Tips for troubleshooting circuit board problems[Copy link]
This post was last edited by qwqwqw2088 on 2018-11-13 08:40 1. Fault characteristics and repair of capacitor damage on industrial control circuit boards Faults caused by capacitor damage are the highest in electronic equipment, especially electrolytic capacitor damage. Capacitor damage is manifested as: 1. Capacitance decreases; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.
Capacitors play different roles in circuits, and the faults they cause also have their own characteristics. In industrial control circuit boards, digital circuits account for the vast majority, and capacitors are mostly used for power supply filtering, while capacitors used for signal coupling and oscillation circuits are relatively rare. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not oscillate and there will be no voltage output; or the output voltage filtering is not good, and the circuit will have logic confusion due to voltage instability, which will manifest as the machine working sometimes well and sometimes badly or failing to turn on. If the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the fault manifestation is the same as above. This is particularly obvious on the computer motherboard. Many computers sometimes fail to turn on after a few years of use, and sometimes can turn on. When the chassis is opened, the electrolytic capacitor can often be seen to be bulging. If the capacitor is removed and the capacity is measured, it is 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, the capacitors that are close to the heat source should be checked, such as the capacitors next to the heat sink and the high-power components. The closer to them, the greater the possibility of damage. I once repaired the power supply of an X-ray flaw detector. The user reported that smoke was coming out of the power supply. After disassembling the chassis, I found that there was a 1000uF/350V large capacitor with something like oil flowing out. After disassembling it, I measured the capacity and found that it was only a few dozen uF. It was also found that only this capacitor was closest to the heat sink of the rectifier bridge, and the others that were far away were intact and had normal capacity. 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. So when inspecting and searching, you should focus on it. Some capacitors have serious leakage, and they are even hot when touched with fingers. Such capacitors must be replaced. When inspecting and repairing a fault that is sometimes good and sometimes bad, except for the possibility of poor contact, most of the faults are generally caused by capacitor damage. So when encountering such a fault, you can focus on checking the capacitor. After replacing the capacitor, it is often a pleasant surprise (of course, you should also pay attention to the quality of the capacitor and choose a better brand, such as Ruby, Black Diamond, etc.).
2. Characteristics and identification of resistor damage It is common to see many beginners struggling with resistors when repairing circuits, disassembling and soldering them. In fact, if you have repaired many times, you don't have to go through so much trouble as long as you understand the characteristics of resistor damage.
Resistors are the most numerous components in electrical equipment, but they are not the components with the highest damage rate. The most common type of resistor damage is open circuit, while resistance increase is less common and resistance decrease is very rare. Common types include carbon film resistors, metal film resistors, wirewound resistors and fuse resistors. The first two types of resistors are the most widely used. The characteristics of their damage are: first, the damage rate of low resistance (below 100Ω) and high resistance (above 100kΩ) is higher, and the intermediate resistance (such as hundreds of ohms to tens of kiloohms) is rarely damaged; second, low resistance resistors are often burned and blackened when damaged, which is easy to find, while high resistance resistors rarely have traces when damaged. Wirewound resistors are generally used for large current limiting and have low resistance. When cylindrical wirewound resistors are burned, some will turn black or have skin peeling and cracks on the surface, while others will have no traces. Cement resistors are a type of wirewound resistors. When burned, they may break, otherwise there will be no visible traces. When the fuse resistor is burned, some will have a piece of skin blown off the surface, while others will have no traces, but they will never be burned and blackened. According to the above characteristics, you can focus on checking the resistors and quickly find the damaged resistors. Based on the characteristics listed above, we can first observe whether the low-resistance resistors on the circuit board have burnt marks. Then, based on the characteristics that most resistors are open circuit or the resistance value increases when they are damaged, and high-resistance resistors are easily damaged, we can use a multimeter to directly measure the resistance value of the high-resistance resistors on the circuit board. If the measured resistance value is larger than the nominal resistance value, then the resistor must be damaged (pay attention to wait until the resistance value display is stable before drawing a conclusion, because there may be a capacitor component in parallel in the circuit, and there is a charging and discharging process). If the measured resistance value is smaller than the nominal resistance value, generally ignore it. In this way, every resistor on the circuit board is measured, even if a thousand are "killed by mistake", not one will be missed. 3. How to judge the quality of operational amplifiers It is difficult for many electronic maintenance workers to judge the quality of operational amplifiers, not only because of their education level (there are many undergraduates under my command, and they will definitely not be able to do it if they are not taught. Even if they are taught, it will take a long time for them to understand. There is also a graduate student who specializes in frequency conversion control with his tutor, and it is also the same!). I would like to discuss this with you and hope it will be helpful to you. The ideal operational amplifier has the characteristics of "virtual short" and "virtual open", which are very useful for analyzing the operational amplifier circuits used in linear applications. In order to ensure linear application, the operational amplifier must work in a closed loop (negative feedback). If there is no negative feedback, the operational amplifier under open-loop amplification becomes a comparator. If you want to judge the quality of a device, you should first distinguish whether the device is used as an amplifier or a comparator in the circuit. According to the principle of virtual short of amplifier, if the operational amplifier works normally, the voltage of its common direction input terminal and reverse direction input terminal must be equal, even if there is a difference, it is at mV level. Of course, in some high input impedance circuits, the internal resistance of the multimeter will have some influence on the voltage test, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier must be broken! (I use FLUKE179 multimeter) If the device is used as a comparator, the common direction input terminal and reverse direction input terminal are allowed to be unequal. If the common direction voltage > reverse voltage, the output voltage is close to the maximum positive value; If the common direction voltage < reverse voltage, the output voltage is close to 0V or the maximum negative value (depending on dual power supply or single power supply). If the voltage detected does not meet this rule, the device must be broken! This way you don't have to use the substitution method or remove the chip from the circuit board to determine whether the operational amplifier is good or bad.
Fourth, a little trick for testing SMT components with a multimeter Some SMD components are very small, and it is inconvenient to use ordinary multimeter probes to test and repair them. First, it is easy to cause short circuits, and second, it is inconvenient to contact the metal part of the component pins on the circuit board coated with insulating coating. Here I will tell you a simple method that will bring a lot of convenience to the detection. Take two smallest sewing needles, (In-depth Industrial Control Maintenance Technology Column) put them close to the multimeter probe, and then take a thin copper wire from a multi-strand cable, use the thin copper wire to tie the probe and sewing needle together, and then solder them with solder. In this way, when using a probe with a fine needle tip to test those SMT components, there is no need to worry about short circuits, and the needle tip can pierce the insulating coating and directly hit the key parts, so you don’t have to bother to scrape those membranes. 5. Inspection and repair methods for short circuit of common power supply of circuit board During circuit board maintenance, if you encounter a short circuit of common power supply, you will often have a headache, because many devices share the same power supply, and every device using this power supply is suspected of short circuit. If there are not many components on the board, you can eventually find the short circuit point by "hoeing the earth". If there are too many components, whether "hoeing the earth" can find the situation depends on luck. Here I recommend a more effective method. Using this method, you can get twice the result with half the effort, and you can often find the fault point quickly. You need a power supply with adjustable voltage and current, voltage 0-30V, current 0-3A, this power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the device power supply voltage level, first adjust the current to the minimum, add this voltage to the power supply voltage point of the circuit, such as the 5V and 0V terminals of the 74 series chip, and slowly increase the current depending on the degree of short circuit. Touch the device with your hand. When you feel that a device is obviously hot, this is often a damaged component. You can remove it for further measurement and confirmation. Of course, the voltage must not exceed the working voltage of the device during operation, and it cannot be connected in reverse, otherwise other good devices will be burned. Six, a small eraser solves big problems There are more and more boards used in industrial control, and many boards use the method of inserting gold fingers into the slot. Due to the harsh industrial site environment, dusty, humid, and corrosive gas environments can easily cause poor contact failures on the boards. Many friends may solve the problem by replacing the boards, but the cost of purchasing boards is very considerable, especially for some imported equipment boards. In fact, you might as well use an eraser to rub the gold finger repeatedly to clean the dirt on the gold finger, and then try the machine. Maybe the problem will be solved! The method is simple and practical. VII. Analysis of intermittent electrical faults In terms of probability, various intermittent electrical faults probably include the following situations: 1. Poor contact Poor contact between the board and the slot, broken cables, poor contact between the plug and the terminal block, and poor soldering of components all belong to this category; 2. Signal interference For digital circuits, faults will only appear under specific conditions. It is possible that the interference is too great and affects the control system, causing it to go wrong. There are also cases where the parameters of individual components of the circuit board or the overall performance parameters have changed, causing the anti-interference ability to approach the critical point, resulting in faults; 3. Poor thermal stability of components From a large number of maintenance practices, the first choice is the poor thermal stability of electrolytic capacitors, followed by other capacitors, transistors, diodes, ICs, resistors, etc.; 4. There is moisture and dust on the circuit board. Moisture and dust can conduct electricity and have a resistance effect. In addition, the resistance value will change during thermal expansion and contraction. This resistance value will have a parallel effect with other components. When this effect is strong, it will change the circuit parameters and cause a fault; 5. Software is also a factor to consider Many parameters in the circuit are adjusted using software. The margin of some parameters is adjusted too low and is in a critical range. When the machine's operating conditions meet the reasons for the software to determine the fault, an alarm will appear.
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