Two major voltage judgment methods for the operation of integrated circuit IC in circuit

How to accurately judge whether the integrated circuit IC in the circuit works, whether it is good or bad is to repair an important content of TV, audio, video equipment, judging is not allowed, often spend a lot of effort to replace the new integrated circuit and the fault still exists, so it is necessary The integrated circuit makes a correct judgment.

1. First, you must master the purpose of the IC in the circuit, the internal structure principle, the main electrical characteristics, etc., and analyze the internal electrical schematic diagram if necessary. In addition to these, if there is a DC voltage, waveform, and DC forward and reverse DC resistance values ​​for each pin, then it provides more favorable conditions for pre-inspection judgment;

2. Then judge the part according to the fault phenomenon, and then find the faulty component according to the location. Sometimes a variety of judgment methods are needed to prove that the device is indeed damaged.

3. Generally, there are two methods for checking and judging the IC in the circuit: First, it is not judged online, that is, the judgment that the IC in the circuit is not soldered into the printed circuit board. In this method, it is difficult to determine the quality of the IC in the circuit without special equipment. Generally, the DC resistance method can be used to measure the forward and reverse resistance values ​​of each pin corresponding to the grounding pin. And compared with a good integrated circuit, you can also use the replacement method to insert the suspect integrated circuit into the position of the same type of integrated circuit of the normal device to determine its good or bad. Of course, it is possible to use the integrated circuit tester to quantitatively check the main parameters, so that the use is more assured. There is also an online check judgment, that is, a method of judging that the integrated circuit is connected to the printed circuit board. Online judgment is the most practical method for overhauling integrated circuits in television, audio and video equipment. The following is a description of several situations:

1. DC working voltage measurement method:

Mainly to measure the DC working voltage value of each pin to the ground; then compare with the nominal value, according to this to judge the quality of the integrated circuit. Using voltage measurement to judge the quality of an integrated circuit is one of the most commonly used methods in maintenance, but it is necessary to pay attention to distinguishing non-faulty voltage errors. When measuring the DC working voltage of each pin of the integrated circuit, if the voltage of the individual pin is inconsistent with the voltage value indicated in the schematic or maintenance technical data, do not rush to conclude that the integrated circuit is damaged. The following factors should be excluded first. Ok again. 1) Whether the nominal voltage provided is reliable, because there are some specifications, the values ​​indicated on the schematic and other data are quite different from the actual voltage, and sometimes even wrong. At this point, you should find more relevant data for comparison. If necessary, analyze the internal schematic diagram and peripheral circuits and then make theoretical calculations or estimates to prove whether the voltage is wrong. 2) To distinguish the nature of the nominal voltage provided, the voltage at which the operating voltage is. Because the individual pins of the integrated block change significantly with the difference of the injected signal, the position of the band or the recording and playback switch can be changed at this time, and then the voltage is normal. If the latter is normal, it means that the nominal voltage is a certain working voltage, and the working voltage means that under a certain condition, that is, the working state of the test is different, and the measured voltage is also different. 3) Pay attention to the change of the pin voltage caused by the variable components of the peripheral circuit. When the measured voltage does not match the nominal voltage, it may be because a single pin or a peripheral circuit associated with the pin is connected to a variable potentiometer or switch (such as volume potentiometer, brightness, contrast, Video, fast forward, fast reverse, recording and playback switch, audio amplitude modulation switch, etc.). These potentiometers and switches are located at different positions, and the pin voltages are significantly different. Therefore, when a voltage difference between a pin occurs, consider the position of the pin or the potentiometer and switch associated with the pin. It can be rotated or pulled to see if the pin voltage is near the nominal value. 4) To prevent errors due to measurement. Errors may occur due to different internal resistance of the multimeter or different DC voltage levels. Generally, the DC voltages marked are tested with the internal resistance of the test instrument greater than 20KΩ/V. When the multimeter with internal resistance less than 20KΩ/V is tested, the measured result will be lower than the original marked voltage. In addition, it should also be noted that the voltages measured on different voltage levels will be different, especially with a large number of files, the deviation of readings is more significant. 5) When the voltage of a certain pin does not match the normal value, it should be analyzed according to whether the voltage of the pin has an important influence on the normal operation of the IC and the corresponding change of the voltage of other pins, in order to judge whether the IC is good or bad. 6) If the voltage of each pin of the IC is normal, the IC is generally considered to be normal; if the voltage of the pin of the IC is abnormal, it should start from the maximum deviation from the normal value, and the imported pump checks whether the external components are faulty. If there is no fault, the IC is likely to be faulty. damage. 7) For dynamic receiving devices, such as televisions, the voltage at each pin of the IC is different when there is no signal. If it is found that the voltage of the pin does not change, the change is large, and the change of the signal size and the position of the adjustable component does not change, and the IC damage can be determined. 8) For devices with multiple working modes, such as video recorders, the voltage of each pin of the IC is different under different working modes. The above points are that when the IC is not faulty in the circuit, the measured result is different from the nominal value for some reason, so in general, a test is required when performing the integrated block DC voltage or DC resistance test. Conditions, especially when it is to be recorded as measured empirical data, pay more attention to this. Usually, each potentiometer is rotated to the middle position of the machine, and the signal source adopts the standard signal under a certain field strength. Of course, if the position of each function switch can be recorded again, it is more representative. If the above-mentioned factors are excluded and the measured individual pin voltages are still not in the nominal value, the reason should be further analyzed, but there are two possibilities. One is caused by the failure of the integrated circuit itself; the other is caused by the peripheral circuit of the integrated block. Identifying these two sources of faults is also the key to repairing integrated circuit home appliances.

2. AC working voltage measurement method:

In order to grasp the changes of the IC AC signal, the AC operating voltage of the IC can be approximated by a multimeter with a dB jack. When testing, the multimeter is placed in the AC voltage block, and the positive pen is inserted into the dB jack. For the multimeter without the dB jack, a 0.1 to 0.5 uF DC blocking capacitor is connected in series with the positive pen. This method is applicable to ICs with relatively low operating frequencies, such as video amplification stages and field scanning circuits of televisions. Since the natural frequencies of these circuits are different and the waveforms are different, the measured data is an approximation or as a presence or absence.