Introduction to the testing method of multimeter
1. In-circuit DC resistance detection method
This is a kind of multimeter ohm stop, directly on the circuit board to measure the IC pins and peripheral components of the positive and negative DC resistance value, and compared with the normal data, to find and determine the fault method. When measuring, pay attention to the following three points:
(1) Disconnect the power supply before measurement to avoid damaging the meter and components during testing.
(2) The internal voltage of the multimeter resistor block should not be greater than 6V, and the range should preferably be R×100 or R×1k block.
(3) When measuring IC pin parameters, pay attention to the measurement conditions, such as the measured model, the sliding arm position of the potentiometer related to the IC, etc., but also consider the peripheral circuit components.
2. DC operating voltage measurement method
This is a kind of in the case of power, with a multimeter DC voltage block on the DC supply voltage, peripheral components of the working voltage measurement; detection IC pins to the ground DC voltage value, and compared with the normal value, and then compressed the scope of the problem, to find out the damaged components. Measurement should pay attention to the following eight points:
(1) The multimeter should have a large enough internal resistance, 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, and if it is a TV set, the signal source should be a standard color bar signal generator.
(3) The meter pen or probe should take anti-slip measures. Because of any instantaneous short-circuit is easy to damage the IC. can be taken as follows to prevent the meter pen sliding: take a section of the bicycle valve core set on the tip of the meter pen, and long out of the tip of the meter pen about 0.5mm or so, which can make the tip of the meter pen good contact with the point being tested, but also effective in preventing slippage, even if it touches the neighboring points will not be a short circuit.
(4) When measured a pin voltage does not match the normal value, the pin voltage should be based on the normal operation of the IC has no significant impact on the other pin voltage and the corresponding change in the analysis, in order to determine the good and bad IC.
(5) IC pin voltage will be affected by peripheral components. When the peripheral components leakage, short circuit, open circuit or change value, or the peripheral circuit is connected to a potentiometer with variable resistance, then the potentiometer sliding arm is in a different position, will make the pin voltage change.
(6) If the IC pin voltage is normal, the IC is generally considered normal; if the IC part of the pin voltage is abnormal, it should start from the largest deviation from the normal value, check the peripheral components have no fault, if there is no fault, the IC is likely to be damaged.
(7) For dynamic reception devices, such as television sets, in the presence or absence of signal, IC pin voltage is different. If you find that the pin voltage should not change instead of large changes, the size of the signal and adjustable components with different positions and changes instead of changes, you can determine the IC damage.
(8) for a variety of operating modes of the device, such as video recorders, in different operating modes, IC pin voltage is also different.
3. AC operating voltage measurement method
In order to grasp the changes in the IC AC signal, you can use a multimeter with a dB jack to approximate the IC AC operating voltage measurement. Detection of the multimeter placed in the AC voltage block, the positive pen inserted into the dB jack; for the multimeter without dB jack, the need for the positive pen in series with a 0.1 ~ 0.5μF isolation capacitor. The method applies to ICs with a relatively low operating frequency, such as the video amplification stage of the television, field scanning circuits and so on. Due to the inherent frequency of these circuits are different, different waveforms, so the measured data is approximate and can only be used for reference.
4. Total current measurement method
This method is to detect the total current of the IC power supply into the line, to determine the IC is good or bad a method. As the vast majority of IC internal direct coupling, IC damage (such as a PN junction breakdown or open circuit) will cause saturation and cut-off of the back stage, so that the total current changes. So by measuring the total current method can determine the IC good or bad. Can also be used to measure the voltage drop of the resistor in the power supply path, using Ohm's law to calculate the total current value.
