Measurement, Testing and Fault Diagnosis
Measurement, Testing and Fault Diagnosis
This domain tests instrument selection, safe probing, measurement interpretation and structured fault-finding in consumer electronic equipment.
Core rules
- A digital multimeter is good for voltage, resistance and continuity; a DSO shows waveform, timing and ripple. Use an insulation tester for insulation resistance.
- Respect CAT ratings: a CAT II meter is not suitable for CAT III distribution work. Check probe rating too.
- True RMS matters for distorted or non-sinusoidal waveforms; average-reading meters mislead.
- Current measurement inserts burden voltage; use the correct range and never measure current across a supply.
- Measure resistance with power off and the component isolated, otherwise parallel paths mislead.
- Half-splitting and signal tracing narrow a fault quickly: verify supply, then control, then signal, then output.
- Compare against a known-good channel or documented voltage where possible.
Worked example
An audio amplifier has no output. First verify the DC supply at the output stage. If it is present, check bias and protection relay operation. Inject a 1 kHz signal at the input and trace it with a DSO: at the preamp output, at the power-amp input, and at the speaker terminals. If the signal disappears at the power-amp input, the fault is upstream; if it is present there but absent at the output, inspect the output stage and protection. This split avoids random replacement of good parts.
Traps
Using a CAT II meter on a CAT III supply; clipping a DSO ground to a live chassis; measuring resistance in-circuit; ignoring probe compensation; trusting a multimeter's continuity beep for low-value resistors.