Testing whether the IC is burned out or short circuit requires professional knowledge. Generally, people without such knowledge and skills can make some simple judgments by carefully observing the appearance of IC: whether the IC surface has burned marks, such as blobs, cracks, discoloration and so on. IC pin break, unsolder. Does IC have burnt paste taste? Whether the original around IC is abnormal. This method is used when IC is not welded into the circuit. Under normal circumstances, a multimeter can be used to measure the positive and reverse resistance values of each pin corresponding to the grounded pin and compare them with the intact IC. In circuit detection This is a method to detect dc resistance, AC/DC voltage and total working current of each pin of IC in circuit (IC in circuit) by means of a multimeter. This method overcomes the limitation of substitution test method and the trouble of disassembling IC. It is the most common and practical method for IC detection. 1. In-circuit DC resistance detection Method This is a method to directly measure the direct current resistance value of each pin and peripheral components of IC on the circuit board using the multimeter ohm block, and compare it with the normal data to find and determine the fault. The following three points should be noted during measurement :(1) disconnect the power supply before measurement to avoid damaging the meter and components during testing. (2) The internal voltage of the electrical barrier of the multimeter should not be more than 6V, and the measuring range should be in R×100 or R×1k. (3) When measuring IC pin parameters, pay attention to the measurement conditions, such as the model under test, the position of the sliding arm of the POTENTIometer related to IC, etc., and consider the quality of the peripheral circuit elements. 2. Dc working voltage measurement method: This is a method to measure the DC supply voltage and the working voltage of peripheral components with the DC voltage shield of a multimeter under the condition of being energized; Test the DC voltage value of IC pins to the ground and compare it with the normal value, then compress the fault range and find out the damaged components. The following eight points should be noted when measuring :(1) the multimeter should have a large enough internal resistance, at least more than 10 times the resistance of the circuit under test, so as not to cause a large measurement error. (2) Usually turn each potentiometer to the middle position. If it is a TV set, the signal source should use the standard color bar signal generator. (3) Anti-skid measures should be taken for the watch pen or probe. Because any instantaneous short circuit is easy to damage IC. Can take the following methods to prevent the sliding of the watch pen: take a section of the bicycle with the valve core set on the table nib, and grow out of the table nib about 0.5mm, which can not only make the table nib good contact with the point being tested, but also effectively prevent slipping, even if hit the adjacent point will not short circuit. (4) If the measured voltage of a pin is inconsistent with the normal value, it shall be analyzed according to whether the voltage of the pin has important influence on the normal operation of IC and the corresponding changes of other pin voltage, so as to judge the quality of IC. (5)IC pin voltage will be affected by peripheral components. When leakage, short circuit, open circuit or variable value occurs in the peripheral components, or the peripheral circuit is connected to a potentiometer with variable resistance value, the position of the potentiometer slider arm is different, will make the pin voltage change. (6) If the voltage of each pin of IC is normal, IC is generally considered as normal; If the pin voltage of the IC part is abnormal, the largest deviation from the normal value should be started, check the peripheral components have no fault, if there is no fault, then the IC is likely to be damaged. (7) For dynamic receiving devices, such as TV sets, IC pin voltage is different when there is no signal. If it is found that the pin voltage should not change but changes greatly, which should not change with the signal size and different positions of the adjustable components, then IC damage can be determined. (8) For devices with multiple working modes, such as video recorders, the voltage of IC pins is also different in different working modes. 3. Ac working voltage Measurement Method In order to grasp the change of IC AC signal, a multimeter with a dB jack can be used to approximate the ac working voltage measurement of IC. When testing, the multimeter is placed in the AC voltage block, and the front watch pen is inserted into the dB jack. For multimeter without dB jack, a 0.1~0.5 F spacer capacitor shall be connected in the front watch pen in series. This method is suitable for IC with low working frequency, such as video amplification stage of TV set, field scanning circuit, etc. Due to the different natural frequencies and waveforms of these circuits, the measured data are approximate and can only be used for reference. 4. Total current measurement Method This method is a method to judge the quality of IC by detecting the total current of the incoming line of IC power supply. Because the vast majority of IC interior is directly coupled, when IC is damaged (such as the breakdown or open circuit of a PN junction), it will cause the post-stage saturation and cut-off, so that the total current will change. Therefore, IC can be judged by measuring the total current. It is also possible to measure the voltage drop of resistance in the supply path and calculate the total current using Ohm’s law. The above detection methods, each has its advantages and disadvantages, it is best to combine all kinds of methods in practical application, flexible application.
