【 Weak current College 】 electronic detection and replacement experience (1) 】

A detection method, resistors and experience:
1 fixed resistors of detection.
A two table pen (without distinction of positive and negative) respectively and the resistance of both pins can detect the actual resistance touch. In order to improve the accuracy of measurement, should be based on the measured resistance nominal value to select a range of sizes. Due to the ohmmeter scale of nonlinear relationship, it's the middle paragraph indexing more fine, so you should make the pointer indicates the value to scale by as much as possible in the position, which is a full scale starting from 20% to 80% in radians, so that a more accurate measurement. According to the resistance of the error class. Readings and nominal resistance between respectively allowed ± 5%, ±10% or ± 20% of the error. If not, is out of range, the resistance value of variable values.
B Note: testing, especially in measuring dozens of k Ω or more resistance of resistance, the hands do not touch the table amount and resistance of conductive parts; the detection of resistance from the circuit of welding down, at least to a head, welding to avoid the other components in the circuit for testing, measurement error caused; color ring resistance of resistance while able to color ring flag to determine, but the use of best or multimeter test their real resistance.
2 cement resistance testing. Detection methods for cement resistors and points for attention and detection of ordinary fixed resistors are identical.
3 detection of fusing resistors. In the circuit, the fuse resistor fuse open circuit, can be judged in the light of experience: If you find the fuse resistor surface blackening or burned, it can be concluded that its overloaded, through its current exceeds the rated value many times; if its surface without any trace of the open circuit, the flow of current is exactly equal to or slightly greater than the rated voltage values. For surface without any trace of fusing resistors orbad judgement, you can use a multimeter R× 1 block to measure, in order to ensure accurate measurement, fuse resistor on welding at one end from the circuit. If the measured resistance is infinite, then the open fuse resistor has expired, if the measured resistance number and nominal value of the far indicating resistance varying values should not be used any more. In maintenance practice found that a small number of fuse resistor in a circuit breakdown short circuit, testing should also be noted.
4 potentiometer detection. Check potentiometers, first turn the rotary knob, see Rotary knobs for smooth, switch is flexible, switch to pass or not pass when you click the "UC" sound is crisp, and listen to the potentiometer internal touch points and resistance of friction sound, such as the "sand" sound quality is not good, description. Multimeter test, according to the size of the measuring potentiometer resistances, choose the appropriate electric barrier multimeter, can then be detected by the following methods. 
A multimeter for ohmmeter measured the "1" and "2" at both ends, the reading should be potentiometer nominal resistance, such as multimeters pointer does not move or resistance comparable to many, the potentiometer is corrupted.
B detect the potentiometer activities arm and resistance of contact is good. Multimeter measured ohmic stalls "1" and "2" (or "2", "3") at both ends of the hinge of the potentiometer counterclockwise rotation to close to the "off" position, when the resistance as small as possible. Then slowly clockwise rotation handle, a resistance value should be gradually increased, the pointers in table headers should move smoothly. When the axis rotation to the extreme position of the handle of "3", the resistance should be close to the potentiometer of nominal value. If the pointer in the multimeter potentiometer axis knobs have a bouncing phenomenon in the process, the activities of the contact with poor contact. 
5 positive temperature coefficient thermistor (PTC). Detection, multimeter R× 1 block, can be divided into two steps:
A room temperature detection (indoor temperature close to 25 ° c); the two table pen contact PTC thermistor two pin measured their real resistance, and with nominal resistance in contrast, both from within the ± 2 Ω is normal. The actual resistance if the difference between the nominal resistance value is too large, then its performance bad or corrupted.
B heating detection; at room temperature testing normal basis, to the second step of the test — a heated monitoring heat source (such as electric iron) * near PTC thermistor to heating, multimeter to monitor its resistance value is with temperature but increased, as is the normal, if the resistance of the thermistor, description of its performance, can not continue to use. Note do not make the heat source and PTC thermistor * too close or direct contact with Thermistors to prevent their very hot. 
6 negative temperature coefficient thermistor (NTC) of testing. 
(1), measuring nominal resistance value Rt multimeter measurement method of NTC thermistor and measurement methods regular fixed resistors of the same, that is, according to the nominal NTC thermistor resistance value Select an appropriate power barrier can be directly measured Rt's actual value. But because the NTC thermistor is sensitive to temperature, the test should note the following:
A Rt is the manufacturers in the ambient temperature is 25 ° c, as measured using a multimeter measuring Rt should also be at ambient temperature close to 25 ° c, in order to guarantee the credibility of the tests.
B measurement of power not exceeding the prescribed value, so that current measurement errors caused by thermal effect.
C Note that correct operation. Test, do not use hand squeeze the thermistor, to prevent the human impact of temperature on the test.
(2), estimation of temperature coefficient α t first measured room temperature T1resistance Rt1, electric iron for heat, * Rt near thermistor, measured resistance value RT2 while thermometers measure in this CTR average RT surface temperature t2 and then evaluated.
7 varistor detection. Multimeter measurement of R×1k retaining varistor is between two pins, the reverse, the insulation resistance is infinity, otherwise, the leakage current. If the measured resistance is small, the varistor is corrupted and cannot be used.
8-detection.
A use a black paper will cover the translucent window photosensitive resistor, when the pointer basic multimeter to remain close to infinity, the resistance. The higher the value, the better the description of photosensitive properties. If this value is very small or close to zero, the description of photosensitive have burn damage, can no longer continue to use.
B willA light source on the translucent window photosensitive resistor, when the pointer should have a multimeter significantly reduced evidently swing, resistance to this value, the less the better description of photosensitive properties. If this value is very large or even infinite, that damage to the internal open-or can no longer continue to use.
C-light window on the incidence of light, with black pieces in the upper window-shade, which breaking Rock light, multimeter pointer should be with the black pieces and sway swaying. If the pointer is always stopped at the multimeter in a location not wiggle with paper and swing, description of photosensitive materials photosensitive resistor has been damaged.
Second, the capacitor of detection methods and experience
Capacitor common markup is direct marking of the common unit has a pF, μ F, very easy to recognize. But some small-capacity capacitor is digital marking method, generally have three digits, the first and second digits as a valid number, and the third digit for the multiples means that many back to 0. For example: 343 represents 34000pF, in addition, if a third digit is 9, said10-1, instead of the 9 th of October, for example: 479 4.7pF said.
Replace the main attention should be paid when the capacitance of the capacitor voltage values generally require no less than the original capacitive voltage requirements. In the requirements more stringent circuits, its capacity is less than the original capacity of ± 20%. In less strict circuit, such as a bypass circuit, General requirements not less than the original capacitance of 1/2 and not greater than the capacitance of the 2 x ~ 6 times. 
1 fixed capacitor testing
A detection of small capacitor 10pF following due to the fixed capacitor 10pF following capacity is too small to be measured using a multimeter, only qualitative check whether there is leakage, the phenomenon of internal short-circuit or breakdown. When measuring, you can use multimeter R×10k retaining, using two tables respectively any received two pin capacitance, the resistance should be infinity. If the measured resistance value (a pointer to the right swing) is zero, then the capacitor leakage damage or internal breakdown.
B testing 10PF ~ 1000 μ F fixed capacitor is charging phenomena, thus it is good or bad. Multimeter R×1k retaining selection. Two transistor of β value of 100 or more, and penetrate current to some optional 3DG6, model composed of composite pipe silicon transistors. Multimeter red and black table respectively and composite pipe of the emitter and collector c e. Due to the magnification composite transistors, the measured capacitance of charging and discharging process be amplified so that the pointer pendulum amplitude increase multimeter to facilitate observation. It should be noted: in test operations, particularly in measuring smaller capacitance, repeatedly swap measured capacitance pin contact A, B, you can clearly see the multimeter pointers.
C for more than 1000 μ F Fixed capacitors, available multimeter R×10k retaining direct testing capacitors have no charging process and there is no internal short-circuit or leakage, and may act according to the pointer to the right margin size estimation of swing out of the capacitor capacity. 
2 Electrolytic Capacitor testing
A capacity for electrolytic capacitor is generally fixed capacitor, so the measurement should be for different size selected appropriate quantum. As a rule of thumb, in General, 1 ~ 47 μ F, available between capacitance R×1k retaining measurement, greater than the capacitance of the 47 μ F R× 100 retaining measurement available. 
B the multimeter red table amount received negative electrode, the black table amount received positive, just contact moments, multimeter pointer is to the right tilt larger skewness (for the same power, the greater the capacity, the greater the swing), then gradually turning to the left until it stops at a certain location. This resistance is Electrolytic Capacitor leakage resistance of forward, this value is slightly larger than the reverse leakage resistance. Actual experience that Electrolytic Capacitor leakage resistance should generally be in more than a few hundred k Ω, otherwise it will not work correctly. In tests, if positive, the reverse is not charging, i.e. table PIN is not moving, then disappear or internal circuit breaker capacity; if the measured resistance is too low or zero, the capacitor leakage current large or breakdown damaged, can no longer be used.
C for the positive and negative signs of electrolytic capacitors, can use the measuring leakage resistance of discrimination. Namely any leakage resistance, to have your mind its size, and then exchange table pen and then measured a resistance. Two measurement resistance value of the time it is being received, the black table by pen received is positive, then red table amount is negative.
D use electric barrier, using a multimeter to a Electrolytic Capacitor, reactive charging method, according to the pointer to the right size of oscillations, you can estimate the capacity of the electrolytic capacitors. 
3 variable capacitor testing
A swirled gently by hand, you should feel very shaft is smooth and should not be felt at times when tight even pine scuffing. Will contain the forward, rear axle, top, bottom, left, right, direction drive, the hinge should be loose.
B with one hand in a swirling hinge, another hand touch dynamic piece group of outer edge, you should not feel any loose phenomenon. The hinge and dynamic between bad variable capacitor, is no longer in use.
C place the multimeter R×10k, hand two tables sum respectively variable capacitor of tablets and tablets of terminations, the other will hinge slowly swirling several pointer back and forth, multimeter should position at infinity. In a swirling the hinge of the process, if the pointer is sometimes point to zero, the motion Tablet and fixed between the shorts point; if encountered an angle, multimeter readout is infinity, but appear a certain resistance, description of the dynamic variable capacitor tablets and tablets between leakage phenomenon.