Practical Repair Techniques for Digital Multimeters
① Locate faults following the principles: from exterior to interior, from simple to complex, divide the whole into parts and focus on key points. The common troubleshooting methods are listed below:
② Sensory Inspection Method Judge fault causes directly with human senses. Visual inspection can detect broken wires, cold solder joints, bridging short circuits, blown fuses, burnt components, mechanical damage, lifted or fractured copper traces on printed circuits, etc. Touch components such as batteries, resistors, transistors and integrated circuits to check temperature rise, and refer to the circuit diagram to find the cause of abnormal heating. In addition, check loose components, firmly inserted IC pins and jamming of the selector switch by hand. Listen for abnormal sounds and smell for peculiar odors.
③ Voltage Measurement Method Measure the operating voltage at each key point to quickly locate faults, such as the supply voltage and reference voltage of the A/D converter.
④ Short-Circuit Method This method is commonly used for checking A/D converters and widely applied in repairing weak-current and microelectronic instruments.
⑤ Open-Circuit Method Disconnect the suspected section from the whole device or unit circuit. If the fault disappears, the defect lies within the disconnected circuit. This method is mainly suitable for circuits with short-circuit faults.
⑥ Component Measurement Method After narrowing down the fault to a certain area or several components, perform in-circuit or off-circuit measurement. If necessary, replace the suspected part with a good component. If the fault disappears, the original component is defective.
⑦ Interference Method Use human 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 unit work properly.
