Methods for Troubleshooting Hidden Faults of Digital Multimeters
1. Waveform Analysis
Use an electronic oscilloscope to observe the voltage waveform, amplitude, period (frequency), etc. of each key point in the circuit. For example, to check whether the clock oscillator is oscillating and whether the oscillation frequency is 40kHz. If there is no output from the oscillator, it indicates that the internal inverter of TSC7106 is damaged, or it may be that external components are open-circuited. The waveform at pin {21} of TSC7106 should be a 50Hz square wave. Otherwise, it is likely that the internal 200-frequency divider is damaged.
2. Measuring Component Parameters
For the components within the fault range, conduct in-circuit or out-of-circuit measurement and analyze the parameter values. When measuring the resistance in-circuit, the influence of the components connected in parallel with it should be considered.
3. Troubleshooting Hidden Faults
Hidden faults refer to the faults that appear and disappear intermittently, and the instrument works properly sometimes and malfunctions at other times. Such faults are relatively complex. Common causes include poor soldering at solder joints (虚焊), looseness, loose connectors, poor contact of transfer switches, unstable component performance, and wires that are on the verge of breaking, etc. In addition, they also include those caused by 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.
4. Appearance Inspection
You can touch the battery, resistors, transistors, and integrated circuits with your hand to check whether the temperature rise is too high. If a newly installed battery gets hot, it indicates that there may be a short circuit in the circuit. In addition, you should also check whether there are broken wires, unsoldered joints, mechanical damages, etc. in the circuit.
5. Detecting the Operating Voltage at Each Stage
Detect the operating voltage at each point and compare it with the normal value. First of all, the accuracy of the reference voltage should be ensured. It is best to use a digital multimeter of the same model or a similar model for measurement and comparison.
