Eight Methods for Multimeter to Judge Common Faults of Instruments and Meters
1. Tap and Press Method
It is common to encounter intermittent faults in instruments during operation. Most such faults are caused by poor contact or cold solder joints. The tap and press method can be adopted for this situation.
The so‑called "tap" means gently tapping the circuit board or component at potential fault points with a small rubber mallet or other tapping tool, to check whether errors or shutdown faults occur.
The so‑called "press" means when a fault occurs, power off the instrument, firmly press the plugged components, plugs and sockets, then power it on again to see if the fault disappears.
If the instrument works normally after tapping the casing but fails again after another tap, re-seat all connectors first for retesting. If this does not work, other troubleshooting methods should be considered.
2. Visual Inspection Method
Use sight, smell and touch for inspection. Sometimes failed components will change color, blister or show burnt spots; burned devices produce a special odor; short‑circuited chips get hot; cold solder joints or open solder joints can also be observed visually.
3. Elimination Method
The elimination method identifies the fault cause by removing or unplugging circuit boards and components inside the instrument. If the instrument returns to normal after removing a certain circuit board or component, the fault is located there.
4. Replacement Method
This method requires two identical instruments or sufficient spare parts. Replace the faulty component on the defective instrument with a good spare component of the same type, and check whether the fault is cleared.
5. Parallel Chip Method (Back‑to‑Back Test)
Also known as parallel connection method. Place a good IC chip on top of the chip under test, or connect good components (resistors, capacitors, diodes, transistors, etc.) in parallel with the component being checked and maintain good contact. If the fault originates from internal open circuit or poor contact of the component, this method can help locate it.
6. Capacitor Bypass Method
When a circuit shows abnormal phenomena such as garbled display, the capacitor bypass method can be used to roughly determine the faulty circuit section. Connect a capacitor across the power pin and ground pin of the IC; for transistor circuits, connect the capacitor between the base input or collector output terminal, and observe its effect on the fault. If bypassing the input end has no effect but bypassing the output end eliminates the fault, the fault occurs at this circuit stage.
7. Comparison Method
Two identical instruments are required, one operating normally. Necessary test equipment such as multimeters and oscilloscopes is also needed. Comparisons include voltage comparison, waveform comparison, static impedance comparison, output result comparison, current comparison, etc.
Procedure: Run the faulty instrument and the normal instrument under identical conditions, then detect signals at selected test points and compare the two groups of signals. Any difference indicates the fault is at this location. This method requires maintenance personnel to have considerable knowledge and skills.
8. Temperature Rise and Fall Method
Sometimes an instrument develops faults after long running hours or under high ambient temperature in summer. It shows normal readings after shutdown inspection, works normally after being left idle for a while, and then fails again after a short operating time.
This phenomenon is caused by individual ICs or components with poor performance whose high‑temperature characteristic parameters fail to meet specifications. The temperature rise and fall method can be used to find the root cause.
The cooling method: when the fault appears, use cotton fiber dipped in anhydrous alcohol to wipe the suspected faulty area to cool it down, and observe whether the fault disappears.
The heating method: artificially raise the ambient temperature. For example, place an electric soldering iron close to the suspected component (caution: do not overheat to avoid damaging good components) and check whether the fault occurs.
