INTRODUCTION
The Elenco" Superhet 108T AM/FM Radio Kit is a
"superheterodyne'; receiver of the standard AM (amplitude
mobulation) and FM (frequency modulation) broadcast
frequencies. The unique design of the Superhet 108
allows you to place the parts over their corresponding
symbol in the schematic drawing on the_surface of the
printeO circuit board during assembly' This technique
maximizes the learning process while keeping the
chances of an assembly error at a minimum. lt 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 9 sections. The
theory of operation'for each section, or stage, should be
read before the assembly is started' This will provide
student with an understanding of what that stage has bt
designed to accomplish, and how it actually works' A
each assembly, you will be instructed to make certain te
and measurements to prove that each section
functioning properly. lf a test fails to produce the pro
results, a-troubleshooting guide is provided to help '
correct the problem. lf test equipment is available, furt
measurements and calculations are demonstrated
allow each student to verify that each stage meets
engineering specifications. After all ol the stages h
been built and tested, a linal alignment procedure
provided to peak the performance of the receiver i
maximize the Superhet 108T's reception capabilities.
GENERAL DISCUSSION
Section 9Section 8FM RADIO
Section 7Section 6
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Section 5Section 4Section 2
Figure 1
Section 1
Section 3
AM RADIO
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 AM detector circuit and the AGC
(automatic gain control) circuit. The AM detector converts
it're amplitude modulated lF (intermediate frequency)
signal t<i a low level audio signal, lhe AGC stage feeds
bdck a DC voltage to the first AM lF amplifier in order to
maintain a near constant level of audio at the detector.
Section 3 is the second AM lF amplifier. The second AM
lF amplifier is tuned to 455kHz (Kilohertz) and has a fixed
gain at this frequency of 50. Section 4 is the first AM lF 2
Implifier which has a variable gain that depends on the
AGb voltage received from the AGC stage. The first AM
lF amplifiei is also tuned to 455k1z. Section 5 includes
the AM mixer, AM oscillator and AM 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. Section 6 it
FM ratio detector circuit. The FM ratio detector has a I
gain of about 20. Section 7 is the second FM lF ampl
The second FM lF amplifier is tuned to 10.71
(Megahertz) and has a set gain of approximately 2O'
3dB- bandwidth of this stage should be approximi
35OkHz. Section 8 is the lirst FM lF amplifier. The firs
lF amplifier is also tuned to 10.7MHz and has a set ga
approximately 10. lt also has a 3dB bandwidth of 350
Section 9 inciudes the FM mixer, FM oscillator, FM RF
the AFC circuits. The incoming radio waves are amp
by the FM RF amplifier, which is tuned to a desired t
siation in the FM frequency bandwidth of 88MH
108MHz. These amplified signals are then coupled tt
FM mixer stage to be changed to a f requency of 10.71
This change, as in AM, is accomplished by heterodl
the radio frequency signal with the oscillator signal'
AFC stage feeds back a DC voltage to the FM oscillal
prevent the oscillator from drifting. Each of these bl
will be explained in detail in the Theory of Operation 1
before the assembly instructions for that stage.
