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Foreword
A digital multimeter (DMM) is a measuring instrument that uses the principle of analog-to-digital conversion to convert a measurement into a digital quantity, and displays the measurement result in digital form. Compared with analog multimeters, digital multimeters are widely used due to their advantages such as high precision, high speed, large input impedance, digital display, accurate reading, strong anti-interference ability, and high degree of measurement automation. However, if it is used improperly, it may cause malfunction.
Digital multimeter troubleshooting should generally start with the power supply. For example, after the power is turned on, if the liquid crystal cell displays, first check whether the voltage of the 9V laminated battery is too low; if the battery lead is disconnected. Searching for faults should follow the order of "first in and out, first in the first, and later in the future." Troubleshooting a digital multimeter can generally be done as follows.
First, the appearance of inspection.
The temperature rise of the battery, resistor, transistor, and manifold can be touched by hand. If the newly installed battery is hot, the circuit may be short-circuited. In addition, it should also be observed whether the circuit is broken, desoldering, mechanical damage.
Second, detect the working voltage at all levels.
Check the working voltage of each point, and compared with the normal value, we should first ensure the accuracy of the reference voltage, it is best to use a same type or similar digital multimeter to measure and compare.
Third, waveform analysis.
Observe the voltage waveform, amplitude, period (frequency) of each key point of the circuit with an electronic oscilloscope. For example, if the clock oscillator is started, the oscillation frequency is 40 kHz. If there is no output from the oscillator, the internal inverter of the TSC7106 is damaged, or the external device may be open circuit. Observe that the waveform of the {21} pin of the TSC7106 should be a 50Hz square wave. Otherwise, the internal 200 divider may be damaged.
Fourth, measuring component parameters.
For on-line measurement or off-line measurement of components within the fault range, the parameter values ​​should be analyzed. For on-line resistance measurements, the effects of the elements in parallel with it should be considered.
V. Hidden troubleshooting.
Recessive faults refer to faults that occur when the fault occurs and when the instrument is in good or bad condition. Such failures are more complex, and common causes include solder joints, loose soldering, loose connectors, poor contact of transfer switches, unstable component performance, and disconnected leads. In addition, it also includes some external factors. If the ambient temperature is too high, the humidity is too high or there are intermittent strong interference signals and so on.
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