Superheterodyne Receiver Questions and Answers

Answer: C

A live audio stage with nothing coming through it points ahead of the detector, and a dead oscillator stops all conversion at once while leaving everything after the mixer perfectly healthy. An open loudspeaker would silence the hiss too. A shorted detector would also kill the hiss, since the audio path would be loaded. And lost gain control bias makes a set overload rather than go silent. Working out how far along the chain a fault must lie, before suspecting any component, is what makes a diagnosis quick.

Answer: D

Sensitivity failing at one end of the band and not the other is a tracking symptom, and the trimmer is the adjustment that governs the high-frequency end. A transformer fault or a leaky audio capacitor would affect every station equally, since they sit after conversion. And a long detector time constant distorts loud modulation rather than reducing sensitivity. Whether a symptom varies with dial position localises the fault to before or after the mixer in one observation.

Answer: A

Repeats at several dial settings mean unwanted frequencies are reaching the mixer, so the preselector is not rejecting them - a misaligned or overloaded front end. A detector fault would distort rather than duplicate. Slow gain control affects level, not frequency. And a misaligned intermediate frequency filter reduces sensitivity and selectivity everywhere at once. Duplicate reception is always a front-end story, because only the front end decides what the mixer sees.

Answer: B

This is the capture effect: the limiter and discriminator together suppress the weaker of two signals on the same channel rather than letting them mix as an AM set would. Blocking involves a signal on a different frequency. A de-emphasis failure changes tone rather than which station is heard. And over-deviation distorts. The effect is a genuine advantage of the architecture with FM, and recognising it as normal behaviour saves chasing a fault that does not exist.

Answer: C

Surviving with no antenna proves the source is inside the set, so it is an internal spurious response, typically oscillator harmonics mixing to land in the passband. An external transmitter would disappear with the antenna. Audio instability would whistle at every dial setting. And a microphonic component responds to vibration rather than to tuning. Disconnecting the antenna is the cheapest diagnostic available and settles the internal or external question outright.

Answer: D

Distortion that appears only with a strong signal points at a stage driven beyond its linear range, usually because the gain control is not reducing gain enough. Insufficient gain would show as poor sensitivity on weak signals, the opposite dependence. An off-frequency oscillator mistunes every station equally. And an open antenna coupling reduces signal rather than distorting it. Noting whether a symptom grows or shrinks with signal strength usually names the stage responsible.

Answer: A

A symptom confined to a distant station and absent on local ones is almost certainly propagation rather than the receiver, since the set treats both signals identically. A gain control fault, an intermittent contact and oscillator drift would all affect local stations too. Asking whether a symptom is selective between signals of different origin is the quickest way to decide whether a receiver is faulty at all, and it saves a great deal of fruitless work.

Answer: B

Both are true and the second is the reason for the first: the image's distance is twice the intermediate frequency, so raising it moves the image where the preselector attenuates more strongly. That is the whole argument for up-conversion, and it is immediately balanced by the fact that adjacent channel filtering gets harder at higher frequencies. Holding both halves of the trade together is what makes the double conversion architecture make sense.

Answer: C

The reason is true but the assertion is false: gain control scales signal and noise together, so the ratio between them is untouched. What it improves is the constancy of the output level and the receiver's resistance to overload, both worth having but neither a noise benefit. This is the same principle that makes extra gain after the first stage useless for noise: once a ratio is set, no amount of scaling recovers it.

Answer: D

Both hold, and the second explains the first exactly: the mixer maps signal and image onto the same intermediate frequency, so nothing downstream can tell them apart. That is why image rejection is entirely a front-end specification and why the preselector earns its place despite contributing almost nothing to selectivity. Wherever a stage maps two inputs onto one output, the discrimination has to happen before it.