Methods and skills in circuit board maintenance

The introduction

Control system due to the high price, so when its failure, in order to pay attention to economic benefits, cost saving, generally use the way of maintenance. But in the following cases, the need to replace a new circuit board: the circuit board has reached the end of life; The circuit board is badly damaged and cannot be repaired; After many times of repeated maintenance, there are problems, indicating that the circuit board exists unstable factors, is no longer suitable for continued use in machine tools. I engaged in the circuit board maintenance work for many years, in the work summed up some maintenance methods and skills, introduced to everyone, and everyone to encourage.

1 observation

When we get a circuit board to be repaired, we first take a close look at its appearance. If the circuit board is burned, then before energizing the circuit board, be sure to carefully check whether the power circuit is normal, to ensure that it will not cause secondary damage before energizing. The observation method is a kind of static inspection method. The following steps are generally followed when using the observation method.

The first step is to observe whether the circuit board is artificially damaged, which is mainly from the following aspects:

(1) to see whether the circuit board has been dropped, resulting in the corner of the board deformation, or the chip on the board was dropped deformation or broken.

Observe the chip socket, to see if there is no special tools, and was forced to pry bad.

(3) observe the chip on the circuit board, if with a socket, first observe whether the chip is inserted wrong, this is mainly to prevent the operator to repair the circuit board when the chip position or direction inserted wrong. If the error is not corrected in time, when the circuit board is energized, it may burn out the chip, causing unnecessary losses.

If there is a short terminal on the circuit board, observe whether the short terminal is inserted wrong.

Circuit board maintenance needs a solid theoretical foundation, work carefully, through the maintenance of careful observation, sometimes in this step can be judged the cause of the problem.

The second step is to see if the components on the circuit board have been burned. Such as resistors, capacitors, diodes are not black, paste the situation. Under normal circumstances, even if the resistance is burned, its resistance value will not change, performance will not change, does not affect the normal use, in this case need to use a multimeter auxiliary measurement. However, if capacitors and diodes are burned, 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 the integrated circuit on the circuit board, such as the 74 series, CPU, coprocessor, AD and other chips, whether there is a bulge, crack, burn, blackening situation. If such a situation occurs, it is almost certain that the chip has been burned out and must be replaced.

The fourth step is to observe whether the circuit line on the circuit board is peeled, burnt and broken. Does the sink hole come off the solder pad?

Step 5: Observe the safety of the circuit board (including the safety tube and thermistor) to see if the fuse is blown. Sometimes because the fuse is too thin, can not see clearly, you can use auxiliary tools – multimeter to determine whether the insurance pipe is damaged.

The occurrence of the above four cases is mostly caused by the large flow of electricity in the circuit. But the specific cause of the current is too large, it is necessary to analyze the specific problems. But the overall idea of finding the problem is to carefully analyze the schematic diagram of the circuit board, and then according to the burned components in the circuit, find its superior circuit, step by step upward derivation, and then by some experience accumulated in the work, analysis of the most prone to problems, find out the cause of the failure.

2 Static measurement method

For most of the circuit boards, through the previous observation method, will not find the problem. As long as a small part of the circuit board will be due to some special reasons physical deformation, easy to find out the cause of the fault, most of the fault circuit board, or need to use the multimeter, some of the main components on the circuit board, key points to carry out orderly measurement, find problems, solve problems.

Before measurement, it is necessary to determine whether the circuit is dominated by analog signals or digital signals. For a circuit board with a schematic, you can tell by looking at the schematic. But for no principle diagram of the circuit board, usually by the following two ways: (1) of the components on printed circuit boards to observe, to see if there is a microprocessor circuit board, whether in early 80, 51 series, is now widely used DSP, as long as the circuit boards of chips, means that the board on the bus structure, digital signal will be part of a large, you can use it as a digital board to deal with. (2) for the circuit board without a microprocessor, observe the components on the board, see the application of 5V power chip. If you have a lot of 5V power chips, you can also use it as a digital circuit for repairs. The maintenance methods of digital circuit and analog circuit are different. Generally speaking, analog circuit is easier to repair. It can be deduced step by step to find out the problems. But for digital circuits, there is no clear relationship between the upper and lower levels because the circuits are all attached to the bus. Therefore, maintenance is more difficult, the following only focuses on the digital circuit static measurement method, maintenance mainly follow the following steps to carry out.

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

The way to check is to find a 5V power chip and measure the two points on the diagonal (for example, a 14-pin chip, measure the 7-pin and 14-pin chip).

For 16-pin chips, measure 8-pin and 16-pin). If there is no short circuit between the two points, the power supply is generally working normally. If there is a short circuit, it is necessary to find the cause through the screening method.

These steps are only the basic ideas of power maintenance, specific to a particularly complex circuit board also need to be specific analysis of specific problems.

The power supply is the basis of the circuit. Only when the power supply works normally can the application of subsequent circuits be discussed. Therefore, the measurement of power supply is very important, and is also a step which is easy to be ignored by maintainers.

Step 2: Use a multimeter to measure the diode and see if it works properly. Normally, the positive and negative electrodes are measured by resistance, with the positive phase measured in tens to hundreds of euros and the negative phase measured in thousands to thousands of euros. Generally, diode damage occurs when the current in the circuit is too high and the diode is broken.

Step 3: Use a multimeter resistance to measure the capacitance, see if there is a short circuit, open circuit, if there is a problem with this part of the circuit. The next step is to determine whether there is a problem with the component itself, or with the circuit to which it is connected, by welding down a foot of the suspect component and seeing if it has a break. This allows you to identify the problem in one step.

The fourth step: the same use of multimeter circuit board on the integrated circuit, transistor, resistor to measure, see whether it conforms to its logical performance. If the circuit board includes the bus structure, generally in the bus, there will be a pull resistor. The measurement of resistance row is a very important step, through its good or bad can initially check the chip hanging on the bus good or bad.

After the inspection of the observation method and static measurement method, most of the problems in the circuit board maintenance can be solved. It is worth noting that the power supply must be ensured to avoid secondary damage to the circuit board after the next step.

Online measurement method

The online measurement method is generally applied to manufacturers of mass production of circuit boards. In order to facilitate maintenance, manufacturers will generally build a more general debugging and maintenance platform, which can conveniently provide the power needed for circuit boards and some necessary initial signals. On-line measurement method mainly solves two problems. The first is to subdivide the problems found in the previous two steps, and finally lock to the component with the problem. 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. Online measurement is mainly carried out through the following steps.

Step 1: Electrify the circuit board. In this step, it should be noted that some circuit board power supply is not a single, may require 5V, plus or minus 12V,24V, etc., do not add the power leakage. After the circuit board is energized, touch the components on the circuit board by hand to see if there are hot components, focus on checking the 74 series of chips, if the component is hot, it means that the component may have been damaged. After replacing the component, check whether the circuit board fault has been resolved.

Step 2: Use oscilloscope to measure the gate circuit on the circuit board and observe whether it conforms to the logic relation. If the output does not conform to logic, two situations should be treated separately. 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 the output should be high level, the actual measurement is low, and can not judge the chip has been damaged, but also need to disconnect the chip and the circuit behind, measurement again, to observe whether the logic is reasonable, to determine the quality of the chip.

Step 3: Measure the crystal vibration 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, the crystal vibration is preliminarily judged to be damaged. If there is output, the chip needs to be removed piece by piece back, installed a piece of a test piece, find out the fault.

Step 4: Digital circuit with bus structure, generally including number, address, control bus three channels. The oscilloscope is used to measure the three-way bus, compare the schematic diagram, observe whether the signal is normal, and find out the problem.

The online measurement method is mainly used for the comparison of two good and bad circuit boards, through comparison, to find problems and solve problems. To complete the maintenance of the circuit board.