who specializes in frequency conversion control, and he faces the same difficulties! I’d like to discuss this topic with everyone here, in the hope that it will be helpful to all of us. An ideal operational amplifier possesses the characteristics of \"virtual short circuit\" and \"virtual open circuit\", and these two characteristics are very useful for analyzing operational amplifier circuits used in linear applications. To ensure linear operation, the op-amp must operate in a closed-loop (negative feedback) configuration. Without negative feedback, an op-amp in open-loop amplification acts as a comparator. To determine whether a device is good or not, one should first figure out whether it is used as an amplifier or as a comparator in the circuit. As can be seen from the diagram, regardless of the type of amplifier, there is always a feedback resistor Rf. During maintenance, we can check this feedback resistor by using a multimeter to measure the resistance between the output terminal and the inverted input terminal. If this resistance is extremely high, such as several MΩ or more, it is likely that the device is intended for use as a comparator. If the resistance is low, ranging from 0Ω to a few dozen kΩ, then we should check whether there is a resistor connected between the output terminal and the inverted input terminal; if so, then the device is definitely designed for use as an amplifier. Based on the principle of short circuit in amplifiers, if an operational amplifier is working properly, the voltages at its non-inverting and inverting inputs must be equal; any difference, if it exists, is on the order of a few millivolts. Of course, in some circuits with high input impedance, the internal resistance of the multimeter can have some impact on voltage measurements, but this effect generally does not exceed 0.2V. If there is a difference of more than 0.5V, then the amplifier is definitely damaged! (I’m using a FLUKE179 multimeter.) If the device is used as a comparator, it is acceptable for the voltages at the non-inverting and inverting inputs to differ, with the non-inverting voltage being higher than the inverting voltage; in such cases, the output voltage will be close to its maximum positive value ； Co-directional voltage