Introduction to detection methods of multimeter
1． On-circuit DC resistance detection method
This is a method of using the ohm block of a multimeter to directly measure the forward and reverse DC resistance values of each pin of the IC and peripheral components on the circuit board, and comparing 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 measuring to avoid damaging the meter and components during testing.
(2) The internal voltage of the multimeter's electrical barrier must not be greater than 6V. It is best to use the R×100 or R×1k range for the measurement range.
(3) When measuring IC pin parameters, pay attention to the measurement conditions, such as the model under test, the position of the sliding arm of the potentiometer related to the IC, etc., and also consider the quality of the peripheral circuit components.
2． DC operating voltage measurement method
This is a method of measuring the DC power supply voltage and the working voltage of peripheral components with a multimeter DC voltage block when power is on; detecting the DC voltage value of each pin of the IC to ground, and comparing it with the normal value, thereby compressing the fault range. Find the damaged component. Pay attention to the following eight points when measuring:
(1) The multimeter must have a large enough internal resistance, at least 10 times greater than the resistance of the circuit being measured, to avoid causing 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 test leads or probes. Any instantaneous short circuit can easily damage the IC. The following method can be used to prevent the test pen from sliding: take a piece of bicycle valve core and put it on the test pen tip, and extend the test pen tip by about 0.5mm. This can not only make the test pen tip well in contact with the tested point, but also effectively prevent slipping. , there will be no short circuit even if it hits adjacent points.
(4) When the measured voltage of a certain pin does not match the normal value, the quality of the IC should be judged by analyzing whether the pin voltage has any important impact on the normal operation of the IC and the corresponding changes in the voltage of other pins.
(5)IC pin voltage will be affected by peripheral components. When peripheral components leak, short circuit, open circuit or change value, or the peripheral circuit is connected to a potentiometer with variable resistance, the pin voltage will change due to different positions of the potentiometer's sliding arm.
(6) If the voltage of each pin of the IC is normal, the IC is generally considered normal; if the voltage of some pins of the IC is abnormal, you should start from the point that deviates the most from the normal value 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 televisions, the voltages of each pin of the IC are different when there is a signal or not. If it is found that the pin voltage should not change but changes greatly, and the pin voltage that should change with the signal size and the position of the adjustable component does not change, it can be determined that the IC is damaged.
(8) For devices with multiple working modes, such as video recorders, the voltages of each pin of the IC are also different in different working modes.
3． AC working voltage measurement method
In order to grasp the changes in the IC AC signal, you can use a multimeter with a dB jack to approximately measure the AC operating voltage of the IC. When testing, put the multimeter in the AC voltage setting 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 to the positive test lead. This method is suitable for ICs with relatively low operating frequencies, such as TV video amplification stages and field scanning circuits. Since these circuits have different natural frequencies and different waveforms, the measured data are approximate and can only be used for reference.
4. total current measurement method
This method is a method to judge the quality of the IC by detecting the total current of the IC power supply line. Since most of the internal parts of the IC are directly coupled, when the IC is damaged (such as a breakdown or open circuit of a certain PN junction), it will cause saturation and cutoff of the subsequent stage, causing the total current to change. Therefore, the quality of the IC can be judged by measuring the total current. You can also measure the voltage drop across the resistor in the power path and use Ohm's law to calculate the total current value.
