Catalog

I Introduction

How to judge accurately whether the IC in the circuit is working or not, good or bad, is an important content of repairing TV, audio and video equipment. If the judgment is not accurate, we often have to spend great efforts replacing the new integrated circuit and at the same time the fault still exists, so we should make a correct judgment on the integrated circuit.

The first thing is to master the use of IC in the circuit, the principle of internal structure, main electrical characteristics, etc., and analyze the internal electrical schematic diagram when necessary. In addition to these, if there are different pins against ground DC voltage, waveform, and resistance value of ground forward and reverse direction DC current, then more favorable conditions for pre-inspection judgment are provided.

Then judge its position according to the fault phenomenon, and then find the fault element according to the position. Sometimes a variety of judgment methods are needed to prove whether the device is indeed damaged or not.

Generally, there are two methods to check and judge the IC in the circuit: one is the online judgment, that is, the IC in the circuit is not welded into the printed circuit board. This method is very difficult to determine whether the circuit IC quality stand is good or not when it's in the absence of special equipment. Generally, we can use DC resistance method to measure all kinds of pins that corresponded to a positive and negative resistance value between the ground pins and compared with the well integrated circuit. Of course, we can also use the substitution method to plug the suspicious IC into the same type IC in the normal equipment to determine its quality.

Of course, if there are conditions, we can also use the IC tester to conduct quantitative testing of the main parameters, so that the use is more guaranteed.

II On-line Judgment

There is also online inspection judgment, which is the judgment method of integrated circuit connection on the printed circuit board. On-line judgment is the most practical method to overhaul integrated circuit in TV, audio and video equipment. The following situations can be explained:

2.1 DC Working Voltage Measurement Method

It mainly measures the DC working voltage value of each pin to the ground; Then compared with the nominal value, the quality of integrated circuit can be judged. It is one of the most commonly used methods to judge whether an integrated circuit is good or bad by using the voltage measurement method. When measuring the DC working voltage of each pin of the integrated circuit, if the voltage of individual pin is inconsistent with the voltage indicated in the schematic diagram or maintenance technical data, do not be so quick to determine that the integrated circuit has been damaged, it should first exclude the following factors before determining.

(1) Whether the nominal voltage provided is reliable. Because there are some specifications, schematic diagrams and other data on the value of the target are different with the actual voltage, and sometimes even wrong. At this point, more relevant data should be found for comparison, and the internal schematic diagram and peripheral circuit should be analyzed and calculated or estimated theoretically to prove whether the voltage is wrong.

(2) To distinguish the nature of the provided nominal voltage, which voltage is the operating state of the voltage. Because the individual pins of the integrated block change obviously with the difference of the input signal, so the position of wave band or video switch can be changed at this time, and then observe whether the voltage is normal or not. If the latter is normal, then it indicates that the nominal voltage belongs to a certain working voltage, and this working voltage refers to a certain condition, that is, the tested working state is different, and the measured voltage is also different.

(3) Note the change of pin voltage caused by peripheral circuit variable elements. When the measured voltage is not in accordance with the nominal voltage, it may be due to an individual pin or the peripheral circuit related to the pin, which is connected with a potentiometer with variable resistance value or a switch (such as the volume potentiometer, brightness, contrast, video, fast forward, quick down, recording switch, audio amplification switch, etc.). The positions of these potentiometers and switches are different, and the voltage of the pins will be obviously different. Therefore, when the voltage of a pin is inconsistent, the position change of the pins or the potentiometers and switches associated with the pins should be considered. The starting point of rotation or pull-out can be used to see whether the voltage of the pins can be near the nominal value.

A Potentiometer

(4) To prevent errors because of measurement. Errors may be caused by different internal resistances of the multimeter or by different DC voltages. The DC voltage marked on general principles with test instrument Ω resistance is greater than 20 k/V test. Resistance is less than 20 k Ω/V multimeter to test, and will make the measured result is lower than the original target voltage. In addition, it should be noted that the voltage measured at different voltage levels will be different, especially with large range, and the effect of reading error is more significant.

(5) When the voltage of a certain pin is found to be inconsistent with the normal value, it shall be analyzed based on whether the voltage of the pin has significant influence on the normal operation of IC and the corresponding changes of other pin voltage, so as to judge whether the IC is good or bad.

(6) If the voltage of each pin of IC is normal, it is generally considered that IC is normal. If the voltage of part of IC pin is abnormal, it shall start from the maximum deviation of normal value. The imported pump shall check the peripheral component for fault free. If no fault is found, the IC is likely to be damaged.

(7) For dynamic receiving devices, such as TV sets, the voltage of each IC pin is different when there is no signal. IC damage can be determined if it is found that the pin voltage does not change significantly, but does not change depending on the signal size and the location of the adjustable element.

(8) For devices with a variety of working modes, such as VCR, the voltage of each IC pin is different under different working modes.

(9) The above points are that when there is no fault of IC in the circuit, the measured result is different from the nominal value due to some reason. Therefore, in general, a test condition should be specified when testing the DC voltage or DC resistance of the integrated block, especially when recording the measured empirical data.The signal source is a standard signal at a certain field intensity. Of course, it is more representative if the switch position of each function can be recorded. If the above factors are excluded, when the voltage of the individual pin is still inconsistent with the nominal value, the reasons need to be further analyzed, but there are only two possibilities. One is the fault of integrated circuit itself. The other is the peripheral circuit of the chip. Identifying these two fault sources is also the key to repairing IC appliances.

2.2 AC Working Voltage Measurement Method 

In order to master the change of IC AC signal, a multimeter with dB jack can be used to approximate the AC working voltage of IC. When testing, the multimeter is placed in the AC voltage catch, and the front pen is inserted into the dB jack. For a multimeter without dB jack, a 0.1-0.5 uF capacitor needs to be connected with the normal pen. The method is applicable to IC with low working frequency, such as video amplifier level of TV, field sweep circuit, etc. Since the natural frequencies of these circuits are different and the waveforms are different, the measured data is approximate, or whether or not.

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