Practical Repair Techniques for Analog Multimeter Circuits
① Fault 1: Blown 250 V / 0.5 A input fuse causes no input current and all ranges to stop working. This is one of the simplest faults during meter repair. It occurs due to improper operation, such as measuring voltage with the current range or resistance range, which blows the input fuse and cuts off current input. The repair procedure is straightforward: disassemble the multimeter, verify that the input fuse is burnt out, and replace it with a fuse of the same model and specification. For Model 47 multimeters, the fuse specification is 250 V / 0.5 A.
② Fault 3: Inaccurate resistance readings or complete failure of the resistance range. Inaccurate measurement may be caused by low voltage
of the 9 V or 1.5 V batteries; replace the batteries to fix the problem. Failure of the resistance range may result from broken wires or cold solder joints for the 9 V and 1.5 V battery connections; inspect and repair the connections. An open-circuit standard resistance in the resistance circuit can also render the resistance range unusable.
③ Fault 4: Loose contact between the brush and circuit board leads to failure of one or multiple measuring ranges and no pointer deflection. The repair method is simple: disassemble the multimeter, clean the circuit board before reinstalling the brush. This fault originates merely from poor contact between the brush and circuit board.
④ Fault 5: Burned shunt resistor in the current range. The shunt resistors for current ranges have low resistance values, ranging from tens to several hundred ohms. Improper operation (measuring voltage on the current range) creates a power short circuit. The resulting short-circuit current burns out the shunt resistor, which requires extra attention.
⑤ Fault 6: Inaccurate resistance zero adjustment. First check whether the battery voltage is low. If the battery and wiring are normal, inspect the standard resistance. If the standard resistance is intact yet zero adjustment still fails, measure the meter head deflection current and adjust the potentiometer. If zeroing is still impossible, the ohm zero-adjustment circuit is defective. Check the potentiometer first. If the potentiometer works well, inspect its wiring. Finally check the variable resistor connected to the potentiometer. The fault may arise when the resistance value of the variable resistor drifts up or down; replace the faulty component after identification.
⑥ Fault 7: Burned voltage-dividing resistors for AC and DC voltage ranges. Voltage-dividing resistors for voltage ranges are generally high-value components. Examples: DC 1000 V with 10 MΩ divider resistor, 500 V with 5 MΩ, 250 V with 4 MΩ, 50 V with 800 kΩ, 10 V with 150 kΩ; AC 1000 V with 2 MΩ, 500 V with 1 MΩ, 250 V with 800 kΩ. These high-value resistors are not easy to damage. The vulnerable parts are voltage-dividing resistors for ranges below 10 V. If high voltage is mistakenly measured with ranges below 10 V, two consequences will occur: first, the input fuse will blow; second, the voltage-dividing resistors are prone to burnout, which should be inspected with priority.
