ISCET Component-Level Troubleshooting 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?

Accepted Answer

An open or dried-out main filter capacitor

Answer

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?

Accepted Answer

Another component or circuit path is in parallel with the resistor being measured.

Answer

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?

Accepted Answer

The coupling capacitor (C2) is leaky.

Answer

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?

Accepted 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.

Answer

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?

Accepted Answer

The collector load resistor is open.

Answer

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.

(ISCET) International Society of Certified Electronics Technicians Practice Test

(ISCET) International Society of Certified Electronics Technicians Practice Test

ISCET Component-Level Troubleshooting Questions and Answers