Five Ways to Diagnose Faults of Digital Multimeters
Troubleshooting of digital multimeters generally starts with the power‑supply circuit. For example, if there is no LCD display after power‑on, first check whether the 9 V stacked‑battery voltage is too low and whether the battery lead is disconnected. Fault‑hunting shall follow the principle: "from internal to external, from simple to complex". Digital‑multimeter troubleshooting can be carried out roughly by the following approaches.
Five General Methods for Troubleshooting Digital Multimeters
1. Visual and Physical Inspection
Touch components such as batteries, resistors, transistors and integrated‑circuit chips by hand to check for excessive temperature rise. If a newly‑installed battery becomes hot, a short‑circuit may exist in the circuit. In addition, inspect the circuit for broken wires, cold solder joints and mechanical damage.
2. Measurement of Operating Voltages at Each Stage
Measure the operating voltage at each test point and compare readings against normal reference values. First ensure the accuracy of the reference voltage. It is recommended to perform measurements and comparisons using another digital multimeter of the same or similar model.
3. Waveform Analysis
Use an oscilloscope to observe the voltage waveform, amplitude, period (frequency) and other parameters at key circuit nodes. For instance, verify whether the clock oscillator starts oscillation and whether its oscillation frequency reaches 40 kHz. If the oscillator produces no output, the internal inverter inside the TSC7106 may be damaged, or external components may be open‑circuited. The waveform at pin 21 of the TSC7106 should be a 50 Hz square‑wave; otherwise, the internal divide‑by‑200 frequency divider is likely faulty.
4. Measurement of Component Parameters
Carry out in‑circuit or off‑circuit measurements for components within the faulty section and analyse their parameter values. When measuring resistors in‑circuit, take into account the interference caused by components connected in parallel.
5. Troubleshooting of Intermittent Faults
Intermittent faults refer to defects that occur sporadically, making the instrument work erratically. Such faults are relatively complicated. Common causes include cold or loose solder joints, loose connectors, poor contact of rotary switches, unstable component performance, and nearly‑broken leads. External factors may also trigger such faults, such as excessively high ambient temperature, excessive humidity, or nearby intermittent strong interference signals.
