General Discussion - Elenco Electronics AM-550K Assembly And Instruction Manual

INTRODUCTION
The Elenco ® Superhet 550 AM Radio Kit is a
"superheterodyne" receiver of the standard AM
(amplitude modulation) broadcast frequencies. The
unique design of the Superhet 550 allows you to
place the parts over their corresponding symbol in
the schematic drawing on the surface of the printed
circuit board during assembly.
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
soldering is attempted.
The actual assembly is broken down into five
sections. The theory of operation for each section, or

GENERAL DISCUSSION

The Superhet 550 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. 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
Section 5
MIXER
LOCAL
OSCILLATOR
This technique
Section 4 Section 3
FIRST SECOND
IF AMPLIFIER IF AMPLIFIER
Figure 1
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
Superhet 550's reception capabilities.
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.
explained in detail in the Theory of Operation given
before the assembly instructions for that stage.
Section 2
DETECTOR
AGC
-4-
Each of these blocks will be
Section 1
AUDIO
AMPLIFIER
Speaker