Standard Troubleshooting Procedures for Digital Multimeters
Troubleshooting digital multimeters should generally begin with the power supply. For example, if nothing appears on the LCD after power-on, first check whether the 9V battery voltage is too low or whether the battery leads are broken. Fault location should follow the sequence: inside first, outside later; easy first, difficult later. Troubleshooting can generally be carried out as follows.
I. Visual and Physical Inspection
Check by touch whether the temperature rise of the battery, resistors, transistors, and integrated circuits is excessively high. If a newly installed battery becomes hot, it may indicate a short circuit in the circuit. In addition, inspect the circuit for broken wires, cold solder joints, mechanical damage, etc.
II. Check Operating Voltages
Measure the operating voltages at various points and compare them with normal values. First ensure the accuracy of the reference voltage, preferably using a digital multimeter of the same or similar model for measurement and comparison.
III. Waveform Analysis
Use an oscilloscope to observe the voltage waveforms, amplitudes, periods (frequencies), etc., at key points in the circuit. For instance, check whether the clock oscillator is running and whether the frequency is 40 kHz. If the oscillator has no output, the internal inverter of TSC7106 may be damaged, or an external component may be open-circuited. The waveform at pin 21 of TSC7106 should be a 50 Hz square wave; otherwise, the internal divide-by-200 counter may be faulty.
IV. Component Parameter Measurement
Perform in-circuit or off-circuit measurement on components within the suspected fault area and analyze the parameter values. For in-circuit resistance measurements, consider the influence of parallel components.
V. Troubleshooting Intermittent Faults
Intermittent faults are those that appear and disappear randomly, causing the meter to work improperly on occasion. Such faults are relatively complex. Common causes include cold solder joints, loose connections, poor contact of selector switches, unstable component performance, and partially broken leads. External factors may also contribute, such as excessively high ambient temperature, high humidity, or intermittent strong interference signals nearby.
