However, other circuits will have to be added to permit reception of CW telegraph
signals as well as special circuits to minimize the effect of internal noise that
most present-day transistors generate.
a. RF Circuit. Coils L1 and L2 along with a ferrite core serve as the antenna.
The tuned circuit (L2-C2) selects the desired frequency and develops maximum
voltage from the received radio waves.
low-impedance winding L1 inductively
couples the signal into the input of transistor Q1.
Transformer T1 provides a
feedback link to the transistor to overcome circuit losses and cause oscillations
at a frequency determined by the tuned circuit formed by C3 and the inductance.
Ganged capacitors C2 and C3 provide frequency tracking between the oscillating tank
circuit and the input tuned circuit to maintain a frequency difference equal to the
IF.
The output frequencies developed by Q1 contain the modulated IF that is
selected by the tuned circuit consisting of C5 and the primary of transformer T2.
It can be seen that Q1 serves as an oscillator, a mixer, and an IF amplifier.
b. IF Circuit.
The two IF stages are conventional-type amplifiers using NPN
transistors Q2 and Q3. Each output circuit consists of a tuned-primary transformer
having a low-impedance secondary. Transformer T3 feeds the second IF stage, while
transformer T4 feeds the IF to diode rectifier Z1. This rectifier serves as both a
detector and a source of AVC voltage.
The AVC voltage, developed across R10, is
furnished to Q2 through R9.
Only the first IF amplifier in this receiver is
controlled by AVC. A stable biasing voltage for Q3 is furnished by the drop across
R13 in the audio stage. A portion of the output from each IF stage is fed back to
the input through C7 and C11.
This feedback system provides an out-of-phase
voltage to prevent the IF amplifier stages from oscillating.
c. AF Circuit. The audio signal developed across volume control R10 (which also
doubles as the AGC voltage source) is coupled into Q4 through C16. The output from
this class A amplifier drives a miniature loudspeaker. Usually a receiver of this
type is equipped with a headphone jack which cuts out the speaker when the
headphone cord is plugged in. Tone controls are not normally included in receivers
of this size, because the reproduction quality of miniature loudspeakers will not
permit the development of all frequencies in high fidelity sound. Hence adjusting
a tone control will not do much to improve sound quality.
5-2.
TROUBLESHOOTING AN AM RECEIVER
When a radio receiver is brought to you for servicing, you must put the set back
in normal operation.
The person submitting the repair order isn't particularly
interested in how you do it.
Solving the problem is all up to you.
Naturally,
you'll want a troubleshooting method that will help you find the trouble in the
least time and with the least effort. Ask yourself the following questions and see
how you rate. If you don't come up to par, remember that as you gain experience
you will be able to give a better answer to many of the following questions.
a. Basic, but important: do you understand AM receivers? Are you familiar with
combination receiver-transmitters?
b. Do you know how to operate the receiver?
322 L5
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