9 common sense of basic inspection of PCB board

It is time to pay attention to some details in the inspection of the PCB board, so as to ensure the quality of the product more prepared. When testing the PCB board, we should pay attention to the following 9 common sense.



1. It is strictly forbidden to use the grounded test equipment to contact the live TV, audio, video and other equipment to detect the PCB without an isolation transformer
It is strictly forbidden to directly test the TV, audio, video and other equipment without power supply isolation transformer with instruments and equipment whose shell is grounded. Although general radio cassette recorders have power transformers, when they come into contact with more special TV or audio equipment with large output power or little understanding of the nature of the power supply, it is first necessary to find out whether the chassis of the radio is live, otherwise it is very easy TVs, stereos, and other equipment that are charged to the backplane cause a short circuit in the power supply, which affects the integrated circuit and further expands the fault.

2. Pay attention to the insulation performance of the electric soldering iron when inspecting the PCB board
It is not allowed to use a soldering iron for welding. It is better to make sure that the soldering iron is not charged. It is best to ground the shell of the soldering iron. You should be more careful with the MOS circuit. It is safer to use a 6~8V low-voltage circuit iron.

3. Before testing the PCB board, you must understand the working principle of the integrated circuit and related circuits
Before checking and repairing the integrated circuit, you must first be familiar with the function of the integrated circuit used, the internal circuit, the main electrical parameters, the role of each pin and the normal voltage, waveform and peripheral components of the pin. If the above conditions are met, analysis and inspection will be much easier.

4. Test the PCB board not to cause short circuit between pins
When measuring voltage or testing waveforms with an oscilloscope probe, do not cause short-circuits between the pins of the integrated circuit due to sliding of the test leads or probes. It is best to measure on the peripheral printed circuit directly connected to the pins. Any instantaneous short circuit is easy to damage the integrated circuit, and more care should be taken when testing flat-pack CMOS integrated circuits.

5. The internal resistance of the test instrument for testing the PCB board should be large
When measuring the DC voltage of integrated circuit pins, a multimeter with an internal resistance greater than 20KΩ/V should be used. Otherwise, there will be a large measurement error for some pin voltages.

6. Check the PCB board should pay attention to the heat dissipation of the power integrated circuit
The power integrated circuit should have good heat dissipation, and it is not allowed to work in a state of high power without a radiator.

7. Reasonable detection of PCB board leads
If it is necessary to add peripheral components to replace the damaged parts inside the integrated circuit, small components should be used, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially to deal with the grounding between the audio amplifier integrated circuit and the pre-amplifier circuit end.

8. Test the PCB board to ensure the welding quality
Welding is indeed firm during welding, and the accumulation of solder and porosity are likely to cause virtual soldering. The soldering time is generally no more than 3 seconds, and the power of the soldering iron should be about 25W internally. The soldered integrated circuit should be carefully checked. It is best to use an ohmmeter to measure whether there is a short circuit between each pin, confirm that there is no solder adhesion, and then turn on the power.

9. Check the PCB board and do not easily judge the damage of the integrated circuit
Do not easily judge that the integrated circuit is damaged. The vast majority of integrated circuits are directly coupled. Once a circuit is abnormal, it may cause multiple voltage changes. These changes are not necessarily caused by damage to the integrated circuit. In addition, in some cases, the voltage of each pin is measured to be normal. When the values ​​match or are close, it may not necessarily mean that the integrated circuit is good. Because some soft faults will not cause changes in DC voltage.

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