Several Principles of Circuit Board Maintenance

Several Principles of Circuit Board Maintenance:

Principle 1: Look first and then measure

The circuit board to be repaired should be visually inspected first. If necessary, it should be observed with the help of a magnifying glass.

Main aspects that need to care:

1. Whether there are broken wires and short circuits, especially whether the printed circuit board connection lines on the circuit board have fractures and adhesions.
2. Whether the related components such as resistors, capacitors, inductors, diodes, transistors, etc. are disconnected.
3. Has anyone repaired it? Which components have been moved? Are there problems such as false soldering, missing soldering, reverse insertion, etc.

After eliminating the above situation, first use a multimeter to measure the resistance between the power supply and the ground of the circuit board. Usually the resistance of the circuit board should not be less than 70Ω. If the resistance is too small, it is only a few or ten ohms. It means that there are components on the circuit board that are broken down or partially broken down, and measures must be taken to find the broken down components. The specific method is to power up the repaired board (At this time, be sure to find out the voltage value of the working voltage of the board and the positive The negative polarity should not be wrongly connected or added higher than the working voltage value. Otherwise, the circuit board to be repaired will be damaged, and the old faults will not be eliminated and new faults will be added) Measure the temperature of each device on the circuit board with a point thermometer, and the temperature rises faster and higher ones are regarded as key suspects.

If the resistance value is normal, use a multimeter to measure the components such as the diode, triode field effect transistor and stripping switch on the board. The purpose is to first ensure that the component being measured is normal. Don’t complicate problems that can be solved with common test tools (such as multimeters, etc.).

 Principle 2: First outside then inside

If the situation allows, it is best to have a good circuit board that is the same as the board to be repaired as a reference, and then use the dual-bang VI curve scanning function of the tester to compare the two boards. The first comparison test point can be started from the port of the circuit board. Then the comparison test of the capacitor is carried out from the outside to the inside, which can make up for the shortcoming that the multimeter is difficult to measure the leakage of the capacitor online.

Principle 3: First easy then difficult

In order to improve the test effect, some technical treatment should be done on the repaired board before the online functional test of the circuit board to minimize the impact of various interferences on the test process. The specific measures are as follows:

1. Preparation before the test:

Short-circuit the crystal oscillator (Note that for the four-pin crystal oscillator, it is necessary to know which two pins are signal output pins, and these two pins can be short-circuited. Remember that under normal circumstances, the other two pins are power pins and must not be short-circuited!) For large-capacity electrolytic capacitors, it is also necessary to solder a foot to make it open, because the charging and discharging of large-capacity capacitors will also cause interference.

2. Test the device using the exclusion method:

During the online test or comparison test of the device, please directly confirm the test result and record the device that passed the test (or relatively normal), and test it again if the test fails (or is out of tolerance). If it still fails, you can confirm the test results first, and keep testing until the devices on the board are tested (or compared). Then deal with those devices that fail the test (or are out of tolerance).

For devices that fail the functional online test, some test instruments also provide a less formal but more practical processing method: because the power supply of the test instrument to the circuit board can also be applied to the corresponding power and ground of the device through the test clip. If the power pins of the device are cut on the pins, the device will be disconnected from the power supply system of the circuit board.

At this time, perform an online functional test on the device; since other devices on the circuit board will not be energized to work to eliminate the interference effect, the actual test effect at this time will be equivalent to the “quasi-offline test”. The accuracy rate will be greatly improved.