PCB News - PCB circuit board maintenance method

PCB circuit board maintenance method:

1. First look and then measure: the PCB to be repaired shall be visually inspected first, and observed with the help of a magnifying glass if necessary.

2. From the outside to the inside: if the situation permits, it is recommended to have a good PCB circuit board as a reference, and then use the double Bang VI curve scanning function of the tester to compare the good and bad of the two boards. The starting comparison test point can start from the port of the PCB circuit board, and then from the outside to the inside, especially the comparison test of the capacitor. This can make up for the defect that it is difficult for the multimeter to detect whether the capacitor is leaking on line.

3. Easy before difficult: in order to improve the test effect, some technical treatment shall be done to the repaired PCB before online function test, so as to minimize the impact of various interferences on the test process. Specific measures are as follows:

Preparation before test: for high-capacity electrolytic capacitors, the next pin should also be welded to open the circuit, because the charge and discharge of high-capacity capacitors will also bring interference.

Use exclusion method to test devices: during the on-line test or comparative test of devices, for those devices that pass the test, please directly confirm the test results and record them. Those that fail the test can be tested again. If they still fail, you can confirm the test results first.

Since most modern circuit boards do not have official schematic diagrams, and there are more and more chip applications with programs, engineers will face challenges in all aspects. A lot of practice has proved that there are still ways to escape these challenges. Below, I will share my maintenance experience over the years with the majority of electronic enthusiasts.

1. Appearance inspection method

That is, through the old method of looking, asking, smelling and cutting, observe whether the circuit board is scorched, whether the copper coating is broken, smell whether there is peculiar smell on the circuit board, whether there is poor welding, whether the interface and golden fingers are moldy and black, etc. Asking customers about the fault phenomenon and the process of fault occurrence can often focus on some parts of the fault. Through the above processing, some problems can often be found.

2. General killing method

That is, all components shall be tested once, and the defective components shall be found and replaced to achieve the purpose of repair. If the components that cannot be detected by the instrument are encountered, the replacement method shall be adopted, that is, whether they are damaged or not, they shall be replaced with new components, so as to finally ensure that all components on the board are good and achieve the purpose of repair. This method is simple and effective, easy to use, and does not require high technical level of engineers, but a high degree of care and sense of responsibility are required during operation. Otherwise, some devices do not remember the position when they are removed, resulting in wrong installation and reverse installation, which is easy to cause maintenance failure. In addition, this method is powerless for problems such as blocked through hole, broken copper coating, improper adjustment of potentiometer and so on. This method cannot be used when encountering chips with programs and data.

3. Comparison method

Comparison method is one of the most commonly used methods for repairing circuit boards without drawings. Practice has proved that it has a very good effect. The purpose of fault detection is achieved by comparing the state of good boards, and abnormalities are found by comparing the curves of each node of two boards. However, many times we don't have a good board for comparison, but this doesn't mean that we can't use this method. For example, we can find circuits with the same attributes in the circuit board. For example, a board has three identical interfaces, and the three interfaces have the same circuit, so we can use the comparison curves between the three identical interfaces to judge the problem. When we can't find the above situation, we can still find circuits with common properties. For example, the buses in the circuit usually have the same curve. Sometimes we can't judge whether an IC is damaged. We may also compare the curve with a good IC by scanning the curve. In short, we need to use our brains to bring the comparison method to the extreme. The curve scanning method is indeed useful in practice, but some beginners do not understand its working principle and complain that the instrument has no practical value. Therefore, I have seen many peer companies basically do not use the circuit board maintenance tester and cry out that they are deceived. Of course, businesses are also suspected of exaggerating this function. In fact, when scanning integrated circuits of COMS process, the curve of each scanning is different, resulting in the situation that it is impossible to judge. This reason is due to the high impedance of CMOS and no discharge channel after inter junction capacitor charging. Therefore, it is necessary to have rich experience in judging CMOS integrated circuits. Finally, it should be added that the curve scanning method is to scan the peripheral devices. If the scanning is an integrated circuit, it is to scan the components connected to the pins inside the integrated circuit. 90% of the integrated circuit damage is the peripheral device damage. However, in case of non peripheral device damage, the problem can not be found when scanning the curve, and the device will have problems when working.

4. State method

The state method is to check the normal working state of each component. If the working state of a component is inconsistent with the normal state, there is a problem with the component or its affected components. For example, when we check the CPU, we will check whether the crystal oscillator works normally and whether the reset signal is normal. Whether the input and output logic of NAND gate is correct can be used to judge whether the device is normal. When using this method, it is often forced to give an excitation signal to the input. For example, use a signal generator to give a signal to the input end of the operational amplifier, and use an oscilloscope to measure the amplitude and distortion of its output signal to judge the problem of the operational amplifier. Force the input level of the logic gate circuit to push up or pull down, and measure whether its output level conforms to the logic of the chip, so as to judge the quality of the chip.

The state method is the most accurate method to judge among all maintenance means, and its operation difficulty is beyond the grasp of ordinary engineers. It requires rich theoretical knowledge and practical experience. In order to meet the state requirements, engineers must do everything possible. This method is superior to the curve scanning method in circuit testing, Both peripheral circuit and non peripheral circuit problems of IC can be found.

5. Substitution method

Replace all ICs with new ones until they are repaired. Today's IC is getting cheaper and cheaper. 74 series and 4000 series chips are less than 1 yuan / piece. Instead of checking problems one by one, it's better to replace them all. Personally, I think this method is a helpless move, and its disadvantages are obvious. Its effect should be lower than the pass through method. The pass through method should at least judge the devices. If it accumulates for a long time, it will have some experience on some circuit boards, and the repair rate of this method depends entirely on luck rather than effort. Even if it is repaired, I don't know where the fault is. However, this method still has its merits. If a circuit board or electronic equipment can be used to verify whether it is normal on the spot, we can replace the IC piece by piece. When we replace it to a certain piece, the fault disappears, which can prove that the one just replaced is broken. If it is an IC in-line package, it is recommended that you install an IC seat immediately after removal. After all, it is only a few cents. Another fatal problem of this method is to ensure that the IC you buy is good. If you accidentally encounter a bad IC during purchase, it will not only fail to repair, but expand the fault and increase the difficulty of repair.

6. Grounding method

The circuit building method is to build a circuit, which can work after installing the missing integrated circuit, so as to verify the quality of the tested integrated circuit. For example, in order to judge the quality of a 555 integrated circuit, we can build a time-based oscillation circuit made of 555 circuit, and then place the tested integrated circuit in the circuit. If the circuit works normally, Prove that 555 is no problem, otherwise it is faulty.

This method can achieve 100% accuracy, but the tested integrated circuits have many types and complex packaging, so it is difficult to build all integrated circuits into one circuit.

7. Principle analysis method

This method is to analyze the working principle of a board. For some boards, such as switching power supply, engineers can know their working principle and details without drawings. For engineers, things that know their schematic diagram are extremely simple to maintain. However, for some relatively complex and rare equipment, there is no ready-made schematic reference. You can manually reverse engineer it and draw the schematic diagram. With the schematic diagram, you can be said to be invincible.

This method requires great patience and careful work attitude of engineers. One effort can benefit forever, but whether to adopt this method depends on the value and quantity of maintenance and the difficulty of doing this work.