ISCET Component-Level Troubleshooting Questions and Answers — Questions and Answers
Question 1: A technician is troubleshooting a linear DC power supply and observes excessive 60/120 Hz AC ripple on the output using an oscilloscope. The DC output voltage is present but is lower than specified. Which component failure is the most likely cause of these symptoms?
- A shorted rectifier diode
- An open-circuited transformer primary
- An open or dried-out main filter capacitor (Correct answer)
- A shorted Zener diode in the regulator section
Correct answer: An open or dried-out main filter capacitor
The main filter capacitor's function is to smooth the pulsating DC from the rectifier into a steady DC voltage. If this capacitor fails open or loses its capacitance (dries out), it can no longer effectively store and release charge, allowing the AC ripple from the rectifier to pass through to the output. [9] A shorted diode would likely blow a fuse or cause a zero-volt output. An open transformer primary would result in no output at all. A shorted Zener diode would pull the output voltage down to nearly zero.
Question 2: When measuring a 10 kΩ resistor in-circuit with a digital multimeter, the reading is stable at 4.7 kΩ. Assuming the multimeter and the resistor's marked value are correct, what is the most probable explanation for this reading?
- The multimeter's battery is low, causing an inaccurate reading.
- The resistor has failed and its value has decreased by more than 50%.
- The circuit's power was accidentally left on during the measurement.
- Another component or circuit path is in parallel with the resistor being measured. (Correct answer)
Correct answer: Another component or circuit path is in parallel with the resistor being measured.
When measuring resistance in a circuit, the multimeter applies a small voltage and measures the resulting current. If other components are in parallel with the resistor under test, they provide an alternate path for the current, resulting in a total measured resistance that is lower than the actual value of the resistor. [8, 11] This is a common pitfall in component-level troubleshooting, often requiring one lead of the component to be lifted from the board for an accurate measurement.
Question 3: A technician is signal tracing an audio amplifier. The signal at the collector of Q1 is a clean sine wave with a +5V DC bias. This signal is fed through a coupling capacitor, C2, to the base of Q2. At the base of Q2, the technician measures a distorted sine wave with a +1.5V DC offset. What is the most likely fault?
- The coupling capacitor (C2) is leaky. (Correct answer)
- The coupling capacitor (C2) has failed open.
- The transistor Q2 has a collector-to-emitter short.
- The transistor Q1 is not amplifying correctly.
Correct answer: The coupling capacitor (C2) is leaky.
A coupling capacitor's primary role is to pass the AC signal while blocking the DC bias voltage from the previous stage. If the capacitor is 'leaky,' it acts like a high-value resistor, allowing some of the DC from Q1's collector to pass through to the base of Q2. [26, 28] This unwanted DC voltage upsets the biasing of Q2, causing it to operate incorrectly and distort the signal.
Question 4: Using a digital multimeter's diode test function to check a known-good NPN Bipolar Junction Transistor (BJT) out-of-circuit, what readings are expected?
- An open circuit (OL) reading between all three terminals in both directions.
- A forward voltage drop (approx. 0.7V) from the base to the collector and from the base to the emitter, and OL when leads are reversed. (Correct answer)
- A low resistance reading between the collector and emitter in both directions, indicating a short.
- A forward voltage drop (approx. 0.7V) from the collector to the base and from the emitter to the base.
Correct answer: A forward voltage drop (approx. 0.7V) from the base to the collector and from the base to the emitter, and OL when leads are reversed.
An NPN BJT behaves like two diodes connected with their anodes at the base. [1, 6] When using the diode test function, placing the positive (red) lead on the P-type base and the negative (black) lead on the N-type emitter or collector will forward-bias the junctions, showing a voltage drop of approximately 0.5V to 0.7V. [2, 4] Reversing the leads should show an open circuit (OL).
Question 5: While signal tracing a common-emitter amplifier stage, a technician confirms a valid AC signal at the transistor's base but finds no AC signal at the collector. A DC voltage measurement at the collector shows it is at the full VCC supply voltage. What is the most probable fault?
- The transistor is shorted from collector to emitter.
- The emitter resistor is open.
- The coupling capacitor at the input is shorted.
- The collector load resistor is open. (Correct answer)
Correct answer: The collector load resistor is open.
In a common-emitter amplifier, the collector voltage is typically biased to about half of VCC. If the collector load resistor opens, it breaks the path for current to flow from VCC through the resistor to the collector. [19] A high-impedance multimeter will then measure the VCC potential at the collector, and since no current can flow, no amplification or signal variation can occur.
Question 6: An electronic device operates normally but fails intermittently when the chassis is tapped or when it undergoes a significant temperature change. This type of symptom is most characteristic of which fault?
- A failed voltage regulator IC.
- A cold solder joint or a hairline crack on the PCB. (Correct answer)
- An electrolytic capacitor that has lost its capacitance.
- A resistor that has drifted out of tolerance.
Correct answer: A cold solder joint or a hairline crack on the PCB.
Intermittent faults that are triggered by mechanical stress (tapping, vibration) or thermal stress (temperature changes causing expansion and contraction) are classic signs of a poor physical connection. [20, 22] A cold solder joint, which is a poorly bonded connection, or a microscopic crack in a copper trace are the most common culprits for this type of behavior.
ISCET Component-Level Troubleshooting Questions and Answers
A technician is troubleshooting a linear DC power supply and observes excessive 60/120 Hz AC ripple on the output using an oscilloscope.
The DC output voltage is present but is lower than specified.
Which component failure is the most likely cause of these symptoms?