General Discussion - Elenco Electronics AM/FM-108TK Manual

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
9
Section
8
FM RADIO
Section
7
Section
6
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Section
5
Section
4
Section
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.