What are the tips for repairing board technology?

First, the fault characteristics of the industrial control circuit board capacitor damage and the fault caused by the repair capacitor damage are the highest in the electronic equipment, especially the damage of the electrolytic capacitor is the most common. The damage of the capacitor is as follows: 1. The capacity becomes smaller; 2. The capacity is completely lost; 3. The leakage occurs; 4. The short circuit.

Capacitors play different roles in the circuit, and the resulting faults also have their own characteristics. In the industrial control circuit board, digital circuits account for the vast majority, and capacitors are often used for power supply filtering, and the capacitance used for signal coupling and oscillation circuits is less. 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 well filtered, the circuit is logically confusing due to voltage instability, and the performance is good or bad when the machine is working. If the capacitor is not between the positive and negative terminals of the digital circuit, the fault will be the same as above. This is especially obvious on the computer motherboard. Many computers have been used for a few years, sometimes they can't be turned on, and sometimes they can be turned on. When you open the case, you can often see the phenomenon of electrolytic capacitor drums. If you remove the capacitor, measure the capacity. It was 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 capacitor life. This rule applies not only to electrolytic capacitors, but also to other capacitors.

Therefore, when looking for faulty capacitors, it is important to check the capacitors that are close to the heat source, 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 reflected that smoke had emerged from the power supply. After disassembling the chassis, I found that a large capacitor of 1000uF/350V had oily things flowing out and removed a certain amount of capacity. Only a few tens of uF, it is found that only this capacitor is closest to the heat sink of the rectifier bridge, and the other distances are intact and the capacity is normal.

In addition, there is a case where the ceramic capacitor is short-circuited, and it is also found that the capacitor is relatively close to the heat-generating component. Therefore, there should be some emphasis when looking for repairs. Some capacitors are more likely to leak, and they may even be hot when touched with a finger. This capacitor must be replaced. In the case of a good or bad fault during maintenance, the possibility of poor contact is ruled out, and most of them are faults caused by capacitor damage. Therefore, when encountering such a fault, you can check the capacitance carefully. It is often surprising to replace the capacitor (of course, pay attention to the quality of the capacitor, choose a better brand, such as ruby, black diamond, etc.).
Second, the characteristics and judgment of resistance damage often see many beginners in the repair circuit when tossing on the resistance, but also the disassembly and welding, in fact, much more repair, you only need to understand the damage characteristics of the resistance, you do not have to pay much attention.

Resistance is the most abundant component in electrical equipment, but it is not the component with the highest damage rate. Resistance damage is the most common with open circuit, the resistance value becomes smaller, and the resistance value becomes very small. Common carbon film resistors, metal film resistors, wirewound resistors and fuse resistors. The first two resistors are the most widely used, and their damage characteristics are low resistance (less than 100Ω) and high resistance (above 100kΩ), and high resistance, intermediate resistance (such as several hundred ohms to several tens of kilo ohms). Very little damage; second, the low resistance value of the resistor is often burnt black, it is easy to find, and the high resistance resistor has few traces when it is damaged. Wirewound resistors are generally used as current limiting currents with little resistance. When the cylindrical wirewound resistor is burnt out, some may be black or have a surface blast, crack, and some have no trace. Cement resistance is a kind of wirewound resistor that may break when burnt out, otherwise there will be no visible traces. When the fuse is burned out, some surfaces will blow up a piece of skin, and some will have no trace, but it will never burnt and black.
According to the above characteristics, when checking the resistance, you can focus on it and quickly find out the damaged resistance. According to the characteristics listed above, we can first observe whether there is any black mark on the low-resistance resistance of the circuit board, and then according to the characteristics that most of the open circuit or resistance value becomes large and the high resistance value is easily damaged according to the resistance damage, We can use the multimeter to directly measure the resistance of the high-resistance resistor on the circuit board. If the resistance is larger than the nominal resistance, the resistor will be damaged (note that the resistance value is stable after the resistance is displayed. In conclusion, because there is a possibility that the capacitor is connected in parallel in the circuit, there is a charge and discharge process. If the resistance is smaller than the nominal resistance, it is generally ignored. In this way, each resistor on the board is measured once, even if "missing" a thousand, it will not let go.
Third, the operational amplifier's good or bad discriminating method The operational amplifier's good or bad judgment has a certain degree of difficulty for a considerable number of electronic repairers, not only the degree of education (there are many undergraduates, not sure if not taught, also teach I haven't learned it for a long time, and there is a graduate student who specializes in frequency conversion control with a tutor. This is also true!), I would like to discuss it with you and hope to help everyone. Ideal op amps have "virtual short" and "dummy" characteristics that are useful for analyzing linearly used op amp circuits. To ensure linear operation, the op amp must operate 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 a comparator in the circuit.

From the figure we can see that no matter what type of amplifier, there is a feedback resistor Rf, we can check this feedback resistor from the circuit during maintenance, check the resistance between the output and the reverse input with a multimeter Value, if the large is outrageous, such as a few MΩ or more, then we can probably be sure that the device is used as a comparator. If the resistance is smaller than 0Ω to tens of kΩ, then check whether there is a resistor connected to the output and the reverse input. Between the ends, there is something to be used as an amplifier. According to the principle of the virtual short of the amplifier, that is, if the operational amplifier is working normally, the voltages of the same input and the reverse input must be equal, even if there is a difference, the mv level, of course, in some high input impedance circuits, the multimeter The internal resistance will have a little effect on the voltage test, but it will not exceed 0.2V. If there is a difference of 0.5V or more, the amplifier will be bad! (I am using the FLUKE179 multimeter). If the device is used as a comparator, it is allowed. The same direction input and reverse input end are not equal, 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 (depending on Dual power supply or single power supply). If the detected voltage does not meet this rule, then the device will be bad! So you don't have to use the substitution method, you can judge the operational amplifier is good without removing the chip on the board.
Fourth, the multimeter test SMT components of a small trick Some patch components are very small, it is very inconvenient to test and repair with a common multimeter test pen, one is easy to cause short circuit, the second is the inadvertent contact with the component tube coated circuit board The metal part of the foot. Here is a simple way to tell you a lot of convenience. Take the two smallest needles, (deep industrial control technology column) and close it with the multimeter pen, then take a thin copper wire in a multi-strand cable, and tie the test leads and the sewing needle with a thin copper wire. Together, solder with solder. In this way, when the SMT components are tested with a small needle tip, there is no short circuit, and the needle tip can pierce the insulating coating and directly smash the key parts, and no need to bother to scrape the film.
V. Maintenance method for circuit board common power supply short-circuit fault In circuit board maintenance , if the short circuit of the common power supply is encountered, the fault is often large, because many devices share the same power supply, and each device using this power supply has a short circuit. There are not many components on the board, and the short-circuit point can be found by means of “squatting the earth”. If there are too many components, it is necessary to rely on luck if the earth is unable to reach the situation. Here, we recommend a method of comparing and using. This method is used to get twice the result with half the effort, and often finds the point of failure very 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. When the hand touches the device, when a certain device is heated, this is often a damaged component, which can be removed for further measurement confirmation. Of course, the voltage must not exceed the operating voltage of the device during operation, and it cannot be reversed, otherwise it will burn out other good devices.
Sixth, a small rubber, solve the big problem . More and more boards are used for industrial control. Many boards use gold fingers to insert slots. Due to the harsh industrial environment, dusty, humid, and corrosive gases are easy. To make the board faulty, many friends may solve the problem by replacing the board, but the cost of purchasing the board is very considerable, especially the board of some imported equipment. In fact, you may wish to use the eraser to rub a few times on the gold finger, clean the dirt on the gold finger, and then try the machine, the problem may be solved! The method is simple and practical.
VII. Analysis of good and bad electrical faults All kinds of good and bad electrical faults may include the following situations in terms of probability: 1. Poor contact between the board and the slot, and the internal disconnection of the cable The wire plug and the terminal block are not in good contact, and the component is soldered, etc.; 2. The signal is disturbed. For the digital circuit, under certain conditions, the fault will be presented, and it may be that the interference is too large. Affected the control system to make it wrong, there are also changes in the individual component parameters or overall performance parameters of the circuit board, so that the anti-interference ability tends to the critical point, resulting in failure; 3. The thermal stability of the components is not good from a large number of maintenance practices Among them, the thermal stability of the first electrolytic capacitor is not good, followed by other capacitors, transistors, diodes, ICs, resistors, etc.; 4. There are moisture and dust on the circuit board. Moisture and dust will be conductive, have a 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 strong, the circuit parameters will be changed to make the fault. 5. The software is also one of the considerations. Many parameters in the circuit are adjusted by software. The margin of some parameters is adjusted too low and is in the critical range. When the machine operating condition meets the reason for the software to determine the fault, then the alarm is Will appear.
Eight, how to quickly find the component information Modern electronic products are diverse, the types of components are increasingly numerous, more than thousands, in the circuit maintenance, especially in the field of industrial circuit board maintenance, many components are never seen, or even unheard of, and even a certain board The information on the components at hand is complete, but the information should be read and analyzed in the 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 too low. I can't get through the pockets of the banknotes. We should be grateful to be born in this great era. The Internet is far from being boundless, making every corner of the world seem to be within reach. Free information is everywhere, and everyone can reach out and welcome. So everyone must make good use of the powerful tool of the Internet to make it easier for you!