Common Faults & Maintenance for Digital Multimeters
(1) No display on the meter
First, check whether the battery voltage is normal (a 9 V battery is generally used; test even new batteries). Second, check if the fuse is blown, the voltage regulator is functional, and whether the current-limiting resistor is open-circuited. Next, inspect the circuit board for corrosion, short circuits or open circuits, especially in the main power supply circuit. If any defects are found, clean the circuit board promptly, followed by drying and soldering work. If the circuit board is intact, measure the two power input pins of the display IC to verify if the supply voltage is normal. If the voltage reading is normal, the display IC is damaged and must be replaced. If the supply voltage is abnormal, check for other short-circuit points. Repair any short circuits found. If no short circuit exists or the fault persists after repair, the IC has an internal short circuit and needs replacement.
(2) Resistance range fails to take measurements
Visually inspect the circuit board first to see if any resistor in the resistance measurement circuit is burnt out. Replace burnt resistors immediately if present. If no burnt components are visible, measure each connected component and replace defective parts. If all peripheral components are normal, the integrated circuit is damaged and shall be replaced.
(3) Inaccurate readings when measuring high voltage with voltage range, or readings drift and become unstable after a short measurement period
This type of fault is mostly caused by insufficient power rating of one or several components. If these components become hot within several seconds after stopping measurement, thermal effects arising from inadequate power capacity are the cause. Such components (or integrated circuits) must be replaced.
(4) Current range cannot perform measurements
Most such faults originate from improper operation. Check whether current-limiting resistors and voltage-dividing resistors are burnt out; replace them if damaged. Then check the connecting wires to the amplifier for damage; reconnect broken wires. If the fault remains unresolved, replace the amplifier.
(5) Unstable readings with digit skipping
Check if the entire circuit board is damp or suffers from leakage current. Clean the board and perform drying treatment if so. Check the input circuit for poor contact or cold solder joints (including test leads). Resolder joints if defects are found. Inspect for degraded resistors or components that get excessively hot right after measurement, which is triggered by reduced power capacity. Replace such components.
(6) Inaccurate readings
This issue is mainly caused by failed resistors or capacitors in the measurement circuit; the faulty resistors or capacitors need replacement. Measure the resistance values in this circuit (including resistance values under thermal response). Replace resistors if their resistance drifts or changes with temperature. Check resistors and capacitors in the reference voltage circuit of the A/D converter and replace damaged parts.
