01 Capacitor failure

　　The failure caused by capacitor damage is the highest in electronic equipment, especially the damage of electrolytic capacitors is the most common.

　　Capacitor damage is manifested as: the capacity becomes smaller, the capacity is completely lost, leakage, and short circuit.

　　The functions of capacitors in the circuit are different, and the faults caused by them also have their own characteristics: in industrial control circuit boards, digital circuits account for the vast majority, capacitors are mostly used for power supply filtering, and there are fewer capacitors used for signal coupling and oscillation circuits. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate and have no voltage output;

　　Or the output voltage filtering is not good, and the logic of the circuit is disordered due to the unstable voltage, which means that the machine works good or bad or cannot be turned on.

　　This is especially obvious on the computer motherboard. Many computers have been used for several years, sometimes they can't be turned on, and sometimes they can be turned on again. When you open the case, you can often see the phenomenon of bulging electrolytic capacitors. If you remove the capacitors, measure the capacity. , found to be much lower than the actual value.

　　The life of the capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the life of the capacitor. This rule applies not only to electrolytic capacitors, but also to other capacitors.

　　Therefore, when looking for faulty capacitors, we should focus on checking the capacitors that are close to the heat source, such as the capacitors next to the heat sink and high-power components. The closer it is, the greater the possibility of damage. Therefore, attention should be paid to the maintenance and search.

　　Some capacitors have serious leakage and even burn your hands when touched with your fingers. Such capacitors must be replaced. In the case of good and bad faults during maintenance, except for the possibility of poor contact, most of the faults are generally caused by capacitor damage. Therefore, when encountering such failures, you can focus on checking the capacitors. After replacing the capacitors, it is often surprising.

　　02 Resistor fault

　　PCB factory often sees many beginners tossing on the resistor when overhauling the circuit, dismantling and soldering. In fact, it is repaired a lot. As long as you understand the damage characteristics of the resistor, you don't have to spend a lot of time.

　　Resistors are the most numerous components in electrical equipment, but not the ones with the highest damage rate. The open circuit is the most common damage to the resistance, and it is rare for the resistance value to increase, and it is very rare for the resistance value to decrease. Commonly used are carbon film resistors, metal film resistors, wirewound resistors and fuse resistors.

　　The first two types of resistors are the most widely used, and their damage characteristics are that the damage rates of low resistance (below 100Ω) and high resistance (above 100kΩ) are relatively high, and the intermediate resistance (such as hundreds of ohms to tens of thousands of ohms) Very little damage; Second, when the low-resistance resistor is damaged, it is often burnt and blackened, which is easy to find, while the high-resistance resistor is rarely damaged.

　　Wirewound resistors are generally used for high current limiting, and the resistance value is not large; when cylindrical wirewound resistors burn out, some will turn black or the surface will explode, crack, and some have no traces;

　　Cement resistors are a kind of wire wound resistors, which may break when burned out, otherwise there will be no visible traces; when the fuse resistors are burned out, some surfaces will blow up a piece of skin, and some have no traces, but they will never burn. blackened.

　　According to the above characteristics, you can focus on checking the resistance and quickly find the damaged resistance.

　　According to the characteristics listed above, we can first observe whether the low-resistance resistors on the circuit board have any traces of burning black, and then according to the characteristics of most open circuits or increased resistance when the resistance is damaged, and the high-resistance resistance is easily damaged, We can use a multimeter to directly measure the resistance at both ends of the high-resistance resistor on the circuit board.

　　If the measured resistance value is larger than the nominal resistance value, the resistance must be damaged (pay attention to wait for the resistance value to display stable before drawing a conclusion, because there may be parallel capacitive elements in the circuit, and there is a charging and discharging process), if the measured resistance value is If the value is smaller than the nominal resistance value, it is generally ignored.

　　In this way, every resistor on the circuit board is measured again, even if one thousand is "killed by mistake", one will not be missed.

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