PCB Fabrication

Three methods of circuit board maintenance

by:A-TECH      2020-04-30
When we get a circuit board to be repaired, we should first observe its appearance carefully. If the circuit board has been burned, then before powering on the circuit board, be sure to carefully check whether the power supply circuit is normal and energize it after ensuring that it will not cause secondary damage. Observation is a kind of static inspection method. When using observation method, the following steps are generally followed. The first step is to observe whether the circuit board has been artificially damaged, which is mainly seen from the following aspects: ① see if the circuit board has been dropped, resulting in deformation of the board angle, or the chip on the board is deformed or broken. (2) observe the socket of the chip to see if it is forced to pry out because there is no special tool. (3) observe the chip on the circuit board. If it has a socket, first observe whether the chip is inserted incorrectly, which is mainly to prevent the operator from inserting the chip in the wrong position or direction when repairing the circuit board. If the error is not corrected in time, when the circuit board is energized, the chip may be burnt out, causing unnecessary losses. (4) If the circuit board has a short-circuit terminal, observe whether the short-circuit terminal is inserted incorrectly. The maintenance of the circuit board needs a solid foundation in theory, careful and earnest work, and through careful observation by the repairer, sometimes the cause of the problem can be judged at this step. The second step is to observe whether the components on the circuit board have been burned out. For example, whether resistors, capacitors and diodes are blackened or pasted. Under normal circumstances, even if the resistor is burnt, its resistance value will not change, its performance will not change, and its normal use will not be affected. At this time, a multimeter is required to assist in measurement. However, if capacitors and diodes are burnt, their performance will change and they will not play their due role in the circuit, which will affect the normal operation of the whole circuit, at this time, new components must be replaced. The third step is to observe whether the integrated circuits on the circuit board, such as 74 series, CPU, coprocessor, AD and other chips, are bulging, cracked, burnt and blackened. If such a situation occurs, it is basically certain that the chip has been burnt out and must be replaced. The fourth step is to observe whether the wiring on the circuit board is peeling or burning. Does the copper hole break away from the pad. Step 5: Observe the insurance on the circuit board (Including safety tube and thermistor)See if the fuse is blown. Sometimes because the fuse is too thin to see clearly, you can use auxiliary tools-Multimeter to determine whether the safety tube is damaged. The occurrence of the above four situations is mostly due to the consequences caused by large current in the circuit. However, the specific reason for the excessive current is to analyze the specific problems. However, the general idea of finding the problem is to carefully analyze the schematic diagram of the circuit board first, and then find its superior circuit according to the circuit where the burned components are located, and deduce it step by step, then, based on some experience accumulated in the work, analyze the places where problems are most likely to occur and find out the causes of the faults. In circuit board maintenance, it is often difficult to find some problems by observation, unless it is obviously burned or deformed. However, most of the problems still require voltmeter measurement before conclusions can be drawn. Circuit board components and related parts shall be tested one by one. The maintenance steps should be operated according to the following procedure. The main practical tool is multimeter. Step 1: detect the short circuit between the power supply and the ground to see the reason. Step 2: Check whether the diode is normal. Step 3: Check whether there is a short circuit or even an open circuit in the capacitor. Step 4: Check the integrated circuit related to the circuit board, as well as the related device indexes such as resistance. We can solve most of the problems in circuit board maintenance by observation and static measurement, which is beyond doubt, but we should ensure that the power supply is normal and no secondary damage can occur during measurement. The on-line measurement method is generally used in manufacturers of mass production circuit boards. In order to facilitate maintenance, manufacturers generally build a more general debugging and maintenance platform, it can conveniently provide the power supply required by the circuit board and some necessary initial signals. The on-line measurement method mainly solves two problems. One is to subdivide the problems found in the previous two steps and finally lock them to the faulty components. Second, through the above two steps of inspection, the problem has not been solved, and the cause of the fault needs to be found through online measurement. The on-line measurement method is mainly carried out through the following steps. Step 1: energize the circuit board. In this step, it should be noted that some circuit board power supplies are not single, may need 5 V, plus or minus 12 V, 24 V, etc, don't leak the power supply. After the circuit board is electrified, touch the components on the circuit board by hand to see if there are components with Perm heat. Focus on checking the 74 series CHiPs. If the components are hot, it means that this component may have been damaged. After replacing components, check whether the circuit board fault has been resolved. Step 2: Measure the gate circuit on the circuit board with an oscilloscope to observe whether it conforms to the logical relationship. If the output does not conform to the logic, it needs to be treated separately in two situations. One is that the output should be low level, and the actual measurement is high level, which can directly judge the chip damage; The other is that the output should be at a high level, and the actual measurement is low, so it cannot be judged that the chip has been damaged. It is also necessary to disconnect the chip from the following circuit and measure it again, observe whether the logic is reasonable and judge whether the chip is good or bad. Step 3: Measure the crystal oscillator in the digital circuit with an oscilloscope to see if it has 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 install the removed chips one by one and install them one by one to find out the fault. Step 4: digital circuit with bus structure generally includes digital, address and control bus. Measure the three-way bus with an oscilloscope, compare the schematic diagram, observe whether the signal is normal, and find out the problem. The on-line measurement method is mainly used for comparing two good and bad circuit boards. Through comparison, problems can be found and solved. So as to complete the maintenance of the circuit board.
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