Common Faults and Solutions of Digital Multimeters
Fault hunting shall follow the principles: from outside to inside, from easy to difficult, divide the whole into parts, and make breakthroughs 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 from overlapping wires, blown fuses, burnt components, mechanical damage, lifted or broken copper traces on printed circuits, etc. Touch to check the temperature rise of batteries, resistors, transistors and ICs, and locate the cause of abnormal temperature rise by referring to the circuit diagram. In addition, manually check whether components are loose, whether IC pins are firmly inserted, and whether the rotary switch is stuck. Listen for abnormal sounds and smell for unusual odors.
② Voltage Measurement Method Quickly locate fault points by measuring the operating voltage at key nodes and verifying whether the values
are normal. Examples include measuring the supply voltage and reference voltage of the A/D converter.
③ Short-circuit Method This method is commonly used in the previously mentioned A/D converter inspection procedures, and 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 within the disconnected circuit. This method is mainly applicable to circuits suffering from short circuits.
⑤ Component Testing Method Once the fault is narrowed down to a certain area or several components, perform in-circuit or off-circuit measurements. If necessary, replace the suspected component with a good one. If the fault disappears, the original component is defective.
⑥ Interference Method Use human body induced voltage as an interference signal and observe changes on the LCD. This method is often used to verify whether the input circuit and display section work properly.
