Fault Diagnosis Methods for Multimeters and Instrumentation
1. Tapping and Manual Pressing Method
Intermittent operation of instruments is most often caused by poor contact or cold solder joints. In such cases, the tapping and pressing method can be used.Tapping means gently tapping a circuit board or component suspected of causing a fault with a small rubber mallet or similar tool to see if errors or shutdowns occur.Manual pressing means turning off the power when a fault appears, firmly re‑pressing all plugged components, connectors and sockets, then restarting the instrument to check whether the fault is eliminated. If the instrument works normally after one tap but malfunctions again after another, it is best to reseat all connectors firmly first. If this does not resolve the problem, other methods must be used.
2. Visual Inspection Method
Use sight, smell and touch. Damaged components may show discoloration, blistering or burn marks; burnt devices often produce distinctive odors; short‑circuited chips become hot; and cold solder joints or detached solder connections can be seen with the naked eye.
3. Elimination Method
The elimination method identifies the cause of a fault by removing plug‑in boards and components inside the instrument. If the instrument returns to normal after removing a certain board or component, the fault is located there.
4. Replacement Method
This requires two instruments of the same model or sufficient spare parts. Replace the faulty component with a known good spare part to see whether the fault is resolved.
5. Comparison Method
This requires two identical instruments, one of which operates normally, as well as necessary equipment such as multimeters and oscilloscopes. Comparisons include voltage, waveform, static impedance, output result and current comparisons.Run the faulty and normal instruments under identical conditions, then measure signals at test points and compare the two sets of readings. Differences indicate the location of the fault. This method requires considerable technical knowledge and skill.
6. Temperature Rise and Fall Method
Some faults appear after prolonged operation or in high summer temperatures, disappear when powered off, and reappear shortly after restarting. This is usually due to poor performance of individual ICs or components whose high‑temperature parameters fail to meet specifications.Cooling: When a fault occurs, wipe the suspected area with anhydrous alcohol using a cotton swab to lower its temperature and see if the fault disappears.Heating: Artificially raise the ambient temperature, for example by placing a soldering iron near the suspicious area (carefully avoid overheating and damaging components), to see whether the fault is induced.
7. Shoulder‑Riding Method (Parallel Method)
Also known as the parallel method. Place a good IC directly on top of the chip being tested, or parallel a good component (resistor, capacitor, diode, transistor, etc.) with the suspected component while maintaining good contact. This method can resolve faults caused by internal open circuits or poor contact inside components.
8. Capacitive Bypass Method
When a circuit behaves abnormally - such as a chaotic display - the capacitive bypass method helps locate the faulty circuit section.Connect a capacitor across the power and ground pins of an IC, or across the base input or collector output of a transistor circuit, and observe the effect on the fault. If bypassing the input has no effect but bypassing the output eliminates the fault, the problem lies in that stage of the circuit.
9. Condition Adjustment Method
Generally, do not randomly adjust components in the circuit before confirming the fault, especially adjustable devices such as potentiometers. However, adjustment is permissible if reference measures are taken in advance, such as marking positions or measuring voltages and resistances before adjustment. Sometimes changing the setting may resolve the fault.
10. Isolation Method
The fault isolation method requires no reference equipment or spare parts and is safe and reliable. Following a fault detection flowchart, the fault search range is gradually narrowed by segmentation and isolation. Combined with signal comparison and component replacement, the fault can usually be located quickly.
