PCB Tech - Common techniques for PCB circuit board repair

PCB circuit board repair

Since most of the modern circuit boards do not have official schematics, and there are more and more applications of chips with programs, engineers are bound to face various challenges. Through a lot of practice, it has been proved that facing these challenges, there are still ways to escape., I will share my maintenance experience over the years with the majority of electronics enthusiasts.

1. Visual inspection method

That is, through the old methods of looking, asking, smelling, and cutting, observe whether the circuit board is burnt, whether there is a broken place in the copper coating, whether there is a peculiar smell on the circuit board, whether there is a bad soldering place, the interface, the gold finger Whether there is mildew and black phenomenon, etc. Ask the customer about the fault phenomenon and the process of the fault, and the fault can often be concentrated in certain parts. Through the above processing, some problems can often be found.

2. Through killing method

That is to check all the components again, find the defective component and replace it to achieve the purpose of repair. If you encounter a component that cannot be detected by the instrument, use the substitution method, that is, replace it with a new component regardless of whether it is damaged or not. Ensure that all components on the board are good to achieve the purpose of repair. This method is simple and effective, easy to use, and does not require high technical level of engineers, but requires a high degree of care and responsibility during operation. Otherwise, some devices will not remember the position when they are removed, and the wrong installation will occur. It is easy to cause maintenance failure. In addition, this method is helpless for problems such as blocked vias, broken copper, and improper adjustment of potentiometers. This method cannot be used when encountering chips with programs and data.

3. Contrast method

The comparison method is one of the most commonly used methods for repairing circuit boards without drawings. Practice has proved to have very good results. The purpose of detecting faults is achieved by comparing the status of the good boards. The abnormalities are found by comparing the curves of the nodes of the two boards. . However, many times we do not have a good board for comparison, but this does not mean that we cannot use this method. For example, we can find circuits with the same properties on the circuit board. For example, a certain board has three identical interfaces. If the three interfaces have the same circuit, we can use the three identical interfaces to compare the curves with each other to judge the problem.

4. State method

The state method is to check the normal working state of each component. If the working state of a certain component does not match the normal state, there is a problem with the device or its affected parts. For example, when we check the CPU, we will check whether the crystal oscillator is working normally and whether the reset signal is normal. Whether the input and output logic of the NAND gate is correct or not can be judged whether the device is normal. When using this method, it is often forced to give an excitation signal to the input. The signal amplitude and distortion are used to judge the problem of the op amp.

5. Substitution method

Replace all IC with new ones until they are repaired. Nowadays ICs are getting cheaper and cheaper. The 74 series and 4000 series chips are less than 1 yuan per piece. Rather than checking the problem piece by piece, it is better to replace them all. I personally think this method is a helpless move, and its shortcomings are obvious. Its effect should be lower than that of the pass-killing method. The pass-killing method must at least judge the device. If accumulated over a long period of time, it will affect some circuit boards. Some experience, and the repair rate of this method depends entirely on luck rather than effort, even if it is really repaired, it is not known where the fault is. However, this method still has its merits. If a certain circuit board or electronic device can be used to verify whether it is normal on the spot, we can replace the IC piece by piece. When the fault disappears when a certain piece is replaced, it can be proved that it is The piece that was just replaced is broken.

6, the circuit method

The circuit method is to make a circuit by hand, and the circuit can work after the integrated circuit is installed, 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 555 The time-base oscillation circuit made by the circuit, and then the integrated circuit under test is placed in the circuit. If the circuit works normally, it proves that 555 is no problem, otherwise it is faulty.

7. Principle analysis method

This method is to analyze the working principle of a board. For some boards, such as switching power supplies, engineers can know the working principle and details without drawings. For engineers, it is very simple to repair things that know the schematics. However, for some relatively complex and rare equipment, there is no ready-made schematic diagram reference to manually reverse engineer it, draw the schematic diagram, and then repair it with the schematic diagram, which can be said to be invincible.