1) Observation method

When we get a circuit board to be repaired, we first observe its appearance carefully. If the circuit board is burned, then before powering on the circuit board, be sure to carefully check whether the power supply circuit is normal, and then power on again after ensuring that it will not cause secondary damage. Observation method 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 mainly depends on whether the circuit board has been dropped, resulting in the deformation of the board corner, or the chip on the board has been deformed or broken. Observe the socket of the chip to see if it is forced to be broken because there is no special tool. Observe the chip on the circuit board. If it is equipped 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 circuit board is powered on, it may burn out the chip and cause unnecessary loss. If there is a short-circuit terminal on the circuit board, observe whether the short-circuit terminal is inserted incorrectly. The maintenance of the circuit board needs a solid theoretical foundation and careful work. Through the careful observation of the repairer, sometimes the cause of the problem can be determined at this step.

The second step is to observe whether the components on the circuit board are burnt out. For example, whether the resistance, capacitance and diode are blackened and pasted. Under normal conditions, even if the resistance is burnt, its resistance value will not change, the performance will not change, and the normal use will not be affected. At this time, a multi-meter is needed for auxiliary measurement. However, if the 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 circuit on the circuit board is bulging, cracked, burnt and blackened. If this happens, it can be determined that the chip has been burned out and must be replaced. The fourth step is to observe whether the wiring on the circuit board has peeling, burning and open circuit. Whether the sink hole is separated from the pad. Step 5 is to observe the fuse on the circuit board to see if the fuse is blown. Sometimes because the fuse is too thin to see clearly, you can use the auxiliary tool to judge whether the fuse tube is damaged.

The occurrence of the above situation is mostly due to the consequences of excessive current in the circuit. But what is the specific reason for the excessive current, it is necessary to analyze the specific problems. But the general idea of finding problems is to first carefully analyze the schematic diagram of the circuit board, and then according to the circuit where the burned components are located, find its superior circuit, and deduce it step by step. Then, with some experience accumulated in the work, analyze the place where the problem is most likely to occur and find out the cause of the fault.

2) Static measurement method

In the maintenance of circuit board, it is difficult to find some problems by observation method, unless it is obviously burnt or deformed. But most of the problems still need to be measured by Voltmeter before the conclusion can be drawn. Circuit board components and related parts should be tested one by one. The main tool for maintenance is a multimeter. The first step is to detect the short circuit between the power supply and the ground and check the cause. The second step is to detect whether the diode is normal, and then check whether there is a short circuit or even an open circuit in the capacitor. Finally, check the related integrated circuit, resistance and other related device indicators of the circuit board. Using observation method and static measurement method can solve most of the problems in circuit board maintenance, but in the measurement to ensure that the power supply is normal, no secondary damage.

3) Online measurement

Online measurement method is generally used in batch production of circuit board manufacturers, manufacturers in order to maintain convenient, generally will build a more general debugging and maintenance platform, it can easily provide the circuit board required power and some necessary initial signal. On line measurement mainly solves two problems. One is to subdivide the problems found in the previous two steps, and finally lock them to the components with problems. Second, through the above two steps of inspection, the problem has not been solved, need to find out the cause of the fault through online measurement. The on-line measurement method is mainly carried out through the following steps.

The first step is to power up the circuit board. In this step, it should be noted that the power supply of some circuit boards is not single. It may need 5V, but also need positive and negative 12V, 24V, etc. don't omit the added power supply. After the circuit board is powered on, touch the components on the circuit board by hand to see if there are hot components. Focus on the 74 series chips. If the components are hot, it indicates that the components may have been damaged. After replacing the components, check whether the circuit board fault has been solved.

The second step is to 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 logic, it needs to be treated separately in two cases. 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 high-level, and the actual measurement is low, which can not judge whether the chip has been damaged. It is also necessary to disconnect the chip from the following circuit, measure again, observe whether the logic is reasonable, and judge whether the chip is good or bad.

Step 3 is to measure the crystal oscillator in digital circuit with oscilloscope to see if it has output. If there is no output, the chip connected to the crystal oscillator should be removed as much as possible before measurement. If there is no output, it is preliminary determined that the crystal oscillator has been damaged; if there is output, it is necessary to install the removed chips one by one, install one piece and measure one piece to find out the fault. As we know that digital circuit contains bus structure, generally including number, address and control bus. So we need to measure the three-way bus with oscilloscope, compare the schematic diagram, observe whether the signal is normal, and find out the problem. Online measurement method is mainly used to compare two good and bad circuit boards. Through comparison, problems can be found and solved. So as to complete the maintenance of the circuit board.