What are the general methods for troubleshooting digital multimeters?
A digital multimeter is a measuring instrument that uses the analog/digital conversion principle to convert the measured quantity into a digital quantity and displays the measurement results in digital form. Compared with pointer multimeters, digital multimeters have the advantages of high precision, fast speed, large input impedance, digital display, accurate readings, strong anti-interference ability, and high degree of measurement automation, and are widely used. However, if used improperly, it may cause malfunction.
Digital multimeter troubleshooting should generally start with the power supply. For example, if the LCD has no display after turning on the power, you should first check whether the voltage of the 9V laminated battery is too low and whether the battery lead is disconnected. When looking for faults, you should follow the order of "first inside then outside, first easy then difficult". Digital multimeter troubleshooting can generally be carried out as follows:
(1) Appearance inspection:
You can touch the battery, resistor, transistor, and integrated block with your hands to see if the temperature is too high. If the newly installed battery is hot, it means the circuit may be short-circuited. In addition, you should also observe whether the circuit is disconnected, desoldered, mechanically damaged, etc.
(2) Detect the working voltage at all levels:
To detect the working voltage at all levels and compare it with the normal value, you should first ensure the accuracy of the reference voltage. It is best to use a digital multimeter of the same model or a similar one for measurement and comparison.
(3)Waveform analysis:
Use an electronic oscilloscope to observe the voltage waveform, amplitude, period (frequency), etc. of each key point in the circuit. For example, test whether the clock oscillator starts oscillating and whether the oscillation frequency is 40 kHz. If the oscillator has no output, it means that the internal inverter of TSC7106 is damaged, or the external component may be open circuit. Observe that the waveform at pin {21} of TSC7106 should be a 50 Hz square wave. Otherwise, the internal 200 frequency divider may be damaged.
(4) Measuring element parameters:
For components within the fault range, perform online or offline measurements and analyze parameter values. When measuring resistance online, the influence of components connected in parallel with it should be considered.
(5) Hidden troubleshooting:
Hidden fault refers to the fault that appears and disappears, and the instrument is good and bad sometimes. This type of failure is relatively complex. Common causes include weak solder joints, loose joints, loose connectors, poor contact of the transfer switch, unstable component performance, and continuous breakage of leads. In addition, it also includes some external factors. For example, the ambient temperature is too high, the humidity is too high, or there are intermittent strong interference signals nearby, etc.
