“How to accurately judge whether the integrated circuit IC in the circuit is “lazy” and not in working state, whether it is good or bad is an important part of repairing TV, audio and video equipment. It’s still there, so make the right judgement about integrated circuits.
How to accurately judge whether the integrated circuit IC in the circuit is “lazy” and not in working state, whether it is good or bad is an important part of repairing TV, audio and video equipment. It’s still there, so make the right judgement about integrated circuits.
1. First, master the purpose, internal structure principle, main electrical characteristics, etc. of the IC in the circuit, and analyze the internal electrical schematic diagram if necessary. In addition to these, if there are more DC voltages to ground, waveforms, and forward and reverse DC resistance values to ground, then it will provide more favorable conditions for the judgment before inspection;
2. Then judge the part according to the fault phenomenon, and then find the faulty component according to the part. Sometimes multiple judgment methods are required to prove whether the device is indeed damaged.
3. Generally, there are two methods for checking and judging the IC in the circuit: one is off-line judgment, that is, the judgment that the IC in the circuit is not soldered into the printed circuit board. In this method, it is very 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 between the pins corresponding to the ground pins. And compared with the intact integrated circuit, the replacement method can also be used to insert the suspicious integrated circuit into the position of the same type of integrated circuit of the normal equipment to determine its quality. Of course, there are conditions to use the integrated circuit tester to quantitatively test the main parameters, so that the use is more guaranteed.
There is also an online inspection and judgment method, that is, a judgment method in which an integrated circuit is connected to a printed circuit board. Online judgment is the most practical way to repair integrated circuits in TV, audio and video equipment. The following situations are described:
1. DC working voltage measurement method:
It is mainly to measure the DC working voltage value of each pin to the ground; then compare it with the nominal value to judge the quality of the integrated circuit. Using the voltage measurement method to judge the quality of the integrated circuit is one of the most commonly used methods in maintenance, but attention should be paid 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 does not match the voltage value marked in the schematic diagram or maintenance technical data, do not rush to conclude that the integrated circuit is damaged, and the following factors should be eliminated first. Confirm again.
1) Whether the provided nominal voltage is reliable, because there are some manuals, schematic diagrams and other data marked values that are quite different from the actual voltage, and sometimes even wrong. At this time, you should find more relevant data for comparison, and if necessary, analyze the internal schematic diagram and peripheral circuits, and then perform theoretical calculations or estimates to prove whether the voltage is wrong.
2) It is necessary to distinguish the nature of the nominal voltage provided, and what kind of working state the voltage belongs to. Because the individual pins of the integrated block change significantly with the injection signal, you can change the position of the band or the recording and playback switch at this time, and then observe whether the voltage is normal. If the latter is normal, it means that the nominal voltage is a certain working voltage, and this working voltage refers to a certain condition, that is, the measured voltage is different under different working conditions.
3) Pay attention to pin voltage changes due to variable components in peripheral circuits. When the measured voltage does not match the nominal voltage, it may be because individual pins or peripheral circuits related to the pins are connected to a potentiometer with variable resistance or a switch (such as volume potentiometer, brightness, contrast, video, fast forward, fast reverse, record and playback switch, audio AM switch, etc.). The positions of these potentiometers and switches are different, and the pin voltage will be significantly different, so when a certain pin voltage does not match, the position change of the pin or the potentiometer and switch associated with the pin should be considered. Rotate or pull the starter to see if the pin voltage is around the nominal value.
4) To prevent errors due to measurement. Errors may occur due to different internal resistances of the multimeter head or different DC voltage ranges. In general, the marked DC voltage is tested with the internal resistance of the test instrument greater than 20KΩ/V. When a multimeter with an internal resistance less than 20KΩ/V is used for testing, the measured result will be lower than the original voltage. In addition, it should also be noted that the voltage measured on different voltage ranges will be different, especially when using a large range range, the effect of reading deviation is more significant.
5) When the measured voltage of a certain pin is inconsistent with the normal value, the quality of the IC should be judged by analyzing whether the voltage of the pin has an important influence on the normal operation of the IC and the corresponding changes of the voltages of other pins.
6) If the voltage of each pin of the IC is normal, it is generally considered that the IC is normal; if the voltage of some pins of the IC is abnormal, it should start from the maximum deviation from the normal value, and check whether the peripheral components are faulty. If there is no fault, the IC is likely to be faulty damage.
7) For a dynamic receiving device, such as a TV, the voltage of each pin of the IC is different when there is a signal or not. If it is found that the pin voltage should not change but changes greatly, and the voltage that changes with the signal size and the different positions of the adjustable components does not change, it can be determined that the IC is damaged.
8) For devices with multiple working modes, such as video recorders, under different working modes, the voltage of each pin of IC is also different.
2. AC working voltage measurement method:
In order to grasp the change of the AC signal of the IC, you can use a multimeter with a dB jack to measure the AC working voltage of the IC approximately. When testing, place the multimeter on the AC voltage block, and insert the positive test lead into the dB jack; for a multimeter without a dB jack, a 0.1-0.5uF DC blocking capacitor needs to be connected in series with the positive test lead. This method is suitable for ICs with relatively low operating frequencies, such as video amplification stages of TV sets, field scanning circuits, etc. Since the natural frequencies of these circuits are different and the waveforms are different, the measured data is an approximation or a presence or absence.
In general, a test condition should be specified when conducting the DC voltage or DC resistance test of the integrated block, especially when it is recorded as the actual measured empirical data. 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, it will be more representative. If the measured individual pin voltage still does not meet the nominal value after excluding the above factors, the reasons need to be further analyzed, but there are only 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 failure is also the key to repairing integrated circuit home appliances.