PCB Technical - Tips for repairing PCB circuit boards without drawings

1. How to repair circuit boards without drawings?

1. To "have a solid picture"

It is necessary to thoroughly understand the principles of some typical circuits and be familiar with them. The drawings are dead, and the thoughts in the head are alive. They can be analogized, reasoned, and inferred. For example, the switching power supply is always inseparable from the oscillating circuit, switching tube, and switching transformer. During the inspection, check whether the circuit is vibrated, whether the capacitor is damaged, whether the transistors and diodes are damaged, and no matter what switching power supply is touched, it can be operated. It’s almost the same, you don’t have to force a circuit diagram;

2. Pay attention to the order of maintenance

Pay attention to the maintenance sequence to find the shortest path to solve the problem, avoid chaotic disassembly, failure to repair, and cause the failure to expand. Maintenance is like a doctor's treatment, but also pay attention to "seeing, hearing and asking."

3. Be good at summarizing the laws.

Generally, after a certain amount of maintenance experience is accumulated, it is necessary to be good at summing up and analyzing the cause of each component damage. Is it improper operation? Lack of maintenance? The design is unreasonable? Poor component quality? Natural aging? With these analyses, if you encounter similar failures next time, even if they are not the same circuit board, you will have a little understanding in your heart.

4. Be good at finding information

Since the advent of the Internet, finding information has become very easy. You can find almost all the equipment principles you don't understand, and the circuit principles you don't understand. Any IC information can be found online.

5. Necessary PCB testing equipment is required

If you treat maintenance as your own business, then a certain amount of equipment investment is necessary. The brand of electric soldering iron, multimeter, and common disassembly tools should not be too bad. If possible, get a 100M dual trace oscilloscope, and if possible, get an online maintenance tester.

Second, the fault characteristics and maintenance of industrial PCB 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 will be no voltage output; or the output voltage is not filtered well, and the circuit is logically chaotic due to voltage instability. 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.

Third, the characteristics and judgment of resistance damage

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 that the damage rate of low resistance (below 100Ω) and high resistance (above 100kΩ) is higher, and the middle resistance value (such as hundreds of ohms to tens of kiloohms) Very little damage; second, when low-resistance resistors are damaged, they are often burnt and blackened, which is easy to find, while high-resistance resistors are rarely 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 kind 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 trace, 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.

Fourth, the judgment method of operational amplifier is good or bad

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.

Five, 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.

Sixth, the repair method of the circuit board public power supply short-circuit fault

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.

Seven, a small eraser, solve big problems

More and more boards are used in industrial control, and many boards use golden fingers to insert into the slots. Due to the harsh industrial site environment, dusty, humid, and corrosive gas environment, the board may 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 golden finger a few times, clean up the dirt on the golden finger, and try the machine again, maybe the problem will be solved! The method is simple and practical.

8. 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, failure of the connection when the cable is broken, poor contact between the plugs and terminals, and false soldering of components are all of this type;

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. The 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 and dust 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.

Nine, 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 PCB 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.