introduction Because of the high price of the control system, when it fails, in order to emphasize economic benefits and save costs, the maintenance method is generally adopted. However, in the following cases, a new circuit board needs to be replaced: the circuit board has reached the end of its life; the board is damaged and cannot be repaired; after repeated repairs, problems continue to occur, indicating that the board does not exist. The stability factor is no longer suitable for continued use in machine tools. I have been working on the maintenance of circuit boards for many years. I have summarized some methods and techniques for maintenance in my work. I will introduce them to you and share them with you. 1 observation method When we get a board to be repaired, we first carefully observe its appearance. If the board is burned, be sure to check the power supply circuit before powering up the board, and then power it up after ensuring that it does not cause secondary damage. The observation method is one of the static inspection methods. When using the observation method, the following steps are generally followed. The first step is to observe whether the board is damaged or not. This is mainly from the following aspects: 1 See if the board is dropped, causing the board corner to be deformed, or the chip on the board being broken or broken. 2 Observe the socket of the chip to see if it is forcibly broken due to the lack of special tools. 3 Observe the chip on the circuit board. If it is with a socket, first observe whether the chip is inserted incorrectly. This is mainly to prevent the operator from inserting the wrong position or direction of the chip when repairing the circuit board. If the error is not corrected in time, when the board is energized, the chip may be burned out, causing unnecessary losses. 4 If there is a shorting terminal on the board, observe if the shorting terminal is inserted incorrectly. The maintenance of the circuit board requires a solid foundation in theory, careful work, and careful observation by the maintainer, sometimes at this step can determine the cause of the problem. The second step is to observe whether the components on the board are burned out. For example, resistors, capacitors, and diodes are not black or smeared. Under normal circumstances, even if the resistance is burnt, its resistance will not change, the performance will not change, and it will not affect the normal use. In this case, a multimeter is needed to assist the measurement. However, if the capacitors and diodes are burnt, their performance will change, and they will not be able to perform their functions 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 circuit on the board, such as the 74 series, CPU, coprocessor, AD and other chips, there are no drums, cracks, burnt, black. If this happens, it is basically OK that the chip has been burned out and must be replaced. The fourth step is to observe whether the traces on the circuit board are peeled or burned. The copper hole is not separated from the pad. Step 5: Observe the fuses on the board (including the fuse and the thermistor) to see if the fuse is blown. Sometimes the fuse is too thin and can't be seen clearly. You can use the auxiliary tool-multimeter to judge whether the fuse is damaged. The occurrence of the above four cases is mostly due to the excessive current in the circuit. However, the specific cause of the current is too large, it is necessary to analyze the specific problem. But the general idea of ​​finding the problem is to first carefully analyze the schematic diagram of the board, and then find out its superior circuit according to the circuit where the burnt component is located, and derive it step by step, and then rely on some experience accumulated in the work to analyze the easiest. Where the problem occurred, find out the cause of the failure. 2 static measurement For most boards, no problems can be found through the previous observations. As long as a small number of boards will be physically deformed for some special reasons, it is easy to find out the cause of the fault. Most of the faulty boards still need to use a multimeter to carry out some major components and key points on the board. Order measurement, find problems, solve problems. Before measuring, it is first necessary to judge whether the circuit is based on analog signals or digital signals. For schematic boards, you can judge by looking at the schematic. However, for a circuit board without a schematic diagram, it is generally judged by the following two methods: 1 observe the components on the circuit board, and see if there is a microprocessor on the circuit board, whether it is the early 80, 51 series, or now widely The DSP of the application, as long as such a chip appears on the circuit board, it means that there is a bus structure on the board, and the digital signal must occupy a large part, so that it can be treated as a digital board. 2 For a circuit board without a microprocessor, observe the components on the board and see how many chips are used for the 5V power supply. If there are a lot of 5V power chips, you can also use it as a digital circuit for repair. The maintenance methods for digital circuits and analog circuits are different. In general, analog circuits are easier to repair and can be derived step by step to find out the problem. However, for digital circuits, since the circuits are all hung on the bus, there is no clear relationship between the upper and lower levels. Therefore, it is more difficult to repair. The following only focuses on the static measurement of digital circuits. The maintenance mainly follows the following steps. Step 1: Use a multimeter to check for shorts between the power supply and ground. The method of inspection is to find a 5V power supply chip and measure two points on the diagonal (such as the 14-pin chip, then measure the 7-pin and 14-pin). For the 16-pin chip, measure 8 and 16 feet). If there is no short circuit between the two points, the power supply is working normally. If a short circuit occurs, you need to find the cause by troubleshooting. These steps are only the basic idea of ​​power supply maintenance. Specific to the particularly complex circuit board, specific problems need to be analyzed. The power supply is the basis of the circuit. Only when the power supply works normally can we talk about the application of the subsequent circuit. Therefore, the measurement of the power supply is very important, and it is also a step that is particularly easy to be ignored by the repairer. Step 2: Use a multimeter to measure the diode and see if it works. Under normal circumstances, the positive and negative poles are measured with a resistance file, and the positive phase is measured from tens to hundreds of ohms, and the reverse phase is from one thousand to several thousand ohms. In general, damage to the diode is caused by excessive current in the circuit causing the diode to break down. Step 3: Use a multimeter resistance file to measure the capacitance to see if there is a short circuit or open circuit. If there is, it indicates that there is a problem with this part of the circuit. The next step is to determine if there is a problem with the component itself, or if there is a problem with the circuit connected to it. The method is to solder the foot of the suspect component to see if the component has an open circuit or an open circuit. This way you can determine the problem in one step. Step 4: Also use a multimeter to measure integrated circuits, transistors, resistors, etc. on the board to see if it meets its own logic performance. If the circuit board includes a bus structure, generally on the bus, there will be a pull-up resistor row. The measurement of the resistance row is a very important step. By its good or bad, the quality of the chip hanging on the bus can be initially checked. After inspection by observation method and static measurement method, most of the problems in circuit board maintenance can be solved. It is worth noting that the power supply should be normal and avoid secondary damage to the board after the next step. 3 online measurement method The online measurement method is generally applied to manufacturers of mass production circuit boards. For the convenience of maintenance, the manufacturer generally builds a relatively common debugging and maintenance platform, which can conveniently provide the power required for the circuit board and some necessary initial signals. Online measurement mainly solves two problems. The first is to subdivide the problems found in the last two steps, and finally lock to the components in question. Second, through the above two steps of inspection, the problem has not been resolved, you need to find the cause of the failure through online measurement. Online measurement is mainly carried out through the following steps. Step 1: Power on the board. In this step, it should be noted that some boards are not single, they may need 5V, and they need positive and negative 12V, 24V, etc. Don't add the added power. It is. After the board is powered, touch the components on the board to see if there are any hot components. Check the 74 series chips. If the components are hot, the components may be damaged. After replacing the component, check if the board failure has been resolved. Step 2: Measure the gate on the board with an oscilloscope and see if it is logical. If the output does not conform to the logic, it needs to be treated separately in two cases. One is that the output should be low level, the actual measurement is high level, and the chip can be directly judged to be damaged; the other is that the output should be high level. The actual measurement is low, and it is not possible to determine that the chip has been damaged. It is also necessary to disconnect the chip from the latter circuit, measure again, observe whether the logic is reasonable, and determine whether the chip is good or bad. Step 3: Use an oscilloscope to measure the crystal in the digital circuit to see if it has an output. If there is no output, the chip connected to the crystal oscillator needs to be removed as much as possible before measurement. If there is no output, it is preliminarily determined that the crystal oscillator has been damaged; if there is output, it is necessary to replace the removed chips one by one, install a piece of the test piece, and find out the fault. The fourth step: digital circuit with bus structure, generally includes three channels of digital, address and control bus. Use an oscilloscope to measure the three-way bus, compare the schematics, observe whether the signal is normal, and find out the problem. The online measurement method is mainly used for comparison of two good and bad circuit boards, and through comparison, problems are found and problems are solved. Thereby completing the maintenance of the board.
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