Component testing skills: precautions for electronic component fault inspection

Date：2022-01-07 12:00:10Views：945

As long as there is no problem in the design of general digital circuits, digital circuits are relatively stable. The most likely problems are those separate components, such as transistors and so on. Therefore, it is best to use more mature circuits and integrated circuit modules in the design, which will ensure the stability of the system. Aiming at the fault phenomenon of electronic products, we can deal with the faulty components through observation, hearing, feeling and touch, eliminating many unnecessary testing methods. If the above methods do not work, instruments and meters need to be used to test the suspicious components and related circuits to find out the fault point. When some components fail, the whole or some parts of the circuit will usually be shut down, making it unable to work, resulting in abnormal changes to electronic products. Therefore, the judgment and maintenance of component failure is a very important step.

1. Components and parts shall not be replaced arbitrarily, especially some key components and parts shall be replaced with those of the original manufacturer in principle, because many components and parts have special safety performance, and this performance can not be seen on the surface, such as power fuse and fuse resistance, which seems to be no different from ordinary fuse and fuse resistance, However, there are often marks on the circuit diagram, some are represented by shadows, some are marked by triangles, and important originals marked in the electrical schematic diagram must be replaced according to the model on the original diagram. However, some unimportant discrete components can be replaced with the same model and specification, such as diode, transistor, ordinary resistor, etc.

2. The quality of replacing new parts must be guaranteed. Some repairmen often buy some processed components. Such components must be screened before use, so as to avoid new trouble to the maintenance of the whole machine due to the poor performance of the replaced components.

3. The capacity of replacement components must meet the original specifications. Of course, it is best to replace components of the same type and model. Do not increase the capacity of components, nor reduce their capacity specifications, nor allow them to be replaced with components of smaller specifications than the original. For example, high-power transistors cannot be replaced by low-power transistors, and delay fuse resistors cannot be replaced by ordinary fuse resistors.

resistor

In any electronic product, the utilization rate of resistors is very high, the proportion of consumption is also very high, and there are many types of resistors, mainly including metal film, wire winding, carbon film, oxide film and other fixed resistors. There are also adjustable resistors, safety resistors, thermistors, photosensitive resistors and various potentiometers.

Fault phenomenon: most of the resistors are in the electronic equipment circuit. If the incoming current exceeds the maximum rated value borne by the circuit, various resistors will be burned out. When the resistors are burned out, some are open circuit, and some surfaces are blackened or charred. In addition, it is found that poor contact leads to desoldering, there are also problems with the product quality of some resistors, resulting in resistance damage. In another case, it may be due to poor lead welding contact. For example, the poor contact of the audio potentiometer will produce a click sound.

Inspection and measurement of resistance: if a suspected burnt resistance is found, find a resistance with the same ohmic power value and connect it to the suspected circuit. If the fault is removed, there is a problem with this resistance. It should be that there are many open circuits. Replace the problem resistance, measure the suspected resistance with a multimeter, and turn off the power of the equipment before measurement, The main purpose is to ensure the safety of the multimeter. Then, short circuit the large electrolytic capacitor in the circuit to the positive and negative ends of the capacitor with the multimeter probe, and you will hear a slap discharge sound. If the measured resistance value is infinite, it indicates that the resistance is open circuit. If the measured resistance value is zero, the resistance is completely short circuited. If the measured resistance value is too far from the marked value, This resistor can no longer be used.

Troubleshooting methods: when the resistance is burnt out, first analyze what causes the resistance to burn out, check whether there is a short circuit in the circuit, look more, smell more and measure more, and do not easily replace the components as soon as they come up, otherwise the fault phenomenon will further expand. Replace the resistance after finding out the cause, and do not replace the resistance with greater power than the original power, It shall be replaced in strict accordance with the specifications marked in the original circuit diagram.

capacitor

The main fault causes of the capacitor are that the capacitor has been used for a long time, the sealing performance becomes worse, and the leakage is serious, and the insulation performance of the oxide film dielectric in the capacitor becomes worse. When it is suspected that the capacitor is faulty during repair, the resistance value can be measured with a multimeter. Except for electrolytic capacitors, the resistance value is generally very large. For non-polar capacitors, the resistance value is generally tens of megohm or more than 100 megohm, and the gauge needle can hardly move. If it is less than a few megohm, the capacitor also has leakage. Judgment on the quality of the capacitor, Electrolytic capacitors with positive and negative electrodes are generally 0.01 μ F and above can be measured with the high resistance gear of the meter. When the two probes touch both ends of the capacitor, usually the probe will swing suddenly (the greater the capacitance, the greater the swing range of the probe), and then the pointer will gradually rise from small to large, and swing to the infinity direction value. When the exchange probe is re measured, the probe will swing again, and then slowly return to the infinity direction from small to large, This proves that the capacitor is intact. If the position of the meter needle swing is small, the capacity of the capacitor is small. If there is a resistance value when the meter needle returns after swing, it means that the leakage resistance of the capacitor is too small. It is difficult to measure the quality of the non-polar capacitor with small capacity (below 6000 PF) with a multimeter, In this case, a capacitor element with the same model and specification can be used to judge whether it is normal.

Transistor fault check

Transistors are the main parts of the circuit. At the same time, the normal operation of these components has a decisive impact on the circuit, which should be the first object to be searched. For the test of various indicators of transistors, the most rapid and accurate way is to use a transistor tester, but this instrument is more expensive. When using a multimeter for simple test and inspection, you can judge the quality of transistors directly on the base plate by testing the voltage at all levels without welding small transistors. It should be noted that the voltage polarities at all levels of NPN tube and PNP tube are opposite. Judge whether the transistor amplifier works normally. You can measure the voltage (bias) between the base and emitter. If the silicon tube is 0.6 ~ 0.7 V and the germanium tube is 0.2 ~ 0.3 V, it is the normal amplification state.

Transistor replacement：

1. To replace the transistor, the power supply must be cut off to prevent short circuit from burning the transistor.

2. The bracket for fastening the transistor shall be well insulated to avoid damaging the transistor due to poor insulation.

3. Transistors are very sensitive to temperature. When the temperature rises, the characteristics of transistors will change. Do not forget to install heat sinks on transistors with high power.

4. The voltage exceeding the rated value shall not be added to the transistor by mistake to avoid tube breakdown.

5. Insulating fins are generally added between high-power transistors and heat sinks.

6. Pay attention to pressure resistance when replacing the secondary pipe.

Fault inspection of integrated circuit

When the integrated block is damaged, like other components, it shall be analyzed and judged first. Carefully measure the DC voltage of each pin according to the data provided by the electrical schematic diagram, and then carefully observe its input and output signal waveform. Because the pin numbers of the general integrated block are too many, it is not easy to remove and measure, and it is of little significance. Unless it is determined that the integrated block is completely damaged, it can be removed and replaced with new components. Generally, do not remove and measure. Of course, the replacement method is the best maintenance method. In the actual repair, check the integrated circuit fault. The more use method is to carefully measure the DC voltage value of each pin of the integrated block and compare it with the normal value marked on the drawing, so as to find out the fault. It should also be carefully analyzed and judged. It must not be easy to identify that the integrated block has a fault if the voltage is abnormal, which will sometimes be counterproductive.

The replacement and disassembly of integrated circuits are troublesome, so pay attention to the following points: it is better to use 20 ~ 30W soldering iron; The soldering iron shell must be grounded; Pay attention to the copper foil falling of the printing plate during operation; Pay attention to the order of pins when installing a new integrated block; Substitution of integrated circuits; Generally speaking, when the integrated circuit model is different, it cannot be replaced. Even if the specifications of the same model are different, the direct replacement effect is also different. Therefore, it is necessary to replace it in strict accordance with the specified model.

Electronic components are the basic elements of electronic products and the basic materials of electronic products. Their performance and quality directly affect the performance and quality of electronic products. We have a certain understanding and understanding of the fault causes and detection methods of resistors, capacitors, transistors and integrated module electronic components, which can also provide a solid foundation for the stability and reliability of electronic equipment.