ISCET Test Equipment Operation & Troubleshooting 5 — Questions and Answers
Question 1: A technician observes that a sine wave on an oscilloscope has a flat top (clipping). This MOST likely indicates:
- The oscilloscope timebase is too fast
- The amplifier stage is saturating or the scope input is overdriven (Correct answer)
- The signal has excessive harmonic content
- The trigger level is set incorrectly
Correct answer: The amplifier stage is saturating or the scope input is overdriven
Flat-topped (clipped) sine waves indicate that an amplifier is being driven into saturation or that the oscilloscope input is overloaded beyond its dynamic range.
Question 2: Which instrument is used to measure the total harmonic distortion (THD) of an audio amplifier output?
- Oscilloscope
- Distortion analyzer or spectrum analyzer (Correct answer)
- LCR meter
- Frequency counter
Correct answer: Distortion analyzer or spectrum analyzer
A distortion analyzer or spectrum analyzer measures the ratio of harmonic energy to the fundamental frequency energy, yielding the THD percentage.
Question 3: When measuring a transistor's beta (hFE) with a DMM in transistor test mode, the reading is 0. This indicates:
- The transistor is a PNP type and needs reversed leads
- The transistor's base-emitter junction is open or the device is faulty (Correct answer)
- The DMM range is set too low
- The transistor requires a higher base current for the test
Correct answer: The transistor's base-emitter junction is open or the device is faulty
A beta reading of 0 indicates the transistor cannot amplify current, pointing to an open base-emitter junction or a completely failed device.
Question 4: What does it mean when a logic probe LED remains continuously lit (not pulsing) on a digital circuit node?
- The node is switching at a very high frequency
- The node is stuck at a constant HIGH logic level (Correct answer)
- The probe battery is low
- The node is in a high-impedance (floating) state
Correct answer: The node is stuck at a constant HIGH logic level
A continuously lit LED on a logic probe indicates the node is held at a constant HIGH logic state rather than toggling.
Question 5: A technician is measuring AC voltage at 60 Hz with a DMM set to DC volts by mistake. The expected reading is:
- The true RMS value of the AC voltage
- Near zero volts due to averaging of the AC waveform (Correct answer)
- Twice the peak voltage
- The peak-to-peak voltage
Correct answer: Near zero volts due to averaging of the AC waveform
A DMM set to DC mode averages the input signal; a symmetrical AC sine wave averages to near zero over a full cycle.
Question 6: Which precaution is MOST important when using a megohmmeter (megger) to test insulation resistance?
- Ensure the circuit under test is energized to provide a reference voltage
- Discharge the equipment under test before and after the megger test (Correct answer)
- Set the megger to the lowest available voltage
- Use clip leads shorter than 6 inches to minimize lead resistance
Correct answer: Discharge the equipment under test before and after the megger test
A megohmmeter applies high DC voltage (typically 500–1000 V) that can charge capacitance in the equipment; the equipment must be discharged before and after the test to prevent shock hazards.
Question 7: On a dual-trace oscilloscope set to ALTernate mode, both channels are displayed by:
- Simultaneously displaying both channels using two separate electron beams
- Alternating between channel 1 and channel 2 on successive sweeps (Correct answer)
- Rapidly switching between channels within a single sweep
- Mixing both channel signals before display
Correct answer: Alternating between channel 1 and channel 2 on successive sweeps
In ALT mode, the oscilloscope displays channel 1 on one sweep and channel 2 on the next sweep, which works well at higher frequencies.
ISCET Test Equipment Operation & Troubleshooting 5
A technician observes that a sine wave on an oscilloscope has a flat top (clipping).
This MOST likely indicates: