Methods and skills in circuit board maintenance

introduction

Since the control system is expensive, when it fails, in order to pay attention to economic benefits and save costs, it is generally repaired. However, in the following situations, a new circuit board needs to be replaced: the circuit board has reached the scrap age; the circuit board is seriously damaged and cannot be repaired; after repeated repairs, problems continue to occur, indicating that the circuit board has unstable factors and is no longer suitable for continued use in the machine tool. I have been engaged in circuit board maintenance for many years. In my work, I have summarized some maintenance methods and techniques, and introduced them to everyone, and I hope everyone will encourage each other.

1 Observation method

When we get a circuit board to be repaired, we first observe its appearance carefully. If the circuit board has been burned, we must carefully check whether the power circuit is normal before powering on the circuit board, and then power on after ensuring that no secondary damage will be caused. The observation method is a type of static inspection method. When using the observation method, the following steps are generally followed.

The first step is to observe whether the circuit board has been damaged by human factors, which mainly depends on the following aspects:

① Check whether the circuit board has been dropped, causing the corners of the board to be deformed, or whether the chip on the board has been deformed or damaged.

② Observe the chip socket to see if it has been damaged by forced prying due to lack of special tools.

③ Observe the chip on the circuit board. If it has a socket, first observe whether the chip is inserted incorrectly. This is mainly to prevent the operator from inserting the chip in the wrong position or direction when repairing the circuit board by himself. If the error is not corrected in time, when the circuit board is powered on, the chip may be burned out, causing unnecessary losses.

④ If there is a short-circuit terminal on the circuit board, check whether the short-circuit terminal is inserted incorrectly.

Circuit board maintenance requires a solid theoretical foundation and careful and conscientious work. Through careful observation by the repairer, the cause of the problem can sometimes be determined at this step.

The second step is to observe whether the components on the circuit board are burnt. For example, whether the resistors, capacitors, and diodes are blackened or fuzzy. Under normal circumstances, even if the resistor is burnt, its resistance value will not change, its performance will not change, and it will not affect normal use. At this time, you need to use a multimeter to assist in measurement. However, if the capacitors and diodes are burnt, their performance will change, and they will not be able to play their due role in the circuit, which will affect the normal operation of the entire circuit. At this time, new components must be replaced.

The third step is to observe the integrated circuits on the circuit board, such as the 74 series, CPU, coprocessor, AD and other chips, to see if there are any bulges, cracks, burns or blackening. If such a situation occurs, it can be basically determined that the chip has been burned and must be replaced.

The fourth step is to observe whether the traces on the circuit board are peeling, burning or disconnected. Whether the copper-plated holes are separated from the pads.

Step 5: Observe the fuses on the circuit board (including fuse tubes and thermistors) to see if the fuses are blown. Sometimes, because the fuses are too thin to see clearly, you can use an auxiliary tool - a multimeter to determine whether the fuses are damaged.

The occurrence of the above four situations is mostly the result of excessive current in the circuit. However, the specific reason for the excessive current requires specific analysis. But the overall idea of ​​finding the problem is to first carefully analyze the schematic diagram of the circuit board, and then find its parent circuit according to the circuit where the burned components are located, deduce upward step by step, and then analyze the most likely place to cause problems based on some experience accumulated in work, and find out the cause of the failure.

2 Static measurement method

For most circuit boards, the above observation method cannot find the problem. Only a small number of circuit boards will be physically deformed due to some special reasons, and the cause of the failure can be easily found. For most faulty circuit boards, it is still necessary to use a multimeter to measure some major components and key points on the circuit board in an orderly manner to find and solve the problem.

Before measuring, we must first determine whether the circuit is mainly analog or digital. For circuit boards with schematics, we can determine this by looking at the schematics. However, for circuit boards without schematics, we can generally determine this by the following two methods: ① Observe the components on the circuit board to see if there is a microprocessor on the circuit board. Whether it is the early 80, 51 series or the widely used DSP now, as long as such a chip appears on the circuit board, it means that there is a bus structure on the board, and digital signals will occupy a large part, so we can treat it as a digital board. ② For circuit boards without microprocessors, observe the components on the board to see if there are many chips using 5V power supply. If there are many 5V power supply chips, we can also treat it as a digital circuit for repair. The repair methods for digital circuits and analog circuits are different. Generally speaking, it is easier to repair analog circuits, and we can deduce step by step to find the problem. However, for digital circuits, since the circuits are all hung on the bus, there is no clear superior-subordinate relationship. Therefore, it is more difficult to repair. The following only focuses on the static measurement method of digital circuits, and the repair mainly follows the following steps.

Step 1: Use a multimeter to check if there is a short circuit between the power supply and the ground.

The inspection method is: find a chip powered by a 5V power supply, and measure two points on the diagonal (for example, for a chip with 14 pins, measure pins 7 and 14.

If the chip has 16 pins, measure pins 8 and 16. If there is no short circuit between the two points, it means that the power supply is working normally. If a short circuit occurs, you need to find the cause through troubleshooting.

These steps are just the basic ideas for power supply maintenance. When it comes to particularly complex circuit boards, specific analysis of specific issues is required.

Power supply is the foundation of the circuit. Only when the power supply works properly can we talk about the application of subsequent circuits. Therefore, the measurement of power supply is very important, but it is also a step that is easily overlooked by maintenance personnel.

Step 2: Use a multimeter to measure the diode and observe whether it works normally. Under normal circumstances, use the resistance range to measure the positive and negative poles. The positive phase measurement is tens to hundreds of ohms, and the reverse phase is one thousand to several thousand ohms. Generally speaking, the diode is damaged because the current in the circuit is too large, causing the diode to be broken down.

Step 3: Use the resistance range of the multimeter to measure the capacitance to see if there is a short circuit or open circuit. If there is, it means that there is a problem with this part of the circuit. The next step is to determine whether the problem is with the component itself or the circuit connected to it. The method is to solder one leg of the suspected component to see if the component is open circuit or open circuit. In this way, the problem can be determined in one step.

Step 4: Also use a multimeter to measure the integrated circuits, transistors, resistors, etc. on the circuit board to see if they meet their own logical performance. If the circuit board includes a bus structure, there will generally be a pull-up resistor row on the bus. The measurement of the resistor row is a very important step. Through its quality, the quality of the chip hanging on the bus can be preliminarily tested.

After inspection by observation and static measurement methods, most of the problems that arise in circuit board maintenance can be solved. One thing worth noting is that you must ensure that the power supply is normal to avoid secondary damage to the circuit board after the next step.

3 Online measurement method

The online measurement method is generally used in manufacturers that mass produce circuit boards. In order to facilitate maintenance, manufacturers generally build a more general debugging and maintenance platform, which can conveniently provide the power supply required by the circuit board and some necessary initial signals. The online measurement method mainly solves two problems. First, the problems found in the above two steps are subdivided, and finally locked to the components with problems. Second, if the problem is not solved through the above two steps, it is necessary to find out the cause of the fault through online measurement. The online measurement method is mainly carried out through the following steps.

Step 1: Power on the circuit board. In this step, it should be noted that some circuit boards do not have a single power supply. They may need 5V, plus or minus 12V, 24V, etc. Do not forget to add the power that should be added. After the circuit board is powered on, touch the components on the circuit board to see if there are any hot components. Focus on checking the 74 series chips. If the components are hot, it means that the components may be damaged. After replacing the components, check whether the circuit board fault has been resolved.

Step 2: Use an oscilloscope to measure the gate circuit on the circuit board to see if it conforms to the logical relationship. If the output does not conform to the logic, it needs to be treated in two different ways. One is that the output should be low level, but the actual measurement is high level, which can directly determine that the chip is damaged; the other is that the output should be high level, but the actual measurement is low, which cannot determine that the chip is damaged. It is also necessary to disconnect the chip from the subsequent circuit and measure again to observe whether the logic is reasonable and determine whether the chip is good or bad.

Step 3: Use an oscilloscope to measure the crystal oscillator in the digital circuit to see if it has output. If there is no output, you need to remove as many chips as possible connected to the crystal oscillator before measuring. If there is still no output, it is preliminarily determined that the crystal oscillator is damaged; if there is output, you need to install the removed chips one by one, and test them one by one to find out the fault.

Step 4: Digital circuits with bus structures generally include three buses: digital, address, and control. Use an oscilloscope to measure the three buses, compare the schematic diagram, observe whether the signal is normal, and find out the problem.

The online measurement method is mainly used to compare two good and bad circuit boards. Through the comparison, problems can be found and solved, thus completing the circuit board repair.