Seven Key Points for Technical Maintenance of Printed Circuit Board

1. Fault Characteristics and Maintenance of Capacitance Damage of Industrial Control Circuit Board

The fault caused by capacitor damage is the highest in electronic equipment, especially in electrolytic capacitor damage.

Capacitance damage is as follows: 1. capacity decreases; 2. total loss of capacity; 3. leakage; 4. short circuit.

Capacitance plays different roles in the circuit, and the faults caused by capacitance have their own characteristics. In industrial control circuit board, digital circuit accounts for the vast majority. Capacitance is mostly used as power filter and less used as signal coupling and oscillation circuit.

If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not oscillate and there is no voltage output. Or the output voltage filter is not good, the circuit logic is chaotic because of voltage instability, which shows that the machine works well or badly or can not turn on. If the capacitor is between the positive and negative poles of the power supply in the digital circuit, the fault performance is the same.

This is particularly evident on the computer motherboard, many computers have been used for several years, sometimes can not turn on, sometimes can turn on the phenomenon, open the chassis, often can see the phenomenon of electrolytic capacitor bulging, if the capacitor is removed to measure the capacity, found that much lower than the actual value.

The life of capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the life of capacitor. This rule applies not only to electrolytic capacitors, but also to other capacitors. Therefore, when looking for fault capacitors, we should focus on checking capacitors close to heat sources, such as capacitors near radiators and high-power components, the closer they are, the greater the possibility of damage.

Once repaired the power supply of an X-ray flaw detector, users reported smoke coming out of the power supply. After opening the chassis, they found that a large capacitor of 1000uF/350V flowed out with oil-like substance, and a volume of capacity was only tens of uF. It was also found that only this capacitor was closest to the radiator of the rectifier bridge and the others were far away. It is intact and has normal capacity. In addition, the short circuit of the ceramic capacitor is found, and the capacitor is close to the heating part. Therefore, it should be emphasized when checking and searching.

Some capacitors leak seriously and even burn hands when touched with fingers. This capacitor must be replaced.

In the maintenance of good or bad faults, excluding the possibility of bad contact, generally most of the fault is caused by capacitor damage. Therefore, when encountering such faults, we can focus on the capacitance inspection. It is often surprising to change capacitance (of course, we should also pay attention to the quality of capacitance, and choose better brands, such as ruby, black diamond, etc.).

2. Characteristics and Discrimination of Resistance Damage

It is often seen that many beginners toss and turn on resistance while repairing the circuit. They are dismantled and welded. In fact, they have been repaired much more. As long as you understand the damage characteristics of resistance, you need not spend much time.

Resistance is the largest number of components 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 value to increase and the resistance value to decrease. Common are carbon film resistance, metal film resistance, winding resistance and insurance resistance.

The first two kinds of resistors are most widely used. Their damage characteristics are that the damage rate of low resistance value (below 100_) and high resistance value (above 100 k) is higher, and the damage rate of intermediate resistance value (for example, several hundred to tens of thousands of euros) is very little. The second is that the damage of low resistance value resistors is often charred and blackened, which is easy to find, while the damage of high resistance value resistors is rarely traced.

Wire winding resistors are generally used as current limiting devices with small resistance values. When the resistance of the cylindrical wire wound is burnt out, some will blacken or the surface will explode and crack, and 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. Sometimes the surface of the fuse resistor will blow off a piece of skin when it burns out. Others have no trace, but they will never burn and blacken. According to the above characteristics, we can focus on checking resistance and quickly find out the damaged resistance.

According to the characteristics listed above, we can first observe whether the low resistance on the circuit board is blackened or not, and then according to the characteristics that most open circuit or resistance values become larger when the resistance is damaged and the high resistance values are easily damaged, we can use the multimeter to directly measure the resistance at both ends of the high resistance value on the circuit board. If the measured resistance value is larger than the nominal resistance value, the resistance will certainly be damaged (it should be noted that the constant resistance value is stable before reaching a conclusion, because there may be a charging and discharging process for 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 resistance on the circuit board is measured once, even if the "wrong killing" 1000, will not let go of one.

3. Discrimination Method of Operational Amplifier

Operational amplifier judgment is difficult for a considerable number of electronic maintainers, not only the relationship between educational level (there are many undergraduate students, if not taught, it will certainly not, it will take a long time to understand, there is also a graduate student specializing in frequency conversion control with the tutor, actually the same!), here we discuss with you. I hope it will be helpful to you all.

The ideal operational amplifier has the characteristics of "virtual short" and "virtual break", which are very useful for analyzing linear operational amplifier circuits. In order to ensure linear operation, the operational amplifier must work in a closed loop (negative feedback). If there is no negative feedback, the OPA under open-loop amplification becomes a comparator. If we want to judge whether the device is good or bad, we 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, that is to say, if this operational amplifier works properly, its voltage at the same input and reverse input must be equal, even if there are differences, it is MV level. Of course, in some high input impedance circuits, the internal resistance of multimeter will have some influence on voltage test, but generally it will not exceed 0.2. V, if there is a difference of more than 0.5V, the amplifier will definitely fail! (I use FLUKE 179 multimeter)

If the device is used as a comparator, it is allowed to vary between the same input and the reverse input. If the same direction voltage > the reverse voltage, the output voltage approaches the maximum positive value; if the same direction voltage < the reverse voltage, the output voltage approaches the maximum value of 0V or negative value (depending on dual or single power supply). If the detected voltage does not conform to this rule, the device must be bad, so you do not need to use substitution method, and you do not need to remove the chip on the circuit board to judge whether the operational amplifier is good or bad.

4. A Tip for Testing SMT Components with Multimeter

Some patch elements are very small, it is very inconvenient to test and repair with ordinary multimeter pen. One is easy to cause short circuit, and the other is inconvenient to contact the metal part of the pin of the element on the circuit board coated with insulation. Here we tell you a simple method, will bring a lot of convenience to the detection.

Take two smallest sewing needles, (Deep Industrial Control Maintenance Technology Column) and close them to the multimeter pen, then take a thin copper wire in a multi-strand cable, tie the pen and sewing needle together with the fine copper wire, and then solder them firmly with solder. In this way, there will be no short circuit when using a stylus with small pinpoint to measure SMT components, and the pinpoint can pierce the insulating coating, directly impact the key parts, and no longer need to bother scraping those membranes.

5, Inspection and Repair Method of Short Circuit Faults in Public Power Supply of PCB

In the maintenance of circuit board, if encounter the fault of short circuit of public power supply, the head is often large, because many devices share the same power supply, each device using this power supply has the suspicion of short circuit. If there are not many components on board, the short circuit point can be found by "hoeing the earth" method. If there are too many components, the "hoeing the earth" energy can be obtained. It depends on luck if you can't hoe the situation. In this paper, we recommend a more useful method, using this method, we can get twice the result with half the effort, and often find the fault point quickly.

There must be a power supply with adjustable voltage and current. The voltage is 0-30V and the current is 0-3A. This power supply is not expensive, about 300 yuan. To adjust the open-circuit voltage to the level of the device power supply voltage, first, to minimize the current, add this voltage to the circuit power supply voltage points such as 5V and 0V of 74 series chips, depending on the degree of short-circuit, slowly increase the current, touch the device by hand, when a device heating obvious, this is often a damaged component, you can take it. Further measurement confirmation is given below. Of course, the operating voltage must not exceed the operating voltage of the device, and can not be reversed, otherwise other good devices will burn.

6, a small rubber to solve big problems

There are more and more boards used in industrial control. Many boards use golden fingers to insert slots. Because of the harsh industrial environment, dusty, humid and corrosive gas environment, the boards are liable to have bad contact faults. Many friends may solve the problem by replacing boards, but the cost of purchasing boards is very high. Significant, especially the boards of some imported equipment. In fact, you may as well use an eraser on the golden finger repeatedly wipe a few times, after cleaning up the dirt on the golden finger, try again, maybe solve the problem! The method is simple and practical.

7. Analysis of Electrical Faults in Time-Warm and Time-Warm

Electrical faults of various time-varying and time-varying types may include the following situations in terms of probability:

1). Poor contact

Bad contact between card and slot, broken cable, bad contact between plug and terminal, virtual welding of components, etc. belong to this category.

2). Signal interference

For digital circuits, under specific conditions, faults will occur. It may be that too much interference affects the control system to make errors. There are also changes in the parameters of individual components or the overall performance parameters of circuit boards, which make the anti-interference ability tend to the critical point, thus causing faults.

3). Poor thermal stability of components

From a large number of maintenance practices, the thermal stability of electrolytic capacitors is not good, followed by other capacitors, transistors, diodes, IC, resistors, etc.

4). There is moisture and dust on the circuit board.

Moisture and dust will conduct electricity and have resistance effect, and the resistance value will change in the process of thermal expansion and cold contraction. This resistance value will have parallel effect with other components. When this effect is strong, circuit parameters will be changed and fault will occur.

5). Software is also a consideration

Many parameters in the circuit are adjusted by software. The margin of some parameters is too low to be in the critical range. When the operating condition of the machine meets the reason of software judging the fault, the alarm will appear.