Six Troubleshooting Techniques for Digital Multimeters
Digital instruments feature high sensitivity and accuracy and are applied in nearly all enterprises. However, instrument faults arise from multiple factors with strong randomness and few predictable patterns, making repair rather difficult. Therefore, the author has compiled repair experience accumulated over years of practical work for reference by peers in this field.
Ⅰ. Repair Methods
Fault locating shall follow the principle: from outside to inside, from simple to complex, break the whole into parts, and focus on key points. The main methods are listed below:
① Sensory Inspection Method Judge the cause of faults directly by human senses. Visual inspection can detect broken wires, cold solder joints, short circuits caused by overlapping wires, blown fuses, burned components, mechanical damage, lifted or fractured copper traces on printed circuits, etc. Touch to check the temperature rise of batteries, resistors, transistors and integrated circuits, and identify the cause of abnormal heating by referring to the circuit diagram. Additionally, manually check loose components, securely inserted IC pins and stuck selector switches. Listen and smell for abnormal noise or odors.
② Voltage Measurement Method Measure the operating voltage at each key point to quickly locate faults, such as measuring the operating voltage and reference voltage of the A/D converter.
③ Short-circuit Method The short-circuit method is commonly used for A/D converter inspection as mentioned above. It is widely applied in repairing weak-current and microelectronic instruments.
④ Open-circuit Method Disconnect the suspected section from the whole device or unit circuit. If the fault disappears, the fault lies in the disconnected circuit. This method is mainly suitable for circuits with short-circuit faults.
⑤ Component Measurement Method When the fault is narrowed down to a certain position or several components, perform in-circuit or off-circuit measurement. Replace with a good component if necessary; if the fault disappears, the original component is defective.
⑥ Interference Injection Method Use human-body induced voltage as an interference signal and observe changes on the LCD display. This method is commonly used to check whether the input circuit and display section function normally.
