Capacitor fault
The fault caused by capacitor damage is the highest in electronic equipment, especially the damage of electrolytic capacitor is the most common. The capacitor damage is manifested as: capacity reduction, complete loss of capacity, leakage and short circuit.
Capacitors play different roles in the circuit, and the faults caused by them also have their own characteristics: in the industrial control circuit board, digital circuits account for the vast majority, capacitors are mostly used for power filtering, and less 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 confused due to the unstable voltage, which shows that the machine works well or fails to start. If the capacitor is between the positive and negative poles of the power supply of the digital circuit, the fault performance is the same as above.
This is particularly obvious on the computer motherboard. After several years of use, many computers sometimes fail to turn on the computer, and sometimes they can turn on the computer. When you open the chassis, you can often see the phenomenon of electrolytic capacitor bulging. If you remove the capacitor and measure the capacity, it is found that it is much lower than the actual value.
The life of the capacitor is directly related to the ambient temperature. The higher the ambient temperature is, the shorter the life of the capacitor is. This law is applicable not only to electrolytic capacitors, but also to other capacitors. Therefore, when looking for faulty capacitors, it is important to check the capacitors close to the heat source, such as the capacitors near the heat sink and high-power components. The closer they are, the greater the possibility of damage. Therefore, we should pay more attention to the maintenance and search.
Some capacitors have serious leakage, even hot hands when touching with fingers, so this kind of capacitor must be replaced. In the maintenance of good and bad faults, excluding the possibility of poor contact, most of them are caused by capacitor damage. Therefore, when encountering this kind of fault, you can focus on checking the capacitor. It is often surprising to replace the capacitor.
Resistance fault
I often see many beginners tossing about the resistance when they are repairing the circuit, dismantling it and welding it. In fact, it’s much easier to repair it. As long as you understand the damage characteristics of the resistance, you don’t have to go to great trouble.
Resistance 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 resistance damage, larger resistance is rare, smaller resistance is rare. The common ones are carbon film resistance, metal film resistance, wire wound resistance and safety resistance.
The first two kinds of resistors are the most widely used. One of the characteristics of their damage is the low resistance (100 Ω Below) and high resistance (100k) Ω The middle resistance (such as hundreds to tens of thousands of Ohm) is rarely damaged; Second, the low resistance resistance is often burnt and blackened when it is damaged, which is easy to find, while the high resistance has little trace when it is damaged.
The winding resistance is generally used for large current limiting, and the resistance value is not large; When the cylindrical wire wound resistance is burnt out, some will be blackened or the surface will be cracked, some will have no trace; Cement resistance is a kind of wire wound resistance, which may break when burned out, otherwise there is no visible trace; When the safety resistor is burnt out, some surfaces will blow off a piece of skin, and some have no traces, but they will never scorch and blacken. According to the above characteristics, when checking the resistance, we can focus on it and quickly find out the damaged resistance.
According to the characteristics listed above, we can first observe whether the low resistance resistance on the circuit board is blackened, and then according to the characteristics that the vast majority of resistance is open or the resistance value becomes larger when the resistance is damaged, and the high resistance resistance value is easy to be damaged, we can directly measure the resistance value at both ends of the high resistance value on the circuit board with a multimeter.
If the measured resistance value is larger than the nominal resistance value, the resistance must be damaged (attention should be paid to wait until the resistance value is stable before drawing a conclusion, because there may be a charging and discharging process in parallel capacitor elements in the circuit). If the measured resistance value is smaller than the nominal resistance value, it is generally ignored. In this way, every resistor on the circuit board is measured once. Even if one thousand resistors are killed by mistake, one resistor will not be missed.
Operational amplifier fault
It’s difficult for quite a lot of electronic repairers to judge the quality of operational amplifiers. It’s not only related to their education level. I’d like to discuss it with you here, hoping to help you.
The ideal operational amplifier has the characteristics of “virtual short” and “virtual break”, which are very useful for the analysis of linear operational amplifier circuits. In order to ensure linear operation, Op Amp must work in closed loop (negative feedback). If there is no negative feedback, the open-loop amplifier becomes a comparator. If you want to judge whether the device is good or bad, you should first distinguish whether the device is used as amplifier or comparator in the circuit.
According to the principle of virtual short of the amplifier, if the operational amplifier works normally, the voltage at the same input and reverse input must be equal, even if there is a difference, it is mv level. Of course, in some high input impedance circuits, the internal resistance of the multimeter will have a little influence on the voltage test, but it will not exceed 0.2V in general. If there is a difference above 0.5V, The amplifier must be broken.
If the device is used as a comparator, the same input and reverse input are allowed to be different. When the same direction voltage > reverse voltage, the output voltage is close to the maximum positive value; Co directional voltage
SMT component failure
Some chip components are very small, so it is very inconvenient to test and repair with the common multimeter probe. One is that it is easy to cause short circuit, and the other is that it is inconvenient for the circuit board coated with insulating coating to touch the metal part of the component pin. Here is a simple method, which will bring a lot of convenience to the detection.
Take two sewing needles of the smallest size and put them close to the multimeter pen. Then take the thin copper wire in a multi strand cable, bind the pen and the sewing needle together with the thin copper wire, and then solder them firmly. In this way, there is no danger of short circuit when using the probe with a small tip to test those SMT components, and the tip can pierce the insulation coating and directly hit the key parts, so that there is no need to scratch those films.
Short circuit fault of public power supply
In the maintenance of circuit board, if there is a short circuit fault of common power supply, it is often a big problem, 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, the short circuit point can be found by “digging the earth”; If there are too many components, “hoeing the earth” can hoe the situation depends on luck. This paper recommends a more effective method, using this method, with half the effort, often can quickly find the fault point.
To have a voltage and current adjustable power supply, voltage 0-30V, current 0-3A, this kind of power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the power supply voltage level of the device, first adjust the current to the minimum, and then add this voltage to the power supply voltage point of the circuit, such as the 5V and 0V terminals of 74 series chips. Depending on the degree of short circuit, the current increases slowly.
If you touch a device by hand, if you feel that a device is heating obviously, it is often a damaged component. You can take it down for further measurement and confirmation. Of course, the operating voltage must not exceed the working voltage of the device, and can not be reversed, otherwise it will burn other good devices.
Board failure
More and more boards are used in industrial control, many of which are inserted into slots with gold fingers. Due to the harsh industrial environment, dusty, humid, corrosive gas environment, it is easy to cause poor contact fault of the board. Many friends may solve the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some imported equipment.
In fact, you might as well use the eraser to wipe the gold finger repeatedly. After cleaning the dirt on the gold finger, try the machine again. Maybe it will solve the problem. The method is simple and practical.
Electrical fault
In terms of probability, various kinds of good and bad electrical faults include the following situations:
Poor contact: poor contact between board and slot, broken cable, poor contact between plug and terminal, false soldering of components, etc;
Signal interference: for digital circuits, the fault will appear only under specific conditions. It may be that the interference is too large, which affects the control system and makes it wrong. Some circuit board individual component parameters or overall performance parameters have changed, which makes the anti-interference ability tend to the critical point and causes the fault;
Poor thermal stability of components: from a large number of maintenance practices, the thermal stability of electrolytic capacitors is the first, followed by other capacitors, triodes, diodes, ICs, resistors, etc;
There are moisture, dust, etc. on the circuit board: moisture and dust will conduct electricity and have resistance effect, and the resistance value will change during the process of thermal expansion and cold contraction. This resistance value will have parallel effect with other components. When the effect is strong, the circuit parameters will be changed and the fault will occur;
Software is also one of the factors to be considered: many parameters in the circuit are adjusted by software, and the margin of some parameters is adjusted too low, which is in the critical range. When the operating conditions of the machine meet the reasons for the software to determine the fault, the alarm will appear.