Multimeter detection integrated circuit
Although integrated circuit replacement is convenient, disassembly is more troublesome after all. Therefore, before dismantling, it should be accurately judged whether the integrated circuit is indeed damaged and the degree of damage, so as to avoid blind disassembly. This article introduces the methods and precautions of off-road and on-road detection integrated circuits that only use a multimeter as a detection tool. The four methods of on-line detection described in the article (measurement of DC resistance, voltage, AC voltage and total current) are practical and commonly used detection methods in amateur maintenance. Here, I also hope that you can provide other practical (integrated circuits and components) discrimination and testing experience.
(1) Off-road detection
This method is carried out when the IC is not welded into the circuit. Generally, a multimeter can be used to measure the forward and reverse resistance values between each pin corresponding to the ground pin, and compare it with the intact IC.
(2) On-road detection
This is a method of detecting the DC resistance of each pin of the IC in the circuit (the IC is in the circuit), the AC and DC voltage to the ground, and the total operating current through a multimeter. This method overcomes the limitations of replacing the IC and the trouble of disassembling the IC in the substitution test method, and is the most commonly used and practical method for detecting ICs.
1. In-circuit DC resistance detection method
This is a method to use the ohm block of a multimeter to directly measure the positive and negative DC resistance values of each pin of the IC and peripheral components on the circuit board, and compare it with the normal data to find and determine the fault. Pay attention to the following three points when measuring:
(1) Disconnect the power supply before measurement, so as not to damage the meter and components during the test.
(2) The internal voltage of the electric block of the multimeter shall not be greater than 6V, and the measuring range is preferably R×100 or R×1k.
(3) When measuring IC pin parameters, attention should be paid to the measurement conditions, such as the tested model, the position of the sliding arm of the potentiometer related to the IC, etc., and the quality of the peripheral circuit components should also be considered.
2. DC working voltage measurement method
This is a way to measure the DC power supply voltage and the working voltage of peripheral components with the DC voltage block of the multimeter when the power is on; detect the DC voltage value of each pin of the IC to the ground, and compare it with the normal value, and then compress the fault range. Locate the damaged component. Pay attention to the following eight points when measuring:
(1) The internal resistance of the multimeter must be large enough, at least 10 times greater than the resistance of the circuit under test, so as not to cause large measurement errors.
(2) Usually turn each potentiometer to the middle position. If it is a TV, the signal source should use a standard color bar signal generator.
(3) Anti-slip measures should be taken for the test leads or probes. The IC is easily damaged due to any momentary short circuit. The following methods can be adopted to prevent the test pen from sliding: take a bicycle valve core and put it on the test pen tip, and extend the test pen tip by about 0.5mm, which can not only make the test pen tip contact with the tested point well, but also effectively prevent slipping , it will not be short-circuited even if it hits a neighboring point.
(4) When the measured voltage of a pin does not match the normal value, it should be analyzed according to whether the pin voltage has an important impact on the normal operation of the IC and the corresponding changes in the voltage of other pins, so as to judge whether the IC is good or bad.
(5) IC pin voltage will be affected by peripheral components. When leakage, short circuit, open circuit or value change occurs in peripheral components, or the peripheral circuit is connected to a potentiometer with variable resistance, the position of the sliding arm of the potentiometer is different, which will cause the pin voltage to change.
(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, you should start from the point where the deviation from the normal value is the largest, and check whether the peripheral components are faulty. If there is no fault, the IC is likely to be damaged. .
(7) For dynamic receiving devices, such as TV sets, 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 that which should change 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, the voltage of each pin of the IC is also different under different working modes.
3. AC working voltage measurement method
In order to grasp the change of the IC AC signal, a multimeter with a dB jack can be used to perform an approximate measurement of the AC operating voltage of the IC. When testing, put the multimeter in the AC voltage block, and insert the positive test lead into the dB jack; for a multimeter without a dB jack, a 0.1-0.5μF DC blocking capacitor needs to be connected in series with the positive test lead. This method is suitable for ICs with relatively low operating frequency, such as video amplifier stages of TV sets, field scanning circuits, etc. Since these circuits have different natural frequencies and waveforms, the measured data are approximate and are for reference only.
4. total current measurement
This method is a method to judge whether the IC is good or bad by detecting the total current of the IC power supply line. Since the vast majority of ICs are directly coupled, when the IC is damaged (such as a PN junction breakdown or open circuit), it will cause saturation and cut-off of the subsequent stage, causing the total current to change. Therefore, the IC can be judged by measuring the total current. It can also be used to measure the voltage drop of the resistor in the power path, and use Ohm's law to calculate the total current value. (http://www.diangon.com/ copyright)
