Experience in testing and replacing electronic components

1. Resistor detection method and experience:
1. Fixed resistor detection.
A. Connect two test leads (regardless of positive and negative) to the two ends of the resistor to measure the actual resistance value. In order to improve the measurement accuracy, the range should be selected according to the nominal value of the measured resistor. Due to the nonlinear relationship of the ohm scale, its middle section is more finely divided, so the pointer indication value should be as close to the middle of the scale as possible, that is, within the 20% to 80% arc range of the full scale, so as to make the measurement more accurate. According to the different resistance error levels. The reading is allowed to have an error of ±5%, ±10% or ±20% between the nominal resistance value. If it does not match and exceeds the error range, it means that the resistance value has changed.
B Note: When testing, especially when measuring resistance above tens of kΩ, do not touch the test leads and the conductive part of the resistor with your hands; when the resistor being tested is soldered off the circuit, at least one end should be soldered off to prevent other components in the circuit from affecting the test and causing measurement errors; although the resistance of the color ring resistor can be determined by the color ring mark, it is best to use a multimeter to test its actual resistance when using it.

2. Detection of cement resistors. The method and precautions for detecting cement resistors are exactly the same as those for detecting ordinary fixed resistors.

3. Detection of fuse resistors. In the circuit, when the fuse resistor is blown and the circuit is disconnected, a judgment can be made based on experience: if the surface of the fuse resistor is found to be black or burnt, it can be concluded that it is overloaded and the current passing through it exceeds the rated value many times; if there is no trace on its surface and the circuit is open, it indicates that the current flowing through is just equal to or slightly greater than its rated fuse value. To judge whether a fuse resistor without any trace on the surface is good or bad, it can be measured with the help of the multimeter R×1 gear. To ensure accurate measurement, one end of the fuse resistor should be welded off the circuit. If the measured resistance value is infinite, it means that the fuse resistor has failed and the circuit is open. If the measured resistance value is far different from the nominal value, it indicates that the resistance value has changed and it should not be used again. In maintenance practice, it is found that a small number of fuse resistors are short-circuited in the circuit, and attention should also be paid during detection.

4. Potentiometer inspection. When inspecting the potentiometer, first turn the handle to see if the handle rotates smoothly, whether the switch is flexible, whether the "click" sound is crisp when the switch is on and off, and listen to the friction sound between the contact point and the resistor body inside the potentiometer. If there is a "rustling" sound, it means that the quality is poor. When testing with a multimeter, first select the appropriate resistance position of the multimeter according to the resistance value of the potentiometer to be tested, and then you can test it according to the following method. 
A Use the ohm range of the multimeter to measure the "1" and "2" terminals. The reading should be the nominal resistance value of the potentiometer. If the pointer of the multimeter does not move or the resistance value is very different, it means that the potentiometer is damaged.
B Check whether the movable arm of the potentiometer is in good contact with the resistor. Use the ohm range of the multimeter to measure the "1" and "2" (or "2" and "3") terminals, and turn the shaft of the potentiometer counterclockwise to a position close to the "off" position. At this time, the smaller the resistance value, the better. Then slowly rotate the shaft handle clockwise, the resistance value should gradually increase, and the pointer in the meter should move smoothly. When the shaft handle is rotated to the extreme position "3", the resistance value should be close to the nominal value of the potentiometer. If the pointer of the multimeter jumps during the rotation of the shaft handle of the potentiometer, it means that the movable contact has a poor contact fault.

5. Testing of positive temperature coefficient thermistor (PTC). When testing, use the multimeter R×1 gear, which can be divided into two steps:
A. Normal temperature test (indoor temperature close to 25℃); touch the two pins of the PTC thermistor with two test leads to measure its actual resistance, and compare it with the nominal resistance. If the difference between the two is within ±2Ω, it is normal. If the actual resistance value differs too much from the nominal resistance value, it means that its performance is poor or it is damaged.
B. Heating test; on the basis of normal normal temperature test, the second step of the test - heating test can be carried out. Place a heat source (such as a soldering iron) close to the PTC thermistor to heat it, and use a multimeter to monitor whether its resistance value increases with the increase of temperature. If so, it means that the thermistor is normal. If the resistance value does not change, it means that its performance has deteriorated and it cannot be used. Be careful not to make the heat source too close to the PTC thermistor or directly contact the thermistor to prevent it from being burned.

6 Testing of negative temperature coefficient thermistors (NTC).
(1) Measuring the nominal resistance value Rt The method of measuring NTC thermistors with a multimeter is the same as the method of measuring ordinary fixed resistors, that is, selecting a suitable resistance block according to the nominal resistance value of the NTC thermistor can directly measure the actual value of Rt. However, since NTC thermistors are very sensitive to temperature, the following points should be noted during testing:
A Rt is measured by the manufacturer at an ambient temperature of 25°C, so when measuring Rt with a multimeter, it should also be done when the ambient temperature is close to 25°C to ensure the reliability of the test.
B The measured power must not exceed the specified value to avoid measurement errors caused by the thermal effect of the current.
C Pay attention to correct operation. During the test, do not pinch the thermistor body with your hands to prevent the body temperature from affecting the test.
(2) Estimation of temperature coefficient αt: First, measure the resistance value Rt1 at room temperature t1, then use a soldering iron as a heat source, close to the thermistor Rt, measure the resistance value RT2, and use a thermometer to measure the average surface temperature t2 of the thermistor RT before calculation.

7. Testing of varistor. Use the R×1k range of the multimeter to measure the forward and reverse insulation resistance between the two pins of the varistor. Both are infinite. Otherwise, it means that the leakage current is large. If the measured resistance is very small, it means that the varistor is damaged and cannot be used.

8. Detection of photoresistor.

A Cover the light-transmitting window of the photoresistor with a piece of black paper. At this time, the pointer of the multimeter remains basically motionless, and the resistance value is close to infinity. The larger this value is, the better the performance of the photoresistor is. If this value is very small or close to zero, it means that the photoresistor has been burned through and damaged, and it can no longer be used.
B Aim a light source at the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter should swing significantly, and the resistance value will obviously decrease. The smaller this value is, the better the performance of the photoresistor is. If this value is very large or even infinite, it means that the internal open circuit of the photoresistor is damaged and it can no longer be used.
C Aim the light-transmitting window of the photoresistor at the incident light, and shake the upper part of the light-shielding window of the photoresistor with a small black paper to make it receive light intermittently. At this time, the pointer of the multimeter should swing left and right with the shaking of the black paper. If the pointer of the multimeter always stops at a certain position and does not swing with the shaking of the paper, it means that the photosensitive material of the photoresistor has been damaged.

2. Capacitor detection method and experience
The common way to mark capacitors is direct marking. The commonly used units are pF and μF, which are easy to recognize. However, some small-capacity capacitors use digital marking, which generally has three digits. The first and second digits are valid numbers, and the third digit is the multiple, which means how many zeros are followed. For example: 343 means 34000pF. In addition, if the third digit is 9, it means 10-1, not 10 to the 9th power. For example: 479 means 4.7pF.

When replacing capacitors, the main thing to pay attention to is that the withstand voltage of the capacitor is generally required to be no lower than the withstand voltage requirements of the original capacitor. In circuits with stricter requirements, its capacity is generally not more than ±20% of the original capacity. In circuits with less stringent requirements, such as bypass circuits, it is generally required to be no less than 1/2 of the original capacitance and no more than 2 to 6 times the original capacitance. 

1 Detection of fixed capacitors 
A Detection of small capacitors below 10pF  Because the capacity of fixed capacitors below 10pF is too small, using a multimeter to measure can only qualitatively check whether it has leakage, internal short circuit or breakdown. When measuring, you can choose the multimeter R×10k block, and use two test pens to connect the two pins of the capacitor at will, and the resistance should be infinite. If the measured resistance (the pointer swings to the right) is zero, it means that the capacitor is leaking or damaged or internally broken down.
B Detect whether the 10PF~1000μF fixed capacitor has charging phenomenon, and then judge whether it is good or bad. The multimeter uses the R×1k block. The β values ​​of the two transistors are both above 100, and the penetration current is relatively small. You can choose 3DG6 and other models of silicon transistors to form a composite tube. The red and black test pens of the multimeter are connected to the emitter e and collector c of the composite tube respectively. Due to the amplification effect of the compound triode, the charging and discharging process of the measured capacitor is amplified, so that the swing amplitude of the multimeter pointer is increased, which is convenient for observation. It should be noted that during the test operation, especially when measuring a capacitor with a smaller capacity, the pins of the measured capacitor should be repeatedly switched to contact points A and B to clearly see the swing of the multimeter pointer.
C For fixed capacitors above 1000μF, the R×10k block of the multimeter can be used to directly test whether the capacitor has a charging process and whether there is an internal short circuit or leakage, and the capacity of the capacitor can be estimated based on the amplitude of the pointer swinging to the right.