8 tips for PCB repair, how many do you know?

时间2017/12/15 15:37:49
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8 tips for PCB repair, how many do you know?

First, the fault characteristics and maintenance of industrial control circuit board capacitor damage


Failures caused by capacitor damage are the highest in electronic equipment, and damage to electrolytic capacitors is the most common.


The performance of capacitor damage is as follows: 1. Capacity becomes smaller; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.


Capacitors play different roles in the circuit, and the faults they cause 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, and less capacitors are 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 there is no voltage output; or the output voltage is not filtered well, the circuit is logically chaotic due to voltage instability, which is manifested as the machine is working well or badly. No matter the machine, if the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the fault will be the same as above. This is especially obvious on computer motherboards. Many computers sometimes fail to turn on after a few years, and sometimes they can be turned on. Open the case, you can often see the phenomenon of electrolytic capacitors bulging, if you remove the capacitors to measure the capacity , Found to be much lower than the actual value.




The lifetime of a capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the lifetime of the capacitor. This rule applies not only to electrolytic capacitors, but also to other capacitors. Therefore, when looking for faulty capacitors, you 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 you are to it, the greater the possibility of damage. I have repaired the power supply of an X-ray flaw detector. The user reported that there was smoke coming out of the power supply. After disassembling the case, it was found that a 1000uF/350V large capacitor had oily things flowing out. Removed a certain amount of capacity. It is only tens of uF, and it is found that only this capacitor is the closest to the heat sink of the rectifier bridge, and the others far away are intact and have 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. Therefore, there should be some emphasis in the inspection and search.


Some capacitors have serious leakage current, and even burn your hands when touched with your fingers. This type of capacitor must be replaced.


In the case of ups and downs during maintenance, except for the possibility of poor contact, most of the failures are generally caused by capacitor damage. Therefore, when encountering such failures, you can focus on checking the capacitors, and it is often pleasant to replace the capacitors (of course, you must also pay attention to the quality of the capacitors, and choose a better brand, such as Ruby, Black Diamond, etc.).


Second, the characteristics and judgment of resistance damage


It is often seen that many beginners are tossing on the resistance when repairing the circuit, and it is dismantled and welded. In fact, there are a lot of repairs. As long as you understand the damage characteristics of the resistance, you don't have to spend a lot of time.


Resistor is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. Open circuit is the most common type of resistance damage. It is rare for the resistance to become larger, and it is rare for the resistance to become smaller. Common ones include carbon film resistors, metal film resistors, wire wound resistors and insurance resistors. The first two types of resistors are the most widely used. One of the characteristics of their damage is the high damage rate of low resistance (below 100Ω) and high resistance (above 100kΩ). Very little damage; second, when low-resistance resistors are damaged, they are often burnt and blackened, which is easy to find, and there are few traces when high-resistance resistors are damaged. Wirewound resistors are generally used for high current limiting, and the resistance is not large. When cylindrical wire wound resistors are burnt, some will turn black or the surface will burst or crack, and some will have no traces. Cement resistors are a type of wire wound resistors, which may break when burned out, otherwise there will be no visible traces. When the fuse resistor burns out, a piece of skin will be exploded on the surface, and some have no traces, but it will never be burnt or blackened. According to the above characteristics, you can focus on checking the resistance and quickly find out 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 that most of the resistors are open or the resistance becomes larger when the resistors are damaged, and the high-resistance resistors are 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 is greater than the nominal resistance, the resistance must be damaged (note that the resistance is stable after the display is stable. In conclusion, because there may be parallel capacitance elements in the circuit, there is a charging and discharging process), if the measured resistance is smaller than the nominal resistance, it is generally ignored. In this way, every resistance on the circuit board is measured once, even if one thousand is "wrongly killed", one will not be missed.




Third, the method of judging whether the operational amplifier is good or bad


It is difficult to judge the quality of operational amplifiers for quite a lot of electronic repairers, not only the education level (there are many undergraduates, if you don’t teach, they won’t. It takes a long time to understand, and there is a special follower The same is true for graduate students whose tutors are studying inverter control!), I would like to discuss with you here, and I hope it will be helpful to everyone.


The ideal operational amplifier has the characteristics of "virtual short" and "virtual break", these two characteristics are very useful for analyzing the linear application of the operational amplifier circuit. In order to ensure linear application, the op amp must work in a closed loop (negative feedback). If there is no negative feedback, the op amp under open-loop amplification becomes a comparator. If you want to judge the quality of the device, you should first distinguish whether the device is used as an amplifier or as a comparator in the circuit.


From the figure, we can see that no matter what type of amplifier, there is a feedback resistance Rf, then we can check this feedback resistance from the circuit when repairing, and use a multimeter to check the resistance between the output terminal and the reverse input terminal. If the value is outrageous, such as a few MΩ or more, we can be sure that the device is used as a comparator. If the resistance is small from 0Ω to tens of kΩ, then check whether there is a resistor connected to the output terminal and reverse input Between the terminals, if there are some, it must be used as an amplifier.


According to the principle of virtual shortness of the amplifier, that is to say, if the operational amplifier works normally, the voltage of the same direction input terminal and the reverse input terminal must be equal. The internal resistance will have a little effect on the voltage test, but generally it will not exceed 0.2V. If there is a difference above 0.5V, the amplifier will undoubtedly be broken! (I am using a FLUKE179 multimeter)


If the device is used as a comparator, the same direction input terminal and the reverse input terminal are allowed to be different, the same direction voltage> reverse voltage, the output voltage is close to the positive maximum value; the same direction voltage <reverse voltage, the output voltage is close to 0V or negative maximum value (depending on dual power supply or single power supply). If it is detected that the voltage does not meet this rule, the device must be broken! In this way, you can judge whether the operational amplifier is good or bad without using the substitution method or removing the chip on the circuit board.




Fourth, a small trick to test SMT components with a multimeter


Some SMD components are very small, and it is inconvenient to use ordinary multimeter test leads to test and repair. One is easy to cause a short circuit, and the other is that it is inconvenient for the circuit board coated with an insulating coating to touch the metal part of the component pin. Here is an easy way to tell everyone, it will bring a lot of convenience to the detection.


Take the two smallest sewing needles, (Deep Industrial Control Maintenance Technology Column), close them to the multimeter pen, and then take a thin copper wire from a multi-strand cable, and tie the pen and the sewing needle with the thin copper wire. Together, use solder to solder firmly. In this way, there is no risk of short circuit when using a test pen with a small needle tip to test those SMT components, and the needle tip can pierce the insulating coating and directly ram the key parts, and there is no need to bother to scrape those membranes.


Fifth, the maintenance method of the short circuit fault of the public power supply of the circuit board


In circuit board maintenance, if you encounter a short circuit of the public power supply, the fault is often serious, 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, use "hoe the earth" After all, you can find the short-circuit point. If there are too many components, it will depend on luck whether the "hoe the earth" can hoe the situation. A more effective method is recommended here. Using this method will get twice the result with half the effort, and the point of failure can often be found quickly.


It is necessary to have 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, depending on the degree of short circuit, slowly increase the current, use Touch the device by hand. When you touch a device that heats up significantly, this is often a damaged component, which can be removed for further measurement and confirmation. Of course, the voltage must not exceed the working voltage of the device during operation, and the connection cannot be reversed, otherwise it will burn out other good devices.


Sixth, a small eraser can solve big problems


More and more boards are used in industrial control. Many boards use golden fingers to insert into the slots. Due to the harsh industrial site environment, dusty, humid, and corrosive gas environments, the boards are likely to have poor contact failures. Friends may have solved the problem by replacing the board, but the cost of buying the board is very considerable, especially the boards of some imported equipment. In fact, you might as well use an eraser to rub the finger repeatedly, clean the dirt on the finger, and try the machine again. The problem may be solved! The method is simple and practical.




Seventh, analysis of electrical faults in good times and bad times


In terms of probability, various electrical faults with good times and bad times may include the following situations:


1. Poor contact


Poor contact between the board and the slot, when the cable is broken internally, the connection and the connection are not good, the contact of the plug and the terminal is not good, and the components are soldered in this category;


2. The signal is interfered


For digital circuits, faults will only appear under certain conditions. It is possible that too much interference has affected the control system and caused errors. There are also changes in individual component parameters or overall performance parameters of the circuit board, which makes anti-interference. Ability tends to the critical point, which leads to failure;


3. Poor thermal stability of components


From a large number of maintenance practices, the thermal stability of electrolytic capacitors is the first to be poor, followed by other capacitors, transistors, diodes, ICs, resistors, etc.;


4. Moisture, dust, etc. on the circuit board


Moisture and dust will conduct electricity and have a resistance effect, and the resistance value will change during the process of 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 malfunctions. happen;


5. Software is also one of the considerations


Many parameters in the circuit are adjusted by software. The margins of some parameters are adjusted too low and are in the critical range. When the operating conditions of the machine conform to the reason for the failure determined by the software, an alarm will appear.


Eighth, how to quickly find component information?


Modern electronic products are diverse, and the types of components are becoming more and more diverse. In circuit maintenance, especially in the field of industrial circuit board maintenance, many components are unseen or even unheard of. In addition, even if the information on the components of a certain board is complete, But if you want to browse and analyze these data one by one in your computer, if there is no quick search method, the maintenance efficiency will be greatly reduced. In the field of industrial electronic maintenance, efficiency is money, and efficiency is the same as pocket money.

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