Elenco AM Radio Kit - Manual (Page 5)

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INTRODUCTION
MIXER
LOCAL
OSCILLATOR
FIRST
IF AMPLIFIER
SECOND
IF AMPLIFIER
DETECTOR
TRANSISTOR
AUDIO
AMPLIFIER
AGC
Figure 1
Speaker
Antenna
Section 1 Section 2 Section 3 Section 4 Section 5
The Elenco ® Dual Audio Superhet 550C AM Radio is
a “superheterodyne” receiver of the standard AM
(amplitude modulated) broadcast frequencies. The
unique design of the Superhet 550C allows you to
place the parts over its corresponding symbol in the
schematic drawing on the surface of the printed circuit
board during assembly. This technique maximizes the
learning process while keeping the chances of an
assembly error at a minimum. It is very important,
however, that good soldering practices are used to
prevent bad connections. The Soldering Guide should
be reviewed before any assembly is attempted.
The actual assembly is broken down into five
sections. The theory of operation for each section, or
stage, should be read before the assembly is started.
This will provide the student with an understanding of
what that stage has been designed to accomplish,
and how it actually works. After each assembly, you
will be instructed to make certain tests and
measurements to prove that each section is
functioning properly. If a test fails to produce the
proper results, a troubleshooting guide is provided to
help you correct the problem. If test equipment is
available, further measurements and calculations are
demonstrated to allow each student to verify that each
stage meets the engineering specifications. After all
of the stages have been built and tested, a final
alignment procedure is provided to peak the
performance of the receiver and maximize the Dual
Audio Superhet 550C’s reception capabilities.
The Dual Audio Superhet 550C can best be understood
by analysis of the block diagram shown in Figure 1.
The purpose of section 1, the Audio Amplifier Stage,
is to increase the power of the audio signal received
from the detector to a power level capable of driving
the speaker. The audio amplifier is switchable
between transistor or IC function.
Section 2 includes the detector circuit and the AGC
(automatic gain control) circuit. The detector converts
the amplitude modulated IF (intermediate frequency)
signal to a low level audio signal. The AGC stage
feeds back a DC voltage to the first IF amplifier in
order to maintain a near constant level of audio at the
detector. Section 3 is the second IF amplifier. The
second IF amplifier is tuned to 455kHz (kilohertz) and
has a fixed gain at this frequency of 100. The 3dB bandwidth of this stage should be approximately
6kHz. Section 4 is the first IF amplifier which has a
variable gain that depends on the AGC voltage
received from the AGC stage. The first IF amplifier is
also tuned to 455kHz and has a 3dB bandwidth of
approximately 6kHz. Section 5 includes the mixer,
oscillator and antenna stages. When the radio wave
passes through the antenna, it induces a small
voltage across the antenna coil. This voltage is
coupled to the mixer, or converter, stage to be
changed to a frequency of 455kHz. This change is
accomplished by mixing (heterodyning) the radio
frequency signal with the oscillator signal. Each of
these blocks will be explained in detail in the Theory
of Operation given before the assembly instructions
for that stage.
GENERAL DISCUSSION
IC AUDIO
AMPLIFIER