An accurate method to determine whether the integrated circuit IC works?
1. First of all, we should master the use, internal structure principle, main electrical characteristics, etc. of the IC in the circuit, and analyze the internal electrical schematic diagram when necessary. In addition to these, if there are DC voltage, waveform and forward and reverse DC resistance to ground of each pin, it will provide more favorable conditions for judgment before inspection;
2. Then judge the position according to the fault phenomenon, and then find the fault component according to the position. Sometimes multiple judgment methods are required to prove whether the device is damaged.
3. Generally, there are two methods to check and judge the IC in the circuit: one is off-line judgment, that is, judgment that the IC in the circuit is not soldered into the printed circuit board. This method is very difficult to determine the quality of the IC in the circuit without special instruments and equipment. Generally, the DC resistance method can be used to measure the forward and reverse resistance between each pin and the grounding pin, and compare with the intact integrated circuit. The replacement method can also be used to insert the suspect integrated circuit into the position of the same type of integrated circuit of the normal equipment to determine its quality. Of course, if possible, we can use the integrated circuit tester to conduct quantitative inspection on the main parameters, so that the use is more guaranteed.
There is also online inspection and judgment, that is, the judgment method of connecting the integrated circuit to the printed circuit board. Online judgment is a practical method for repairing integrated circuits in TV, audio and video equipment.
The following situations are described:
1. DC working voltage measurement method: mainly measure the DC working voltage value of each pin to the ground; Then compare with the nominal value to judge whether the integrated circuit is good or not. Using voltage measurement method to judge the quality of integrated circuits is one of the methods often used in maintenance, but attention should be paid to distinguish non fault voltage errors. When measuring the DC working voltage of each pin of the integrated circuit, if the voltage of individual pin does not conform to the voltage value marked in the schematic diagram or maintenance technical data, do not rush to determine that the integrated circuit is damaged. The following factors should be eliminated before determining.
1) Whether the nominal voltage provided is reliable, because there are some instructions, schematic diagrams and other data marked with a large difference from the actual voltage, sometimes even wrong. At this time, more relevant data should be found for comparison. If necessary, the internal schematic diagram and the external circuit should be analyzed, and then the theoretical calculation or estimation should be carried out to prove whether the voltage is wrong.
2) It is necessary to distinguish the nature of the nominal voltage provided and its voltage is the voltage under which operating state. Because individual pins of the integrated block change obviously with different injection signals, the position of the band or the recording/playing switch can be changed at this time, and then observe whether the voltage is normal. If the latter is normal, it means that the nominal voltage belongs to a certain working voltage, and this working voltage means that the measured voltage is different under a specific condition, that is, the tested working state is different.
3) Pay attention to the pin voltage changes caused by the variable elements of the peripheral circuit. When the measured voltage does not conform to the nominal voltage, it may be because an individual pin or the peripheral circuit related to this pin is connected with a potentiometer or switch with variable resistance (such as volume potentiometer, brightness, contrast, video recording, fast forward, fast reverse, recording/playback switch, audio frequency amplitude modulation switch, etc.). These potentiometers and switches are located in different positions, and the pin voltage will be significantly different. Therefore, when a pin voltage does not match, the position change of the pin or the potentiometer and switch associated with the pin should be considered. Turn or pull the head to see whether the pin voltage is near the nominal value.
4) The error caused by measurement shall be prevented. The internal resistance of multimeter head is different or the DC voltage range is different, which will cause errors. In general, the marked DC voltage is tested with the internal resistance of the test instrument greater than 20K Ω/V. When the multimeter with internal resistance less than 20K Ω/V is tested, the measured result will be lower than the original marked voltage. In addition, it should also be noted that the voltage measured on different voltage ranges will be different, especially with a large range range, the reading deviation will have a more significant impact.
5) When the measured voltage of a pin is inconsistent with the normal value, the IC can be judged by analyzing whether the pin voltage has an important impact on the normal operation of the IC and the corresponding changes of other pin voltages.
6) If the voltage of each IC pin is normal, it is generally considered that the IC is normal; If the IC part pin voltage is abnormal, start from the place that deviates from the normal value. The inlet pump checks whether the peripheral components have faults. If there is no fault, the IC is likely to be damaged.
7) For dynamic receiving devices, such as TV, the voltage of each IC pin is different when there is a signal or not. If it is found that the pin voltage does not change but changes greatly, and does not change with the signal size and the different positions of the adjustable components, it can be determined that the IC is damaged.
8) For devices with multiple working modes, such as video recorders, IC pin voltages are different under different working modes.
The above points are that when there is no IC fault in the circuit, the measured result is different from the nominal value for some reason. In general, a test condition should be specified when testing the DC voltage or DC resistance of the integrated block, especially when recording it as the measured experience data. Generally, all potentiometers are rotated to the middle position of the machine, and the standard signal under a certain field strength is used as the signal source. Of course, it is more representative if the position of each functional switch can be recorded again. If the measured voltage of individual pins still does not conform to the nominal value after excluding the above factors, further analysis is required, but there are two possibilities. One is caused by the failure of the integrated circuit itself; The other is the peripheral circuit of the integrated block. Distinguishing these two fault sources is also the key to repair IC household appliances.
AC working voltage measurement method: In order to master the change of IC AC signal, you can use a multimeter with a dB jack to approximate the AC working voltage of the IC. When testing, the multimeter is placed in the AC voltage gear, and the positive probe is inserted into the dB jack; For a multimeter without a dB jack, it is necessary to connect a 0.1~0.5uF DC isolating capacitor in series with the positive probe. This method is suitable for IC with low operating frequency, such as video amplifier stage of TV, field scanning circuit, etc. Because the natural frequencies of these circuits are different and the waveforms are different, the measured data are approximate values or whether they are available or not.
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