1. Pre-qualification test
This is mainly through the "warming" and the local "voltage calibration" after warming up to determine whether the logic function of the digital voltmeter is normal.
If “+â€, “one†polarity conversion, or “+†or “one†voltage calibration can occur when “zeroingâ€, only the display number is inaccurate, and even the voltage number displayed by one of them is correct. This indicates that the logic function of the digital voltmeter is normal.
Conversely, if the zero cannot be adjusted or the voltage is not displayed, the logic function of the whole machine is abnormal.
2. Measure the power supply voltage
The output voltage of various DC stabilized power supplies inside the digital voltmeter is inaccurate or unstable, and the Zener diode (2DW7B, 2DW7C, etc.), which is the source of the “reference voltageâ€, has no regulated output, which is the logic function of the digital voltmeter. One of the main causes of disorder. Therefore, when troubleshooting, it is necessary to first check whether the various DC regulated outputs and the reference voltage source inside the digital voltmeter are accurate and stable. If problems are found and repaired, it is often possible to troubleshoot and restore the logic function of the digital voltmeter to normal.
3. Variable adjustable device
Semi-variable devices in the internal circuit of the digital voltmeter, such as "reference voltage" source trimmer varistor, differential amplifier operating point trim varistor, and transistor regulated power supply modulating piezoelectric device, etc., because the sliding ends of these semi-adjustable devices are in poor contact, or Its wirewound resistance is mild, and there are often faults such as inaccurate, unstable, and unmeasurable values ​​of the digital voltmeter. Sometimes a slight change in the relevant semi-adjustable device can often eliminate the problem of poor contact and return the digital voltmeter to normal.
It must be pointed out that the parasitic oscillation generated by the transistor regulated power supply itself often causes the digital voltmeter to display an unstable fault phenomenon.
Therefore, under the condition that the logic function of the whole machine is not affected, the piezoelectric positioner can be slightly changed to eliminate the parasitic oscillation.
4. Observing the working waveform
For a faulty digital voltmeter, observe the signal waveform of the integrator output, the signal waveform of the clock generator output, the working waveform of the ring step trigger circuit, and the ripple voltage waveform of the regulated power supply by means of a suitable electronic oscilloscope. It is very helpful to find the fault location and analyze the cause of the fault.
5. Research circuit principles
If the above troubleshooting procedures fail to find the problem, the circuit principle of the digital voltmeter must be further studied, that is, to understand the working principle and logical relationship of each component circuit part, in order to analyze the circuit parts that may cause malfunction, and to develop an inspection. Test plan for the cause of the failure.
6. Develop a test plan
A digital voltmeter is a sophisticated electronic measuring instrument with complicated circuit structure and logic functions. Therefore, on the basis of in-depth study of its working principle of the whole machine, according to the preliminary analysis of the possible fault causes, the test plan can be drawn to effectively determine the fault location and detect the damaged and variable value devices to achieve the purpose of repairing the instrument.
7. Test and update the device
There are many devices used in the circuit of the digital voltmeter, in which the Zener as the reference voltage source, that is, the standard Zener diode, such as 2DW7B, 2DW7C, etc., the integrated operational amplifier in the reference amplifier and the integrator circuit, the ring stepping trigger The switching diodes in the circuit, as well as the integrated blocks or switching transistors in the registered bistable circuit, often suffer from damage and variable values. Therefore, if the device in question must be tested, cannot be tested, or has been tested but still has problems, the device must be updated to resolve the problem quickly.
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