What is the procedure for testing integrated circuits with a multimeter?
There are ways to replace integrated circuits, but disassembly is troublesome. Before disassembly, it is necessary to accurately determine whether the integrated circuit has indeed been damaged and the extent of damage to avoid blind disassembly. The measurement of DC resistance, voltage, AC voltage, and total current is one of the four methods for online detection of integrated circuits using a multimeter.
(1) Off line detection:
This method is performed when the IC is not soldered into the circuit. Generally, a multimeter can be used to measure the forward and reverse resistance values between each pin and the ground pin, and compare them with a intact IC.
(2) Online detection:
This is a detection method that uses a multimeter to detect the DC resistance, AC/DC voltage to ground, and total working current of each pin of the IC online (IC in the circuit). This method overcomes the limitations of the substitution test method that requires replaceable ICs and the trouble of disassembling ICs, and is the most commonly used and practical method for detecting ICs.
① Online DC resistance detection method: This is a method of using a multimeter ohmmeter to directly measure the forward and reverse DC resistance values of IC pins and peripheral components on the circuit board, and comparing them with normal data to discover and determine faults. When measuring, the following three points should be noted:
a. Before measuring, disconnect the power supply to avoid damaging the meter and components during testing.
b. The internal voltage of the resistance range of the multimeter should not exceed 6 V, and the range should preferably be R × 100 or R × 1 K gear.
c. When measuring IC pin parameters, attention should be paid to measurement conditions, such as the tested model, the position of the sliding arm of the potentiometer related to the IC, and the quality of peripheral circuit components.
② DC working voltage measurement method: This is a method of measuring the DC supply voltage and the working voltage of peripheral components using a multimeter's DC voltage range when powered on; Detect the DC voltage values of each IC pin to ground and compare them with normal values to compress the fault range and identify damaged components. When measuring, pay attention to the following eight points:
a. The multimeter should have a sufficiently large internal resistance, which should be at least 10 times greater than the resistance of the circuit being tested, to avoid causing significant measurement errors.
b. Usually, rotate each potentiometer to the middle position. If it is a television, the signal source should use a standard color bar signal generator.
c. The pen or probe should be equipped with anti slip measures. Any momentary short circuit can easily damage the IC. The following methods can be used to prevent the pen from slipping: take a section of bicycle valve core and cover it on the pen tip, and grow it about 0.5 mm longer than the pen tip. This not only ensures good contact between the pen tip and the tested point, but also effectively prevents slipping, even if it touches adjacent points, it will not short-circuit.
d. When the voltage of a certain pin does not match the normal value, it should be analyzed based on whether the voltage of that pin has a significant impact on the normal operation of the IC and the corresponding changes in the voltage of other pins in order to determine the quality of the IC.
e. The voltage of IC pins is affected by peripheral components. When there is leakage, short circuit, open circuit, or variation in the peripheral components, or when the peripheral circuit is connected to a variable resistance potentiometer, the position of the potentiometer sliding arm is different, which will cause the pin voltage to change.
f. 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 in the IC is abnormal, the peripheral components should be checked for faults starting from the point where the deviation from the normal value is maximum. If there are no faults, the IC is likely to be damaged.
g. For dynamic receiving devices such as televisions, the voltage at each pin of the IC varies with or without a signal. If it is found that the pin voltage should not change but changes greatly, and does not change with the signal size and different positions of adjustable components, it can be determined that the IC is damaged.
h. For devices with multiple working modes, such as video recorders, the voltage of each pin of the IC varies under different working modes.
③ AC working voltage measurement method: To understand the changes in the AC signal of the IC, a multimeter with a dB socket can be used to approximate the AC working voltage of the IC. When testing, place the multimeter in the AC voltage range and insert the positive probe into the dB socket; For a multimeter without a dB jack, it is necessary to connect a 0.1-0.5 lead in series with the positive lead μ F isolation capacitor. This method is applicable to ICs with relatively low operating frequencies, such as video amplification stages and field scanning circuits in televisions. Due to the different natural frequencies and waveforms of these circuits, the measured data is approximate and can only be used as a reference.
④ Total current measurement method: This method is a method of determining the quality of an IC by detecting the total current of the incoming line of the IC power supply. Due to the fact that the vast majority of ICs are directly coupled, damage to the IC (such as a breakdown or open circuit of a PN junction) can cause subsequent saturation and cutoff, resulting in a change in the total current. So, by measuring the total current, the quality of the IC can be judged. You can also measure the voltage of the resistance in the power supply path and calculate the total current value using Ohm's law.
