Elenco Electronics | AM/FM-108K | Instruction manual | Elenco Electronics AM/FM-108K Instruction manual

AM/FM RADIO KIT
MODEL AM/FM-108K
INTEGRAL CIRCUIT, 9 TRANSISTORS, 4 DIODES
Assembly and Instruction Manual
Elenco Electronics, Inc.
TM
TM
Copyright © 2003, 1989 by Elenco Electronics, Inc. All rights reserved.
Revised 2003
REV-X
No part of this book shall be reproduced by any means; electronic, photocopying, or otherwise without written permission from the publisher.
753508
The AM/FM Radio project is divided into two parts, the AM Radio Section and the FM Radio Section. At this time, only
identify the parts that you will need for the AM radio as listed below. DO NOT OPEN the bags listed for the FM radio. A
separate parts list will be shown for the FM radio after you have completed the AM radio.
PARTS LIST FOR THE AM RADIO SECTION
If you are a student, and any parts are missing or damaged, please see instructor or bookstore.
If you purchased this kit from a distributor, catalog, etc., please contact Elenco Electronics (address/phone/email is at the back of this manual) for additional assistance, if needed. DO NOT contact your place of purchase
as they will not be able to help you.
TM
RESISTORS
Qty.
1
1
2
1
1
1
2
1
1
1
1
1
1
1
Symbol
Value
Color Code
Part #
R45
R44
R38, 43
R41
R37
R42
R33, 36
R40
R32
R35
R39
R31
R34
Volume/S2
10W 5% 1/4W
47W 5% 1/4W
100W 5% 1/4W
470W 5% 1/4W
1kW 5% 1/4W
2.2kW 5% 1/4W
3.3kW 5% 1/4W
10kW 5% 1/4W
12kW 5% 1/4W
27kW 5% 1/4W
39kW 5% 1/4W
56kW 5% 1/4W
1MW 5% 1/4W
50kW / SW
brown-black-black-gold
yellow-violet-black-gold
brown-black-brown-gold
yellow-violet-brown-gold
brown-black-red-gold
red-red-red-gold
orange-orange-red-gold
brown-black-orange-gold
brown-red-orange-gold
red-violet-orange-gold
orange-white-orange-gold
green-blue-orange-gold
brown-black-green-gold
Pot/SW with nut and washer
121000
124700
131000
134700
141000
142200
143300
151000
151200
152700
153900
155600
171000
192522
CAPACITORS
Qty.
1
1
2
5
1
1
4
1
2
Symbol
Value
Description
Part #
C1
C30
C31,
C29,
C44
C28
C32,
C34
C39,
Variable
150pF
.01mF
.02mF or .022mF
.047mF
.1mF
10mF
100mF
470mF
Tuning Gang AM/FM
Discap (151)
Discap (103)
Discap (203) or (223)
Discap (473)
Discap (104)
Electrolytic Radial (Lytic)
Electrolytic Radial (Lytic)
Electrolytic Radial (Lytic)
299904
221510
241031
242010
244780
251010
271045
281044
284744
38
33, 35, 36, 37
40, 41, 42
43
SEMICONDUCTORS
Qty.
1
3
1
Symbol
Value
Description
Part #
D4
Q7, 8, 9
U1
1N4148
2N3904
LM-386
Diode
Transistor NPN
Integrated Circuit
314148
323904
330386
COILS
Qty.
1
1
1
1
1
MAGIC WAND
Symbol
Color
Description
Part #
Qty.
Description
Part #
L5
T6
T7
T8
L4
Red
Yellow
White
Black
AM
AM
AM
AM
AM
430057
430260
430262
430264
484004
1
1
4”
Iron Core
Brass Core
Shrink Tubing
461000
661150
890120
Oscillator
IF
IF
IF
Antenna with Holders
MISCELLANEOUS
Qty.
1
1
1
1
1
1
1
1
1
1
Description
PC Board
Switch
Battery Holder
Speaker
Speaker Pad
Knob (pot)
Knob (dial)
Earphone Jack with Nut
Radio Stand
Earphone
Part #
Qty.
517055
541023
590096
590102
780128
622017
622030
622130 or 622131
626100
629250
3
3
3
1
1
8
1
1
8”
1
Description
Part #
Screw 2-56 x 1/4”
Screw #3
Nut 2-56
Plastic Washer
Socket 8-Pin
Test Point Pin
Label AM/FM
Manual
Wire 22 insulated
Solder
641230
641310
644201
645108
664008
665008
723508
753508
814520
9ST4
**** SAVE THE BOX THAT THIS KIT CAME IN. IT WILL BE USED ON PAGES 23 & 52. ****
-1-
IDENTIFYING RESISTOR VALUES
Use the following information as a guide in properly identifying the value of resistors.
BAND 1
1st Digit
Color
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Gray
White
Digit
0
1
2
3
4
5
6
7
8
9
Multiplier
BAND 2
2nd Digit
Color
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Gray
White
Resistance
Tolerance
Color
Multiplier
Black
1
Brown
10
Red
100
Orange
1,000
Yellow
10,000
Green
100,000
Blue
1,000,000
Silver
0.01
Gold
0.1
Digit
0
1
2
3
4
5
6
7
8
9
Color
Silver
Gold
Brown
Red
Orange
Green
Blue
Violet
Tolerance
+10%
+5%
+1%
+2%
+3%
+.5%
+.25%
+.1%
BANDS
2
1
Multiplier
Tolerance
IDENTIFYING CAPACITOR VALUES
Capacitors will be identified by their capacitance value in pF (picofarads), nF (nanofarads), or mF (microfarads). Most
capacitors will have their actual value printed on them. Some capacitors may have their value printed in the following
manner. The maximum operating voltage may also be printed on the capacitor.
Multiplier
For the No.
0
1
2
3
Multiply By
1
10
100
1k
Note: The letter “R” may be used at times
to signify a decimal point; as in 3R3 = 3.3
4
5
8
10k 100k .01
9
0.1
First Digit
Second Digit
Multiplier
103K
Tolerance
100
The letter M indicates a tolerance of +20%
The letter K indicates a tolerance of +10%
The letter J indicates a tolerance of +5%
Maximum Working Voltage
The value is 10 x 1,000 = 10,000pF or .01mF 100V
-2-
INTRODUCTION
TM
The Elenco Superhet 108 AM/FM Radio Kit is a
“superheterodyne” receiver of the standard AM (amplitude
modulation) 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 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 soldering is
attempted.
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 108’s reception
capabilities.
The actual assembly is broken down into 9 sections. The
theory of operation for each section, or stage, should be read
GENERAL DISCUSSION
FM RADIO
Section 9
FM RF
AMPLIFIER
Section 8
Section 7
Section 6
Figure 1
FM MIXER
1ST FM IF
AMPLIFIER
FM
OSCILLATOR
2ND FM IF
AMPLIFIER
FM
DETECTOR
Speaker
AFC
AUDIO
AMPLIFIER
AM MIXER
1ST AM IF
AMPLIFIER
2ND AM IF
AMPLIFIER
AM
OSCILLATOR
Section 5
AM
DETECTOR
AGC
Section 4
Section 3
Section 2
Section 1
AM RADIO
The purpose of Section 1, the Audio Amplifier Stage, is to
increase the power of the audio signal received from either
detector to a power level capable of driving the speaker.
Section 2 includes the AM detector circuit and the AGC
(automatic gain control) stage. The AM 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 AM IF amplifier in order to maintain a near
constant level of audio at the detector. Section 3 is the
second AM IF amplifier. The second AM IF amplifier is tuned
to 455kHz (Kilohertz) and has a fixed gain at this frequency
of 50. Section 4 is the first AM IF 2 amplifier which has a
variable gain that depends on the AGC voltage received from
the AGC stage. The first AM IF amplifier is also tuned to
455kHz. 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 is the FM ratio
detector circuit. The FM ratio detector has a fixed gain of
about 20. Section 7 is the second FM IF amplifier. The
second FM IF amplifier is tuned to 10.7MHz (Megahertz) and
has a set gain of approximately 20. The 3dB bandwidth of
this stage should be approximately 350kHz. Section 8 is the
first FM IF amplifier. The first FM IF amplifier is also tuned to
10.7MHz and has a set gain of approximately 10. It also has
a 3dB bandwidth of 350kHz. Section 9 includes the FM
mixer, FM oscillator, FM RF (Radio Frequency) amplifier, AFC
(Automatic Frequency Control) stage, and the FM antenna.
The incoming radio waves are amplified by the FM RF
amplifier, which is tuned to a desired radio station in the FM
frequency bandwidth of 88MHz to 108MHz. These amplified
signals are then coupled to the FM mixer stage to be
changed to a frequency of 10.7MHz. This change, as in AM,
is accomplished by heterodyning the radio frequency signal
with the oscillator signal. The AFC stage feeds back a DC
voltage to the FM oscillator to prevent the oscillator from
drifting. Each of these blocks will be explained in detail in the
Theory of Operation given before the assembly instructions
for that stage.
-3-
CONSTRUCTION
Introduction
The most important factor in assembling your Superhet 108 AM/FM Radio Kit is good soldering techniques.
Using the proper soldering iron is of prime importance. A small pencil type soldering iron of 25 - 40 watts is
recommended. The tip of the iron must be kept clean at all times and well tinned.
Safety Procedures
• Wear eye protection when soldering.
• Locate soldering iron in an area where you do not have to go around it or reach over it.
• Do not hold solder in your mouth. Solder contains lead and is a toxic substance. Wash your hands
thoroughly after handling solder.
• Be sure that there is adequate ventilation present.
Assemble Components
In all of the following assembly steps, the components must be installed on the top side of the PC board unless
otherwise indicated. The top legend shows where each component goes. The leads pass through the
corresponding holes in the board and are soldered on the foil side. Foil Side
Use only rosin core solder of 63/37 alloy.
DO NOT USE ACID CORE SOLDER!
Mount Part
What Good Soldering Looks Like
Bend Leads to
Hold Part
Solder and
Cut Off Leads
Types of Poor Soldering Connections
A good solder connection should be bright, shiny,
smooth, and uniformly flowed over all surfaces.
1.
Solder all components from
the copper foil side only.
Push the soldering iron tip
against both the lead and
the circuit board foil.
1. Insufficient heat - the
solder will not flow onto the
lead as shown.
Soldering Iron
Component Lead
Foil
Soldering iron positioned
incorrectly.
Circuit Board
2.
3.
4.
Apply a small amount of
solder to the iron tip. This
allows the heat to leave the
iron and onto the foil.
Immediately apply solder to
the opposite side of the
connection, away from the
iron.
Allow the heated
component and the circuit
foil to melt the solder.
Allow the solder to flow
around the connection.
Then, remove the solder
and the iron and let the
connection cool.
The
solder should have flowed
smoothly and not lump
around the wire lead.
Rosin
2. Insufficient solder - let the
solder flow over the
connection until it is
covered. Use just enough
solder
to
cover
the
connection.
Soldering Iron
Solder
Foil
Solder
Gap
Component Lead
Solder
3. Excessive solder - could
make connections that you
did not intend to between
adjacent foil areas or
terminals.
Soldering Iron
Solder
Foil
4. Solder bridges - occur
when solder runs between
circuit paths and creates a
short circuit. This is usually
caused by using too much
solder.
To correct this,
simply drag your soldering
iron across the solder
bridge as shown.
Here is what a good solder
connection looks like.
-4-
Soldering Iron
Foil
Drag
SEMICONDUCTOR PARTS FAMILIARIZATION
This section will familiarize you with the proper method used to test the transistors and the diode.
TRANSISTOR TEST
Refer to the parts list and find transistors. These are
NPN transistors. Refer to Test A for locating the
Emitter, Base and Collector. Using an Ohmmeter,
connect the transistor as shown in Test A. Your meter
should be reading a low resistance. Switch the lead
from the Emitter to the Collector. Your meter should
again be reading a low resistance.
Using an Ohmmeter, connect the transistor as shown
in Test B. Your meter should be reading a high
resistance. Switch the lead from the Emitter to the
Collector. Your meter should again be reading a high
resistance. Typical results read approximately 1MW
to infinity.
Low Resistance
High Resistance
W
W
NPN
W
COM
NPN
W
COM
EBC
EBC
TEST B
TEST A
DIODE TEST
Refer to the parts list and find a diode. This is a
silicon 1N4148 diode. Refer to Test C for locating the
Cathode and Anode. The end with the band is the
cathode. Using an Ohmmeter, connect the diode as
shown in Test C. Your meter should be reading a low
resistance. Using an Ohmmeter, connect the diode
as shown in Test D. Your meter should be reading a
high resistance. Typical results read approximately
1MW to infinity.
Low Resistance
High Resistance
W
W
COM
W
COM
W
Diode
Diode
TEST C
TEST D
-5-
SECTION 1
AUDIO AMPLIFIER
The purpose of the Audio Amplifier is to increase the
audio power to a level sufficient to drive an 8 ohm speaker.
To do this, DC (direct current) from the battery is
converted by the amplifier to an AC (alternating current) in
the speaker. The ratio of the power delivered to the
speaker and the power taken from the battery is the
efficiency of the amplifier. For the Audio Amplifier, we use
the integrated circuit (IC) LM-386. In Figure 2, you can
see equivalent schematic and connection diagrams.
In a Class A amplifier (transistor on over entire cycle), the
maximum theoretical efficiency is .5 or 50%. But, in a
Class B amplifier (transistor on for 1/2 cycle), the
maximum theoretical efficiency is .785 or 78.5%. Since
transistor characteristics are not ideal in a pure Class B
amplifier, the transistors will introduce crossover
distortion. This is due to the non-linear transfer curve near
zero current or cutoff. This type of distortion is shown in
Figure 3.
Equivalent Schematic and Connection Diagrams
VS
6
15kW
7
BYPASS
15kW GAIN
8
GAIN
1
15kW
1.35kW
2
3
– INPUT
+ INPUT
50kW
50kW
4
GND
Dual-In-Line and Small Outline Packages
GAIN
– INPUT
In order to eliminate crossover distortion and maximize
efficiency, the transistors of the audio amplifier circuit are
biased on for slightly more than 1/2 of the cycle, Class AB.
In other words, the transistors are working as Class A
amplifiers for very small levels of power to the speaker, but
they slide toward Class B operation at larger power levels.
+ INPUT
GND
1
8
2
7
3
6
4
5
Top View
To make the LM-386 a more versatile amplifier, two pins
(1 and 8) are provided for gain control. With pins 1 and
8 open, the 1.35kW resistor sets the gain at 20 (see
Figure 4a). The gain will go up to 200 (see Figure 4b) if
a capacitor is placed between pins 1 and 8. The gain can
be set to any value from 20 to 200 if a resistor is placed
in series with the capacitor. The amplifier with a gain of
150 is shown in Figure 4c.
Figure 2
The amplifier in our kit with a gain of 150 is shown in
Figure 5. Capacitor C40 couples the audio signal from the
volume control to the input of the audio amplifier.
Capacitor C43 blocks the DC to the speaker, while
allowing the AC to pass.
Figure 3
Typical Applications
Amplifier with Gain = 200
Amplifier with Gain = 20
Minimum Parts
10mF
+
VS
VS
2
2
1
8
5
LM386
VIN
3
10kW
6
–
7
+
+
1
LM386
VIN
3
10kW
.05mF
6
–
7
+
4
4
8
5
+
.05mF
BYPASS
10W
10W
Figure 4a
Figure 4b
Amplifier with Gain = 150
VS
2
3
1
7
8
5
+
4
Figure 4c
+
10mF
6
–
LM386
VIN
10kW
47W
5
VOUT
150W
+
10W
BYPASS
.05mF
Figure 5
-6-
GAIN
BYPASS
VS
VOUT
ASSEMBLY INSTRUCTIONS
We will begin by installing resistor R43. Identify the resistor by its color and install as shown on page 4. Be
careful to properly mount and solder all components. Diodes, transistors and electrolytic capacitors are
polarized, be sure to follow the instructions carefully so that they are not mounted backwards. Check the box
when you have completed each installation.
Test Point Pin
Lytic Capacitor
NPN Transistor
Flat
Side
Polarity Mark
(–)
Foil Side
of PC Board
EBC
B
(+)
Electrolytics have a polarity marking
indicating the (–) lead. The PC board
is marked to show the lead position.
Figure A
E
C
Mount so E lead is
in the arrow hole
and flat side is in
the same direction
as shown on the
top legend. Leave
1/4” between the
part and PC board.
Figure C
Figure B
Diode
Be sure that the band is
in the correct direction.
Anode
Integrated Circuit
Band
Cathode
Insert the IC socket into the PC board with the notch
in the direction shown on the top legend. Solder the
IC socket into place. Insert the IC into the socket
with the notch in the same direction as the notch on
the socket.
Figure D
Notch
Figure E
R43 - 100W Resistor
(brown-black-brown-gold)
C39 - 470mF Lytic
(see Figure B)
TP2 - Test Point Pin
(see Figure A)
C40 - 10mF Lytic
(see Figure B)
C42 - 10mF Lytic
(see Figure B)
C41 - 10mF Lytic
(see Figure B)
R44 - 47W Resistor
(yellow-violet-black-gold)
C43 - 470mF Lytic
(see Figure B)
U1 - IC Socket 8-pin
U1 - LM386 Integrated Circuit
(see Figure E)
C44 - .047mF Discap (473)
TP1 - Test Point Pin
(see Figure A)
R45 - 10W Resistor
(brown-black-black-gold)
J1 - Jumper Wire
(use lytic lead)
1/8”
TP-15 - Test Point Pin
(see Figure A)
-7-
ASSEMBLY INSTRUCTIONS
Note: Mount the Pot/SW, earphone
jack, and speaker to the foil side of the
PC board.
Battery Holder
3 Screws 2-56 x 1/4”
3 Nuts 2-56
Solder and cut off
excess leads.
Volume/S2
(50kW Pot / SW)
with Nut & Washer
Plastic Washer
Knob (pot)
(see Figure F)
Knob
Nut
Washer
Earphone Jack
with Nut
(see Figure H)
Cut off
locating pin
Speaker
Speaker Pad
Wire #22 Insulated
(see Figures G & I)
Plastic Washer
Solder all 5 tabs to PC board
Figure F
Figure G
Pad
If the speaker pad has center and outside pieces, then
remove them. Peel the backing off of the speaker pad and
stick the pad onto the speaker. Then stick the speaker
onto the solder side of the PC board as shown.
Backing
Backing
Remove
Speaker
Figure I
Figure H
Your kit may contain a different type of earphone jack. Before installing
the jack, determine which one you have.
Nut
3
1
1 - GND
2 - Tip
3 - N.C. Tip
1.5” Wire
2” Wire
1.5” Wire
Foil Side
2
Foil Side
2
2” Wire
Nut
GND
Pad
3 1
Part # 622130
1 - GND
2 - Tip
3 - N.C. Tip
GND
Pad
1” Wire
1” Wire
Part # 622131
Part # 622130
Mount the jack with the nut from the foil side of the PC board (terminal #1
on the GND pad of the PC board). Be sure to line up the tab with the pad
on the copper side of the PC board. Solder terminal #1 to the pad of the
PC board.
From Terminal 3
Part # 622131
Cut three wires 1”, 1.5” and 2” and strip 1/4” of insulation off
of both ends. Solder the 3 wires as shown.
*** Save the extra wire for the FM Section. ***
-8-
STATIC MEASUREMENTS
POWER TEST
For all measurements, connect your equipment GND
to circuit GND TP15. Set your VOM (Volt-OhmMillimeter) to read 2 amps DC. Connect the meter to
the circuit as shown in Figure 6. Make sure that the
volume control is in the OFF position (turned fully
counter-clockwise). While watching your VOM, turn
the volume to the ON position (rotate clockwise until
a “click” is heard). The VOM should indicate a very
low current. Adjust your meter for a more accurate
reading if necessary. If the current is greater than 20
milliamps, immediately turn the power off. The
current should be less than 10 milliamps. This is the
current drawn by the battery when no input signal is
present (the “idle current”). Turn OFF the power. If
your circuit fails this test, check that all of the parts
have been installed correctly, and check for shorts or
poor solder connections.
–
– +
Amps
+
Amps COM
Figure 6
OUTPUT BIAS TEST
Put the battery into the holder.
V
COM V
TP15
Figure 7
Adjust your VOM to read 9 volts and connect it as
shown in Figure 7. Make sure that the battery, or a 9
volt power supply (if available), is properly connected
and turn the power ON. The voltage at TP1 should
be between 3 to 6 volts. If you get this reading, go on
to the next test. If your circuit fails this test, turn the
power OFF and check that the integrated circuit is
correctly inserted in the correct location. The notch
of the IC must be in the same direction as marked on
the PC board. Check that all resistor values are the
correct value and not interchanged. All static tests
must pass before proceeding to the Dynamic Tests or
the next section.
-9-
If you do not have an audio generator, skip the following test and go directly to Section 2.
DYNAMIC MEASUREMENTS
GAIN
Connect the VOM and audio generator to the circuit
as shown in Figure 8.
lead of your VOM to the Jumper J3. Record the AC
input voltage to the amplifier here:
Normally the AC gain is measured at a frequency of
1kHz. Your VOM however, may not be able to
accurately read AC voltages at this frequency.
Therefore, it is recommended that this test be
performed at 400Hz. Set the audio generator at
400Hz and minimum voltage output. With the power
ON, set your VOM to read an AC voltage of 1 volt at
test point TP1. Increase the volume control about
half way. Slowly increase the amplitude of the audio
generator until your VOM reads 1 volt AC. Leave the
audio generator at this setting and move the positive
Vin = _________ volts.
You may have to change scales on your VOM for the
most accurate reading. Turn the power OFF. The AC
voltage gain of your audio amplifier is equal to the AC
output voltage divided by the AC input voltage, or
1/Vin.
Calculate the gain. The gain should be 100–180.
Gain = _________
Generator
Hz
V
TP15
COM
TP15
Figure 8
-10-
V
If an oscilloscope is not available, skip the following test and go directly to Section 2.
AC BANDWIDTH
or 2.8 divisions. This frequency is called the low
frequency 3dB corner. Record your answer.
Connect the oscilloscope and audio generator to
your circuit as shown in Figure 9. Set the audio
generator for a frequency of 1kHz and minimum
voltage output. Set the oscilloscope to read .5 volts
per division. Turn on the power and slowly increase
the volume control to a comfortable level. Increase
the amplitude of the audio generator until the
oscilloscope displays 2 volts peak to peak, (Vpp), at
TP1. It may be necessary to adjust the volume
control. Move the oscilloscope probe to jumper J3
and record the input voltage here:
(f low 3dB) = __________ kHz.
Calculate the AC bandwidth:
(f high 3dB – f low 3dB) = __________ kHz.
AC Bandwidth = __________
Your calculated answer should be greater than
30kHz.
DISTORTION
Vin = _______ Vpp
Connect the generator and oscilloscope as shown in
Figure 9. Set the generator at a frequency of 1kHz,
turn the power ON. Adjust the generator output and
turn the volume until the peaks of the sinewave at
TP1 are clipped for maximum signal as shown in
Figure 10A. One side of the sinewave may clip
before the other depending on the DC centering at
TP1. If oscillations are seen, connect a clip lead from
the GND of your generator to the GND of the circuit.
(at this point, you may want to verify the AC gain).
Move the oscilloscope probe back to TP1 and slowly
increase the frequency from the audio generator until
the waveform on the oscilloscope drops to .7 of its
original reading 1.4Vpp or 2.8 divisions. The
frequency of the generator when the output drops to
.7 of its original value is called the high frequency 3
decibel (dB) corner. Record this frequency here:
Measure the maximum voltage peak to peak when
clipping first occurs and record that value here:
(f high 3dB) = __________ kHz.
Vclp = _______ Vpp.
Slowly decrease the frequency of the generator until
the output drops to .7 of its original reading, 1.4Vpp
Turn the power OFF.
Battery
Generator
Hz
TP15
TP15
Figure 9
-11-
MAXIMUM POWER OUTPUT
The maximum power output before distortion due to
“clipping” can be calculated using the voltage Vclp Clipped
obtained in the Distortion Step as follows:
Crossover
Distortion
Vpeak (Vp) = Vclp/2
Vroot mean squared (Vrms) = Vp x .7
Max power out = (Vrms)2/8 ohms = (Vclp x .35)2/8
Maximum power output should be greater than 200
milliwatts.
A
Figure 10
B
EFFICIENCY
By measuring the DC power taken from the battery
at the maximum power output level, the efficiency to
the audio amplifier can be calculated. Power from
the battery is equal to the current taken from the
battery times the voltage of the battery during
maximum power output. Efficiency can then be
calculated as follows: Eff = Max audio power/Battery
power. It is best to use a power supply (if available)
to prevent supply voltage from changing during these
measurements.
Connect the generator, oscilloscope, power supply
(or battery) and current meter as shown in Figure 11.
Set your current meter to read 1 amp DC. Turn the
power ON and rotate the volume control to
maximum. Slowly increase the amplitude of the
audio generator until the output is clipped as shown
in Figure 10A.
Record Vclp here:
Vclp = _________ Vpp.
This should be equal to Vclp in the Distortion Step.
Record the DC current drawn from the 9 volt supply
here:
Current (I) max = ________ A.
Measure the supply voltage and record the V supply
here:
V supply = ________ volts.
Turn the power OFF.
Calculate the maximum power output as done in the
Maximum Power Output Step.
Record your answers on page 13.
-12-
Generator
If you do not have a power supply, use a 9
volt battery instead.
Power Supply
Hz
Amps
Amps COM
TP15
TP15
Figure 11
Vp = Vclp/2
Vp = ______
Vrms = Vp x .7
Vrms = ______
Max power out = (Vrms)2/8
Max power out = ______
Since the battery power equals the battery voltage times the current taken from the battery; calculate the battery
power:
Battery power = Imax x V supply
Battery power = ______
Since the efficiency (N) is equal to the Max power out divided by the Battery power, we can now calculate the
efficiency of the audio amplifier.
N = Max power out/Battery power
N = _______
N in % = N x 100
N = _______%
Your calculated answer should be around .6 or 60%.
-13-
SECTION 2
AM DETECTOR AND AGC STAGE
The purpose of the detector is to change the
amplitude modulated IF signal back to an audio
signal. This is accomplished by a process called
detection or demodulation. First, the amplitude
modulated IF signal is applied to a diode in such a
way as to leave only the negative portion of that
signal (see Figure 12). The diode acts like an
electronic check valve that only lets current pass in
the same direction as the arrow (in the diode symbol)
points. When the diode is in conduction (On
Condition), it will force the capacitors C33 and C38 to
charge to approximately the same voltage as the
negative peak of the IF signal. After conduction
stops in the diode (Off Condition), the capacitors will
discharge through resistors R36 and R42. The
discharge time constant must be small enough to
follow the audio signal or high frequency audio
distortion will occur. The discharge time constant
must be large enough, however, to remove the
intermediate frequency (455kHz) and leave only the
audio as shown in Figure 12.
Figure 12
The purpose of the automatic gain control (AGC)
circuit is to maintain a constant level at the detector,
regardless of the strength of the incoming signal.
Without AGC, the volume control would have to be
adjusted for each station and even moderately strong
stations would clip in the final IF amplifier causing
audio distortion. AGC is accomplished by adjusting
the DC bias of the first IF amplifier to lower its gain
as the signal strength increases. Figure 12 shows
that the audio at the top of the volume control is
actually “riding” on a negative DC voltage when
strong signals are encountered. This negative DC
component corresponds to the strength of the
incoming signal. The larger the signal, the more
negative the component. At test point five (TP5), the
audio is removed by a low pass filter, R36 and C32,
leaving only the DC component. Resistor R35 is
used to shift the voltage at TP5 high enough to bias
the base of transistor Q8 to the full gain position
when no signal is present. Resistors R35 and R36
also forward bias diode D4 just enough to minimize
“On Condition” threshold voltage.
ASSEMBLY INSTRUCTIONS
Switch
J2 - Jumper Wire
(use lytic lead)
1/8”
-14-
ASSEMBLY INSTRUCTIONS
C34 - 100mF Lytic
(see Figure B)
R38 - 100W Resistor
(brown-black-brown-gold)
T6 - AM IF Coil
(Yellow Dot)
TP3 - Test Point Pin
(see Figure A)
R35 - 27kW Resistor
(red-violet-orange-gold)
T8 - AM IF Coil
(Black Dot)
TP5 - Test Point Pin
(see Figure A)
D4 - 1N4148 Diode
(see Figure D)
C32 - 10mF Lytic
(see Figure B)
C38 - .01mF Discap (103)
R42 - 2.2kW Resistor
(red-red-red-gold)
R36 - 3.3kW Resistor
(orange-orange-red-gold)
C33 - .02mF Discap (203)
or .022mF Discap (223)
STATIC MEASUREMENTS
AGC ZERO SIGNAL BIAS
With the power turned OFF, connect your VOM to TP5 as shown in Figure 13. Make sure that the AM/FM switch
is in the AM position.
Check that the VOM is adjusted to
read 9 volts DC and turn the power
ON. The voltmeter should read
approximately 1.5 volts DC. If your
reading varies by more than .5 volts
from this value, turn the power OFF
and check the polarity of D4. Also
check R36 and R35 and check that
transformer T6 is properly installed.
V
COM
V
TP15
Figure 13
T8 TEST
With the power turned OFF, connect the positive lead
of the VOM to TP3 and the negative lead to ground
pin TP15. Make sure that the VOM is set to read 9
volts DC and turn the power ON. The voltage on the
VOM should be the same as your battery voltage or
power supply voltage. If not, turn the power OFF and
check that T8 is properly installed. Turn the power
OFF.
If you do not have an RF generator, skip to Section 3.
-15-
DYNAMIC MEASUREMENTS
AM DETECTOR AND AGC TEST
Connect your VOM and RF generator as shown in Figure 14.
.001mF
V
GENERATOR
COM V
TP15
Hz
Figure 14
Set the VOM to accurately read 2 volts DC and
the output of the RF generator for 455kHz,
modulation, and minimum voltage output. Turn
power ON and slowly increase the amplitude of
set
no
the
the
TP15
generator until the voltage at TP5 just starts to drop.
This point is called the AGC threshold with no IF
gain. Make a note of the amplitude setting on the RF
generator here: ____________.
If your RF generator does not have amplitude modulation and
you do not have an oscilloscope, skip to Section 3.
SYSTEM CHECK
Connect your equipment as shown in Figure 15.
.001mF
GENERATOR
TP15
Hz
TP15
Figure 15
Set the RF generator at 455kHz, 1kHz at 80%
modulation and minimum voltage output. Turn the
power ON and set the volume control at maximum.
Slowly adjust the amplitude of the RF generator
output until you hear the 1kHz tone on the speaker.
If this test fails, turn the power OFF and check R42
and D4. Turn the power OFF.
-16-
AM DETECTOR BANDWIDTH TEST
Connect your test equipment as shown in Figure 15.
Set the generator at 455kHz with 80% modulation at
a modulation frequency of 1kHz.
Set the
oscilloscope to read .1 volts per division. Turn the
power ON and set the volume at the minimum.
Increase the amplitude of the generator until the
signal on the oscilloscope is 4 divisions peak to
peak. Check the signal to make sure that it is free of
distortion. Leave the frequency of the generator at
455kHz, but increase the modulation frequency until
the output drops to .28Vpp. Record the modulation
frequency on the generator here:
__________
This frequency should be greater than 5kHz. Turn
the power OFF.
SECTION 3
SECOND AM IF AMPLIFIER
The purpose of the second IF amplifier to increase
the amplitude of the intermediate frequency (IF) and
at the same time provide SELECTIVITY. Selectivity
is the ability to “pick out” one radio station while
rejecting all others. The second IF transformer (T8)
acts as a bandpass filter with a 3dB bandwidth of
approximately 6kHz.
The amplitude versus
frequency response of the second IF amplifier is
shown in Figure 16.
separate the radio stations.
The gain at 455kHz in the second IF amplifier is fixed
by the AC impedance of the primary side of
transformer T8, and the DC current in Q9. The
current in Q9 is set by resistors R39, R40 and R41.
Both C36 and C37 bypass the 455kHz signal to
ground, making Q9 a common emitter amplifier. The
signal is coupled from the first IF amplifier to the
second IF amplifier through transformer T7. The IF
transformers not only supply coupling and selectivity,
they also provide an impedance match between the
collector of one stage and the base of the next stage.
This match allows maximum power to transfer from
one stage to the next.
Both IF amplifiers are tuned to a frequency of
455kHz and only need to be aligned once when the
radio is assembled. These amplifiers provide the
majority of the gain and selectivity needed to
kHz
kHz
kHz
Figure 16
-17-
ASSEMBLY INSTRUCTIONS
R39 - 39kW Resistor
(orange-white-orange-gold)
T7 - AM IF Coil
(White Dot)
C36 - .02mF Discap (203)
or .022mF Discap (223)
TP4 - Test Point Pin
(see Figure A)
Q9 - 2N3904 transistor
(see Figure C)
R40 - 10kW Resistor
(brown-black-orange-gold)
C37 - .02mF Discap (203)
or .022mF Discap (223)
R41 - 470W Resistor
(yellow-violet-brown-gold)
STATIC MEASUREMENTS
Q9 BIAS
Connect your VOM as shown in Figure 17. Set the
VOM to read 9 volts DC and turn the power ON. The
voltage at the emitter of Q9 should be approximately
1 volt. If your reading is different by more than .5
volts, turn the power OFF and check components
R39, R40, R41 and Q9.
V
COM
TP15
Figure 17
If you do not have an RF generator and oscilloscope, skip to Section 4.
-18-
V
DYNAMIC MEASUREMENTS
AC GAIN
Connect your test equipment as shown in Figure 18.
.001mF
GENERATOR
TP15
Hz
TP15
Figure 18
BANDWIDTH
Set the generator at 455kHz, no modulation and
minimum voltage output. Set the oscilloscope at 1
volt per division. The scope probe must have an
input capacitance of 12pF or less or it will detune T8.
Turn the power ON and slowly increase the
amplitude of the generator until 4 volts peak to peak
are seen on the scope. With an alignment tool or
screwdriver, tune T8 for a peak on the scope while
readjusting the generator’s amplitude to maintain 4
Vpp at the oscilloscope. After T8 is aligned, move
the scope probe to the base of Q9 and record the
peak to peak amplitude of the signal here:
Reconnect your test equipment as shown in Figure 18.
Turn the power ON and adjust the generator for 4
volts peak to peak at TP3. Realign T8, if necessary,
for maximum output while adjusting the output of the
generator to maintain 4Vpp at TP3. Slowly decrease
the frequency of the RF generator until the signal at
TP3 drops to .707 of its original value or 2.8Vpp.
Record the frequency of the RF generator here:
Fl = _______kHz.
Now increase the frequency of the generator past the
peak to a point where the signal drops to .707 of its
peak value. Record that frequency here:
Vb=________ Vpp.
Turn the power OFF. The AC gain of the second IF
amplifier at 455kHz is equal to 4/Vb and should be
greater than 100. If your value is less than 50 check
components R39, R40, R41, C36 and C37. Also
make sure that Q9 is properly installed. Turn the
power OFF.
Fh = __________kHz.
By subtracting the frequency of the lower 3dB corner
from the frequency of the higher 3dB corner you get
the bandwidth of the second IF amplifier.
Calculate the bandwidth by (FI–Fh)
Bandwidth = __________kHz.
Your results should be similar to the values shown in
Figure 16. Turn the power OFF.
-19-
SECTION 4
FIRST AM IF AMPLIFIER
The operation of the first IF amplifier is the same as
the second IF amplifier with one important difference.
The gain of the first IF amplifier decreases after the
AGC threshold is passed to keep the audio output
constant at the detector and prevent overload of the
second IF amplifier. This is accomplished by making
the voltage on the base of transistor Q8 lower as the
signal strength increases. Since the voltage from
base to emitter is fairly constant, the drop in voltage
at the base produces a similar drop in voltage at the
emitter of Q8. This drop lowers the voltage across
R37 and thus, reduces the DC current through R37.
Since all of the DC current from the emitter of Q8
must go through R37, the DC current in Q8 is
therefore lowered. When the DC current in a
transistor is lowered, its effective emitter resistance
increases. The AC gain of transistor Q8 is equal to
the AC collector load of Q8 divided by its effective
emitter resistance. Raising the value of the effective
emitter resistance, thus, lowers the AC gain of Q8.
ASSEMBLY INSTRUCTIONS
R34 - 1MW Resistor
(brown-black-green-gold)
TP6 - Test Point Pin
(see Figure A)
Q8 - 2N3904 Transistor
(see Figure C)
CAUTION: Test point must
not touch can of IF Coil.
C35 - .02mF Discap (203)
or .022mF Discap (223)
R37 - 1kW Resistor
(brown-black-red-gold)
STATIC MEASUREMENTS
Q8 BASE BIAS
Q8 CURRENT
Connect your VOM to the circuit as shown in Figure 13.
Set your VOM to read 2 volts DC and turn the power
ON. The voltage at TP5 should be approximately 1.5
volts. If your circuit fails this test, check Q8 and R37.
Turn the power OFF.
Connect the positive lead of your VOM to the emitter
of Q8 and connect the negative lead to ground point
TP15. Turn the power ON. The voltage should be
approximately .8 volts. Since the current in Q8 is
equal to the current in R37, I(Q2) = .8/R37 or
approximately .8 milliamps. Turn the power OFF.
If you do not have an RF generator and oscilloscope, skip to Section 5.
-20-
DYNAMIC MEASUREMENTS
Short TP3 to R38 as shown below.
.001mF
Generator
Hz
TP15
TP15
Figure 19
AC GAIN
probe to the base of Q8 and record the peak to peak
level of the 455kHz signal here:
Connect your test equipment as shown in Figure 19.
Vb=___________Vpp.
The scope probe must have an input capacitance of
12pF or less, otherwise it will detune transformer T7.
Using a clip lead, short TP3 to R38 as shown. This
short prevents the AGC from lowering the gain of the
first IF amplifier. Set the generator to 455kHz, no
modulation, and minimum voltage output. Set the
scope to read 1 volt per division and turn the power
ON. Increase the amplitude of the generator until
approximately 4Vpp is seen on the scope. Retune
the IF transformer T7 to maximize the 455kHz at
TP4. After tuning T7, adjust the generator amplitude
in order to keep 4Vpp at TP4. Now move the scope
The AC gain of the first IF amplifier is equal to 4/Vb.
The AC gain should be greater than 100. DO NOT
TURN THE POWER OFF, GO TO THE NEXT TEST.
AGC ACTION
Move the scope probe back to TP4 and adjust the
generator for 4Vpp if necessary. Remove the clip
lead shorting TP3 to R38. The AGC should reduce
the signal level at TP4 to approximately .8 volts. Turn
the power OFF.
SECTION 5
AM MIXER, AM OSCILLATOR, AND AM ANTENNA
In a superheterodyne type receiver, the radio wave at
the antenna is amplified and then mixed with the
local oscillator to produce the intermediate frequency
(IF). Transistor Q7 not only amplifies the RF signal,
but also simultaneously oscillates at a frequency
455kHz above the desired radio station frequency.
Positive feedback from the collector to the emitter of
Q7 is provided by coil L5 and capacitor C31. During
the heterodyning process the following four
frequencies are present at the collector of Q7.
1. The
2. The
3. The
4. The
-21-
local oscillator frequency, OF.
RF carrier or radio station frequency.
sum of these two frequencies, OF + RF.
difference of these two frequencies, OF – RF.
The “difference frequency” is used as the
intermediate frequency in AM radios. The collector of
Q7 also contains an IF transformer (T6) tuned only to
the difference frequency. This transformer rejects all
frequencies except those near 455kHz. T6 also
couples the 455kHz signal to the base of Q8 to be
processed by the IF amplifiers. The antenna and the
oscillator coils are the only two resonant circuits that
change when the radio is tuned for different stations.
Since a radio station may exist 455kHz above the
oscillator frequency, it is important that the antenna
rejects this station and selects only the station
455kHz below the oscillator frequency.
The
frequency of the undesired station 455kHz above the
oscillator is called the image frequency. If the
selectivity of the antenna (Q factor) is high, the image
will be reduced sufficiently.
The oscillator circuit must also change when the
radio is tuned in order to remain 455kHz above the
tuning of the desired radio station. The degree of
accuracy in keeping the oscillator frequency exactly
455kHz above the tuning of the antenna is called
tracking accuracy.
ASSEMBLY INSTRUCTIONS
R31 - 56kW Resistor
(green-blue-orange-gold)
C28 - .1mF Discap (104)
C30 - 150pF Discap (151)
L5 - AM Oscillator Coil
(Red Dot)
J1 - Jumper Wire
(use lytic lead)
1/8”
TP7 - Test Point Pin
(see Figure A)
C31 - .01mF Discap (103)
Q7 - 2N3904 Transistor
(see Figure C)
R32 - 12kW Resistor
(brown-red-orange-gold)
R33 - 3.3kW Resistor
(orange-orange-red-gold)
C29 - .02mF Discap (203)
or .022mF Discap (223)
L4 - AM Antenna w/ holders
(see Figures J & K)
C1 - Tuning Gang Capacitor
2 Screws #3
(see Figure L)
Knob (dial)
Screw #3
Label AM/FM
(See Figure M)
Note: Mount the tuning gang
capacitor to the foil side of
the PC board.
Figure J
Resistance measurements
will be used to check the
configuration of the coil. Slide
one holder off the ferrite core
of the antenna assembly.
Then slide the coil off the the
ferrite core.
Measure the
resistance of the coil. Your
readings should match the
approximate values as shown.
3 Wire
4 Wire
White
Black
Red
White
}
R=9 - 11W
}
R=1 - 1.5W
Black
Red
}
}
Green
-22-
R=9 - 11W
R=1 - 1.5W
IMPORTANT: Before installing the antenna coil, determine if you have a 3 wire coil or a 4 wire coil. Assemble it to the
PC board as shown below. Mount the antenna assembly to the PC board.
Put the tab of the first holder into the right hole and twist the tab 90O.
Put the tab of the second holder into the left hole and twist the tab 90O. Punch out one antenna shim from the front flap of the box.
Insert the cardboard antenna shim between the ferrite core and the
Slide the ferrite core through the holders.
antenna coil. This will temporarily hold the coil in place.
Slide the antenna coil through the ferrite core.
Note: If the end of a wire from the antenna should break off, strip the
insulation off the end with a hot soldering iron. Lay the wire down on a hard
surface and stroke the wire with your iron. The insulation should come off
very easily. CAUTION: The soldering iron will burn the hard surface that you
are working on.
C (white)
B Twisted Together
C (white)
B (black)
B Twisted Together
Black
C (white)
Black
A (red)
OR
Red
Red
Tabs
A (green)
Tabs
A (green)
3 Wire Type Antenna: Solder the 3 colored wires to
the PC board:
Wire A (red) to the hole marked 4 Wire Type Antenna: Solder the 4 colored wires to the PC board: Wire A (green) to the
“RED”, Wire B (black) to the hole marked “BLK” and hole marked “RED”, Wire B (red and black twisted together) to the hole marked “BLK” and
Wire C (white) to the hole marked “WHT”.
Wire C (white) to the hole marked “WHT”.
Figure K
It is important to know which of the two types of the tuning gang capacitor you have received with your kit. Look at the
gang capacitor that you have.
AM SIDE
AM SIDE
AM Antenna
Trimmer
FM Antenna
Trimmer
FM Antenna
Trimmer
AM Antenna
Trimmer
FM Oscillator
Trimmer
AM Oscillator
Trimmer
FM SIDE
FM SIDE
FM Oscillator
Trimmer
AM Oscillator
Trimmer
Locator Lead
Locator Lead
Mount the tuning gang capacitor to the foil side of the PC board with the
AM and FM sides in the correct direction. Fasten the gang in place with
two screws from the front of the PC board. Solder the leads in place and
cut off the excess leads coming through the PC board on the front side.
Figure L
-23-
Knob Post
Screw Holes
ASSEMBLY INSTRUCTIONS
Figure M
Screw
Knob
Fasten the knob to the shaft of the
capacitor with a screw.
Rotate the knob fully clockwise.
Peel off the protective backing on
the label. Line up the long white
lines on the label with the arrows
on the PC board.
PC Board Stand
Insert the PC board into the stand as shown.
-24-
STATIC MEASUREMENTS
Q7 BIAS
Connect your VOM to the circuit as shown in Figure 20.
Short TP6 to the
collector of Q7
as shown.
V
COM
V
TP15
Figure 20
Connect a clip lead from TP6 to the collector of Q7.
This short prevents Q7 from oscillating. Set the VOM
to read 2 volts DC and turn the power ON. The DC
voltage at TP7 should be about 1.6 volts. If the
voltage in your circuit differs by more than .5 volts,
leave the power ON and check the battery voltage. If
the battery voltage is greater than 8.5 volts, check
components R31, R32, R33 and Q7.
Turn the power OFF.
If you do not have an oscilloscope, skip to the AM Final Alignments.
DYNAMIC MEASUREMENTS
AM OSCILLATOR CIRCUIT
Connect your test equipment to the circuit as shown in Figure 21.
TP15
Figure 21
Set the scope to read 1 volt per division and turn the
power ON. The scope should display a low voltage
sinewave. The frequency of the sinewave should
change when the tuning gang is turned. If your
circuit fails this test, check components Q7, gang
capacitor, C28, C29, C30, C31, L4 and L5. Turn the
power OFF.
-25-
AM FINAL ALIGNMENTS
AM ALIGNMENT
EQUIPMENT
There are two different AM alignment procedures.
The first alignment procedure is for those who do not
have test equipment and the second is for those who
do have test equipment.
WITHOUT
TEST
It is best to use an earphone for this procedure.
Make sure that the switch is in the AM position. With
an alignment tool or screwdriver, turn coils L5, T6, T7
and T8 fully counter clockwise until they stop. DO
NOT FORCE THE COILS ANY FURTHER. Turn
each coil in about 1 1/4 to 1 1/2 turns. Set the AM
antenna coil about 1/8” from the end of its ferrite rod.
Refer to Figure K.
Included in your kit is a special device called a
“magic wand” which is used for aligning resonant
circuits. It usually has a piece of brass on one end
and a piece of iron on the other. When the brass end
of the “magic wand” is placed near the AM antenna,
the antenna coil will react as if inductance has been
removed. Likewise, when the iron end of the “magic
wand” is placed near the AM antenna, the antenna
coil will react as if inductance has been added.
Therefore, when either brass or iron is placed near
the antenna coil, it will change the inductance of the
antenna coil. This change in the inductance will
cause the resonant frequency of the circuit to
change, thus changing the frequency at which the
antenna was selective. When aligning the antenna
and oscillator circuits, coils L4 and L5 are adjusted at
the lower end of the band, while the oscillator and
antenna trimmer capacitors are adjusted at the
higher end of the band. This is done so that the
antenna and the oscillator will track correctly.
IF ALIGNMENT
Turn the power ON and adjust the volume to a
comfortable level. Turn the dial until a weak station is
heard. If no stations are present, slide the antenna
back and forth on its ferrite core, and retune the dial
if necessary. Adjust T6 until the station is at its
loudest. Reduce the volume if necessary. Adjust T7
until the station is at its loudest and reduce the
volume if necessary. Adjust T8 until the station is at
its loudest and reduce the volume if necessary.
Retune the radio for another weak station and repeat
this procedure until there is no more improvement
noticed on the weakest possible station. This
process peaks the IF amplifiers to their maximum
gain.
OSCILLATOR ALIGNMENT
Tune the radio until a known AM station around
600kHz is heard. It may be necessary to listen to the
station until their broadcast frequency is announced.
If no stations are present at the low side of the AM
band, adjust L5 until a station is heard. Once a
station is found and its broadcast frequency is
known, rotate the dial until the white pointer is
aligned to that station’s frequency marking on the
dial. Adjust L5 until the station is heard. Tune the
radio until a known station around 1400kHz is heard.
It may be necessary to listen to the station until their
broadcast frequency is announced. If no stations are
present, adjust the AM oscillator trimmer on the gang
until a station is heard (refer to Figure L). Once a
station is found and its broadcast frequency is
known, rotate the dial until the white pointer is
aligned to that station’s frequency marking on the
dial. Adjust the AM oscillator trimmer on the gang
until the station is heard. Repeat these 2 steps until
the oscillator alignment is optimized. This process
sets the oscillator range at 955kHz to 2055kHz.
Soldering Iron Tip
Shrink Tubing
Iron Slug
Brass Slug
Magic Wand Assembly
Place the piece of brass inside the end of the
shrink tubing, with 1/4” outside. Heat the brass up
with your soldering iron until the tubing shrinks
around the brass. Assemble the iron piece to the
other end in the same manner.
-26-
ANTENNA ALIGNMENT
Tune the radio for a station around 600kHz. With the
“magic wand” place the brass end near the antenna
coil as shown in Figure 22. If the signal heard at the
output increases, it means that the antenna coil needs
less inductance. To remove inductance, carefully slide
the antenna coil along it’s ferrite core in the direction
shown in Figure 22. Place the iron end of the “magic
wand” near the antenna coil. If the signal heard at the
output increases, this means that the antenna coil
needs more inductance. To add more inductance,
carefully slide the antenna coil along its ferrite core in
the direction shown in Figure 22. Repeat these steps
until the signal heard decreases for both ends of the
“magic wand”. Tune the radio for a station around
1400kHz. With the “magic wand”, place the brass end
near the antenna coil. If the signal heard at the output
increases, it means that the antenna coil needs more
capacitance. Adjust the antenna trimmer on the back
of the gang until the signal is at its loudest. Refer to
Figure L for the location of the antenna trimmer. Place
the iron end of the “magic wand” near the antenna
coil. If the signal heard at the output increases, it
means that the antenna coil needs less capacitance.
Adjust the antenna trimmer on the back of the gang
until the signal is at its loudest. Repeat these steps
until the signal heard decreases for both ends of the
“magic wand”. Since the adjustment of both the
antenna trimmer and antenna coil will effect the
antenna alignment, it is advisable to repeat the entire
procedure until the antenna alignment is optimized.
This process sets the tracking of the AM radio section.
Once the antenna is properly aligned, CAREFULLY
APPLY CANDLE WAX or glue to the antenna coil and
the ferrite rod to prevent it from moving (see Figure 23).
Cut the shim flush with the antenna.
This concludes the alignment of the AM radio
section. If no stations are heard, verify that AM
signals are present in your location by listening to
another AM radio placed near the Superhet 108. If
the AM section is still not receiving, go back and
check each stage for incorrect values and for poor
soldering. Proceed to the FM assembly section.
Magic Wand
Antenna
Shim
Antenna Coil
Ferrite Core
Antenna Holder
If the antenna needs:
• More inductance, slide the coil
• Less inductance, slide the coil
Figure 22
Wax
Wax
Figure 23
AM ALIGNMENT WITH TEST EQUIPMENT
IF ALIGNMENT
the AM position. Place a short from the collector
of Q7 to TP6. This short “kills” the AM oscillator.
Connect your RF generator and oscilloscope as
shown in Figure 24. Make sure that the switch is in
Figure 24
.001mF
GENERATOR
TP15
Hz
TP15
-27-
collector of Q7 to TP6.
Set the RF generator at 455kHz, modulation of
400Hz 80% and minimum voltage out. Set the
oscilloscope to read .1 volts per division and turn the
power ON. Increase the amplitude of the generator
until the oscilloscope shows a 400Hz sinewave 5
divisions or .5 volts pp. With an alignment tool or
screwdriver adjust T6 for a peak. Reduce the
generator amplitude so that 5 divisions are
maintained. Adjust T7 for a peak and reduce that
amplitude again if necessary. Repeat these steps to
optimize the IF alignment. This process aligns the IF
amplifiers to 455kHz.
Calculate the bandwidth: __________kHz.
OSCILLATOR ALIGNMENT
Set the RF generator at 540kHz, 400Hz 80% AM
modulation and a low level of output. Turn the power
ON and set the volume control to a comfortable level.
Turn the tuning knob counter-clockwise until the
white pointer is aligned at the 540kHz marking on the
dial. With an alignment tool or screwdriver adjust L5
until a 400Hz tone is heard. Adjust L5 for a peak on
the oscilloscope. Adjust the amplitude of the RF
generator to maintain a level of .5 volts peak to peak
or less. After peaking L5, set the generator
frequency to 1600kHz. Turn the tuning knob
clockwise until the white pointer is aligned to the
1600kHz marking on the dial. With an alignment tool
or screwdriver, adjust the AM oscillator trimmer on
the back of the tuning gang until a 400Hz tone is
heard. Adjust the trimmer for a peak on the
oscilloscope. Refer to Figure L for the location of the
AM oscillator trimmer. Repeat these steps to
optimize the oscillator alignment. This process sets
the oscillator range at 955kHz to 2055kHz.
After the IF alignment is complete, lower the
frequency of the generator until the voltage drops
.707 of its peaked value or .35Vpp. Record the
frequency of the lower 3dB corner here:
Fl = _________kHz.
Increase the frequency of the generator past the
peak until the voltage seen on the scope drops .707
of its peaked value or .35Vpp. Record the frequency
of the high 3dB corner here:
Fh = __________kHz.
ANTENNA ALIGNMENT
The bandwidth of the IF is equal to BW = Fh - Fl. The
IF’s bandwidth should be around 6kHz. Turn the
power OFF and remove the short from the
With the power turned OFF, connect your test
equipment as shown in Figure 25.
Generator
Battery
Hz
Wire loop
close to
antenna
TP15
Figure 25
-28-
brass end of the “magic wand” near the antenna coil.
If the signal increases, it means that the antenna coil
needs less capacitance. Adjust the antenna trimmer
for a peak. Refer to Figure L for the location of the
AM antenna trimmer. Since the adjustment of both
the antenna alignment is optimized. This process
sets the AM tracking of the Superhet 108.
Once the antenna is properly aligned, carefully apply
candle wax or glue the antenna coil to the ferrite rod
to prevent it from moving as shown in Figure 23. Cut
the shim flush with the antenna. Proceed to the FM
assembly section.
Set the generator at 600kHz, 400Hz 80%
modulation, moderate signal strength. Set the
oscilloscope to read .1 volts per division. Turn the
tuning knob fully counter-clockwise and turn the
power ON. Slowly turn the tuning knob clockwise
until a 400Hz sinewave is seen on the scope. Adjust
the volume control to a comfortable level. If a station
exists at 600kHz, then lower the frequency of the
generator and repeat the previous steps. With the
“magic wand”, place the brass end near the antenna
coil as shown in Figure 22. If the signal on the scope
increases, it means that the antenna coil needs less
inductance. To add more inductance, carefully slide
the antenna coil along it’s ferrite core in the direction
shown in Figure 22. Repeat these steps until the
signal seen decreases for both ends of the “magic
wand”. Increase the frequency of the generator to
1400kHz and turn the tuning knob clockwise until a
400Hz sinewave is seen on the scope. If a station
exists at 1400kHz, increase the frequency of the
generator and repeat the previous steps. Place the
This concludes the alignment of the AM radio
section. If no stations are heard, verify that AM
signals are present in your location by listening to
another AM radio placed near the Superhet 108. If
the AM section is still not receiving, go back and
check each stage for incorrect values and for poor
soldering. Proceed to the FM assembly section.
AM RADIO HIGHLIGHTS
6. The process of adding the audio waves to the
radio waves is called modulation, and the
process of removing the radio wave from the
audio wave is called demodulation, which is
performed in an AM radio by the detector.
7. The amount of signal picked up by the antenna
will depend on the power of the signal transmitted
and the distance the signal travelled.
8. Rectification is the process of removing half the
signal, while filtering is the process of smoothing
that signal.
9. Heterodyning is the process of mixing two signals
(the incoming RF signal and the RF signal from
the local oscillator) to produce a third signal (the
IF signal).
1. The number of vibrations (or cycles) per second
produced by a sound is called the frequency, and
is measured in hertz.
2. The distance between peaks of sound waves is
called the wavelength.
3. Sound waves are produced as a certain number
of vibrations per second. The more vibrations per
second, the higher the frequency; the fewer
vibrations, the lower the frequency.
4. Waves of very high frequency are called radio
waves and travel great distances through the air
without the use of wires.
5. Carrier waves are radio waves used by broadcast
stations to carry audio waves.
DC VOLTAGES
The voltage readings below should be used in troubleshooting the AM section. (Switch at AM position).
Q7
B
E
C
1.5
1.0
8.8
TP5 (AGC)
1.4
Q8
1.4
.8
8.8
1.7
1.0
9.0
Q9
B
E
C
B
E
C
U1
TP1
1
2
3
4
5
6
7
8
Test Conditions
1.3
0
0
0
4.5
9.0
4.6
1.3
1. Volume set to minimum.
2. Connect side of capacitor C29 (that goes
to L4) to TP15 with a jumper wire.
3. Battery voltage = 9V
4. All voltages are referenced to circuit
common.
4.5
5. Voltage readings can vary +10%
-29-
QUIZ - AM SECTION
INSTRUCTIONS: Complete the following examination, check your answers carefully.
1. The number of cycles produced per second by a
source of sound is called the ...
A) amplitude.
B) vibration.
C) sound wave.
D) frequency.
6. When the two metal plates on a variable
capacitor are unmeshed the ...
A) capacitance is minimum.
B) capacitance is maximum.
C) capacitance is not affected.
D) inductance is increased.
2. The radio frequencies used by AM broadcast
stations are between ...
A) 20kHz and 400kHz.
B) 5kHz and 20kHz.
C) 2400kHz and 6000kHz.
D) 550kHz and 1600kHz.
7. The process of mixing two signals to produce a
third signal is called ...
A) filtering.
B) detecting.
C) rectification.
D) heterodyning.
3. The process of removing the audio wave from
the radio wave is called ...
A) demodulation.
B) frequency reduction.
C) modulation.
D) vibrating.
8. The magic wand is used to determine ...
A) whether more or less inductance is
required in a tuned circuit.
B) whether more or less capacitance is
required in a tuned circuit.
C) the gain of an RF amplifier.
D) whether the oscillator is functioning.
4. When an electromagnetic wave (modulated radio
wave) passes an antenna, it ...
A) induces a voltage and current in the
antenna.
B) changes an audio wave into a radio
wave.
C) changes the carrier frequency.
D) produces sidebands.
9. The IF frequency of your AM radio is ...
A) 1600kHz.
B) 455kHz.
C) 550kHz.
D) 910kHz.
10. The purpose of the AGC circuit is to ...
A) automatically control the frequency of
the oscillator circuit.
B) control the band width of the IF stages.
C) reduce distortion in the audio circuit.
D) maintain a constant audio level at the
detector, regardless of the strength of
the incoming signal.
5. The power of the signal transmitted by the
broadcast station and the distance, the signal
travelled from the transmitter to the receiver,
determine the ...
A) frequency of the modulation.
B) wavelength of the audio waves.
C) amount of signal picked up by the
antenna.
D) type of filter that is used.
Answers: 1. D, 2. D, 3. A, 4. A, 5. C, 6. A, 7. D, 8. A, 9. B, 10. D
-30-
PARTS LIST FOR FM SECTION
RESISTORS
Qty.
2
1
1
5
1
2
2
7
2
2
3
2
Symbol
Value
Color Code
Part #
R9, 23
R25
R3
R18, 22, 24, 26, 27
R11
R15, 6
R2, 7
R10,12,14,16,19,20,28
R1, 8
R4, 5
R13, 17, 21
R29, 30
100W 5% 1/4W
220W 5% 1/4W
470W 5% 1/4W
1kW 5% 1/4W
1.8kW 5% 1/4W
2.2kW 5% 1/4W
6.8kW 5% 1/4W
10kW 5% 1/4W
22kW 5% 1/4W
33kW 5% 1/4W
47kW 5% 1/4W
390kW 5% 1/4W
brown-black-brown-gold
red-red-brown-gold
yellow-violet-brown-gold
brown-black-red-gold
brown-gray-red-gold
red-red-red-gold
blue-gray-red-gold
brown-black-orange-gold
red-red-orange-gold
orange-orange-orange-gold
yellow-violet-orange-gold
orange-white-yellow-gold
131000
132200
134700
141000
141800
142200
146800
151000
152200
153300
154700
163900
CAPACITORS
Qty.
Symbol
Value
Description
Part #
1
1
1
1
2
3
3
10
1
1
1
1
C9
C10
C6
C11
C4, 5
C3, 7, 27
C2, 8, 12
C13 - 22
C23
C26
C25
C24
15pF
30pF
33pF
220pF
470pF
.001mF
.005mF
.01mF
.02mF or .022mF
.1mF
10mF
470mF
Discap (15)
Discap (30)
Discap (33)
Discap (221)
Discap (471)
Discap (102)
Discap (502)
Discap (103)
Discap (203) or (223)
Discap (104)
Electrolytic Radial (Lytic)
Electrolytic Radial (Lytic)
211510
213010
213317
222210
224717
231036
235018
241031
242010
251010
271045
284744
Qty.
Symbol
Value
Description
Part #
D1
D2, 3
Q1 - Q6
1N60
2N3904
Varactor Diode
Diode
Transistor
310176
311065
323904
Symbol
Value
Description
Part #
T5
T4
T2, 3
T1
L1
L2
L3
Blue
Pink
Green
Orange
6 Turns
2 Turns
5 Turns
FM
FM
FM
FM
FM
FM
FM
430110
430120
430130
430140
430160
430170
430180
Symbol
Description
Part #
Antenna FM
Screw 2-56 x 1/4”
Antenna Screw M2
Nut 2-56
Test Point Pin
Coil Spacer
484005
641230
643148
644201
665008
669108
SEMICONDUCTORS
1
2
6
COILS
Qty.
1
1
2
1
1
1
1
Detector
Detector
IF
Mixer
RF Amp
RF Amp
Oscillator
MISCELLANEOUS
Qty.
1
2
2
2
7
1
TP8 - 14
-31-
SECTION 6
THE FM RADIO
Section 6 begins the construction of the FM radio.
The stages that we will build are shown in the block
diagram below. We will begin with the FM Ratio
Detector and work back to the FM Antenna. Each
stage will be tested before proceeding to the next
stage.
FM RADIO
Section 9
FM RF
AMPLIFIER
Section 8
Section 6
Section 1
Speaker
FM MIXER
1ST FM IF
AMPLIFIER
FM
OSCILLATOR
Section 7
2ND FM IF
AMPLIFIER
FM
DETECTOR
AUDIO
AMPLIFIER
AFC
FM RATIO DETECTOR
Thus, no current is drawn through C23 resulting in
zero audio output voltage. When the incoming signal
is modulated, the current through one diode will be
greater than the other. This causes a current to flow
in C23 which will produce an audio voltage across
C23. If the modulation is of opposite direction than
before, more current will flow in the other diode,
which will again cause current to flow in C23 in the
opposite direction resulting in an audio voltage being
produced across C23. The large current drawn from
the audio which causes the battery voltage to vary.
The ratio detector is decoupled further by the resistor
R23 and capacitor C21.
In the AM DETECTOR section we observed that the
audio was detected from changes in the amplitude of
the incoming signal. In FM detection, the audio is
detected from changes in frequency of the incoming
signal. The RATIO DETECTOR has built-in limiting
action which limits the signal so that any noise riding
on the FM carrier will be minimized. The RATIO
DETECTOR is redrawn below for ease of
explanation.
When an incoming signal is present at T4 and T5, a
current flows through D2, R26, R28, R27 and D3. At
no modulation, the current through the diodes D2
and D3 are equal because T5 is center tapped.
mF
.02mF
.01mF
mF
Figure 26
-32-
ASSEMBLY INSTRUCTIONS
R24 - 1kW Resistor
(brown-black-red-gold)
R23 - 100W Resistor
(brown-black-brown-gold)
C21 - .01mF Discap (103)
C23 - .02mF Discap (203)
or .022mF Discap (223)
R21 - 47kW Resistor
(yellow-violet-orange-gold)
R25 - 220W Resistor
(red-red-brown-gold)
C19 - .01mF Discap (103)
C24 - 470mF Lytic
(See Figure B)
T3 - FM IF Coil
(Green Dot)
D2 - 1N60 Diode
(see Figure D)
TP9 - Test Point Pin
(see Figure A)
R26 - 1kW Resistor
(brown-black-red-gold)
TP8 - Test Point Pin
(see Figure A)
R28 - 10kW Resistor
(brown-black-orange-gold)
Q6 - 2N3904 Transistor
(see Figure C)
C25 - 10mF Lytic
(see Figure B)
R20 - 10kW Resistor
(brown-black-orange-gold)
R27 - 1kW Resistor
(brown-black-red-gold)
R22 - 1kW Resistor
(brown-black-red-gold)
D3 - 1N60 Diode
(see Figure D)
C22 - .01mF Discap (103)
T5 - FM Detector Coil
(Blue Dot)
T4 - FM Detector Coil
(Pink Dot)
(see Figure 27)
Lytic Capacitor
Test Point Pin
NPN Transistor
Flat
Side
Polarity Mark
Foil Side
of PC Board
( )
Be sure that the negative lead is in
the correct hole on the PC board.
Figure A
EBC
B
(+)
E
Figure B
C
Mount so E lead is
in the arrow hole
and flat side is in
the same direction
as shown on the
top legend. Leave
1/4” between the
part and PC board.
Diode
Band
Anode
Be sure that the band is in the correct
direction.
Figure C
Figure D
Test Point Pin
Notch
Note: Notch must be pointing to the top of the
PC board. Some FM detector coils have a red
line or part number in place of the notch.
-33-
Cathode
Figure 27
STATIC MEASUREMENTS
V
Battery
COM V
TP15
Figure 28
FM VOLTAGE
Connect your VOM as shown in Figure 28. Switch
the AM/FM switch to the FM position. Set your VOM
to read 9 volts DC. Turn the power ON. The voltage
at this point should be between 7 and 9 volts. Turn
the power OFF. If you do not get this reading, check
R25, C24 and the battery voltage.
TRANSISTOR CURRENT TEST
V
COM
V
TP15
Figure 29
Connect your VOM to the circuit as shown in Figure 29.
Turn the power ON. The voltage at the emitter of Q6
should be about .7 volts. Record the voltage here:
Since the current through resistor R22 is equal to the
current through transistor Q6, calculate the current
through Q6 as follows:
V(Q6) = ________.
Current (I) = V(Q6) / R22
Turn the power OFF. If your answer is greater than 2
volts, check R20, R21, R22, R24, Q6 and the battery.
Your calculated answer should be between .0005
amps (.5 milliamps) and .0011 amps (1.1 milliamps).
Current (I) = __________.
-34-
If you don’t have an RF generator and oscilloscope, skip to Section 7.
DYNAMIC MEASUREMENTS
AC GAIN
The AC gain of the ratio detector is set by the AC
impedance of the primary side of T4 and the current
through Q6. The current is set by R20, R21 and R22.
Capacitors C22 and C19 bypass the AC signal to
ground.Connect your RF generator and oscilloscope
to the circuit as shown in Figure 30.
Your scope probe must have an input capacitance of
12pF or less, otherwise the probe will detune T4
causing an incorrect measurement of the AC gain.
Set the generator for 10.7MHz no modulation and
minimum voltage output. Set the scope to read
50mV/division. Turn the power ON and slowly
increase the amplitude of the generator until 3
divisions or 150mVpp are seen on the scope. With
an alignment tool or screwdriver, adjust T4 for a
peak. Reduce the generator input to maintain
150mVpp on the scope. Move the scope probe to
the base of Q6 and record the voltage here:
Vb = __________ mVpp.
Turn the power OFF. The AC gain can be calculated
as follows:
AC Gain = 150mV / Vb
Your calculated answer should be approximately 20.
Record your calculation:
AC Gain = __________
.001mF
Generator
Hz
TP15
TP15
Figure 30
RATIO DETECTOR ALIGNMENT
METHOD #1
ALIGNMENT WITH NO TEST EQUIPMENT
With an alignment tool or a screwdriver, turn both
coils T4 and T5 fully counter-clockwise until they
Generator
stop. DO NOT FORCE THE COILS ANY FURTHER.
Now turn both coils in about 1 1/4 to 1 1/2 turns.
.001mF
Hz
TP15
Figure 31
-35-
TP15
METHOD #2
ALIGNMENT OF RATIO DETECTOR USING A
RF GENERATOR AND OSCILLOSCOPE
Turn the power ON and set the volume control to a
minimum. Increase the amplitude of the sweep
generator until an “S” curve is seen (refer to Figure 32).
Using an alignment tool or screwdriver, adjust the blue
coil T5 until the “S” curve is centered, until each half of
the “S” is equal. Repeat these steps until the
alignment is optimized. Turn the power OFF.
Connect the RF generator and oscilloscope to the
circuit as shown in Figure 31. Set the generator for
10.7MHz modulated at 1kHz, 22.5kHz deviation with
minimum voltage out. Turn ON the radio and turn the
volume control to the minimum. Slowly increase the
amplitude of the generator until a 1kHz sinewave is
seen on the scope. With an alignment tool or
screwdriver, peak the pink coil T4 for maximum
amplitude. Now peak the blue coil T5 for minimum
optimized. Turn the power OFF.
METHOD #3
ALIGNMENT OF RATIO DETECTOR USING A
SWEEP GENERATOR AND OSCILLOSCOPE
Connect the sweep generator and oscilloscope to the
circuit as shown in Figure 31. Set the sweep
generator for 10.7MHz and minimum voltage out.
Figure 32
SECTION 7
SECOND FM IF AMPLIFIER
The purpose of the 2nd IF amplifier is to increase the
amplitude of the intermediate frequency (IF) while
also providing Selectivity. Selectivity is the ability to
“pick out” one station while rejecting all others. T3
acts as a bandpass filter that only passes signals
around 10.7MHz. The resistor R19 is used to widen
the 3dB bandwidth of the 2nd FM IF amplifier.
The gain at 10.7MHz is fixed by the AC impedance of
the primary side of T3 and the current in Q5. The
current is fixed by R16, R17 and R18. Capacitors
C18 and C17 bypass the AC signal to ground. C20
is a bypass capacitor from V+ to ground.
.707
10.775MHz
10.625MHz
10.7MHz
Figure 33
-36-
ASSEMBLY INSTRUCTIONS
R17 - 47kW Resistor
(yellow-violet-orange-gold)
R19 - 10kW Resistor
(brown-black-orange-gold)
Q5 - 2N3904 Transistor
(see Figure C)
TP10 - Test Point Pin
(see Figure A)
C20 - .01mF Discap (103)
T2 - FM IF Coil
(Green Dot)
C18 - .01mF Discap (103)
C17 - .01mF Discap (103)
R16 - 10kW Resistor
(brown-black-orange-gold)
R18 - 1kW Resistor
(brown-black-red-gold)
STATIC TESTS
Q5 BIAS
OFF. If you do not get this reading, check R17, R16,
R18, Q5 and T2.
Connect your VOM to the circuit as shown in Figure 34.
Turn the power ON. The voltage at the base of Q5
should be approximately 1.4 volts. Turn the power
V
COM V
TP15
Figure 34
If you don’t have an RF generator and oscilloscope, skip to Section 8.
-37-
.001mF
GENERATOR
Hz
TP15
TP15
Figure 35
AC GAIN
BANDWIDTH
Connect the RF generator and oscilloscope to the
circuit as shown in Figure 35. The scope probe must
have an input capacitance of 12pF or less otherwise
the probe will detune T3 resulting in a false reading
of the AC gain. Set the generator at 10.7MHz no
modulation and minimum voltage output. Set the
scope to read 50mV per division and turn the power
ON. Slowly increase the generator until 150mVpp or
3 divisions are seen on the scope. With an alignment
tool or screwdriver adjust T3 for a peak. Reduce the
generator until 150mVpp or 3 divisions are seen on
the scope. With an alignment tool or screwdriver
adjust T3 for a peak. Reduce the generator input to
maintain 3 divisions on the scope. Move the probe to
the base of Q5 and record the input voltage here:
With the power turned OFF, connect your test
equipment as shown in Figure 35. Set your
generator at 10.7MHz no modulation and minimum
voltage output. Set the scope to read 50mV per
division. Turn the power ON and slowly adjust the
generator amplitude until 150mVpp is seen on the
scope. Realign T3, if necessary, for maximum output
while adjusting the generator to maintain 150mVpp.
Slowly decrease the frequency of the generator until
the voltage drops .707 of its original value, 2.1
divisions or 106mVpp. Record the frequency of the
lower 3dB drop-off point here:
Fl = _________MHz.
Increase the frequency until the voltage drops to .707
of its original value, 2.1 divisions or 106mVpp.
Record the frequency of the high frequency 3dB
drop-off point here:
Vb = __________ mVpp.
Turn the power OFF. The AC gain can be calculated
as follows:
Fh = ___________MHz.
AC Gain = 150mV / Vb
Your calculated answer should be about 20.
The bandwidth of the 2nd IF can be calculated as
follows:
Record your calculation:
Bandwidth = Fh - Fl
AC Gain = __________
Your results should be between 300 - 500kHz.
Record your calculation:
Bandwidth = __________
-38-
SECTION 8
FIRST FM IF AMPLIFIER
The operation of the first IF amplifier is the same as
the second IF amplifier except that the gain is
different. The gain is set by the AC impedance of the
primary side of T2 and the current in Q4. The current
in Q4 is set by the resistors R12, R13 and R15.
Capacitors C14 and C15 bypass the AC signal to
ground. C13 and C16 are bypass capacitors from V+
to ground to prevent feedback on the V+ line. R19 is
used to widen the bandwidth of the transformer T2.
ASSEMBLY INSTRUCTIONS
C13 - .01mF Discap (103)
R13 - 47kW Resistor
(yellow-violet-orange-gold)
C14 - .01mF Discap (103)
R14 - 10kW Resistor
(brown-black-orange-gold)
T1 - FM Mixer Coil
(Orange Dot)
TP11 - Test Point Pin
(see Figure A)
R12 - 10kW Resistor
(brown-black-orange-gold)
Q4 - 2N3904 Transistor
(see Figure C)
R15 - 2.2kW Resistor
(red-red-red-gold)
C16 - .01mF Discap (103)
C15 - .01mF Discap (103)
STATIC TESTS
Q4 BIAS
Connect your VOM as shown in Figure 36. Turn the
power ON. The voltage at the base of Q4 should
approximately be 1.4 volts. If you do not get this
reading, check R12, R13, R15, Q4 and T1.
V
COM V
TP15
Figure 36
-39-
If you don’t have an RF generator and oscilloscope, skip to Section 9.
.001mF
GENERATOR
TP15
Hz
TP15
Figure 37
BANDWIDTH
Connect the RF generator and oscilloscope and
oscilloscope to the circuit as shown in Figure 37. The
scope probe must have an input capacitance of 12pF
or less otherwise the probe will detune T2 causing an
incorrect measurement of AC gain. Set the
generator at 10.7MHz no modulation and minimum
voltage output. Set the scope to read 20mV per
division and turn the power ON. Slowly increase the
amplitude of the generator until 3 divisions or
60mVpp are seen on the scope. With an alignment
tool or screwdriver, adjust T2 for a peak. Reduce the
generator input to maintain 3 divisions on the scope.
Move the scope probe to the base of Q4 and record
the input voltage here:
Connect your test equipment as shown in Figure 37.
Set your generator at 10.7MHz no modulation and
minimum voltage output. Set the scope to read
20mV per division. Turn the power ON and slowly
increase the amplitude of the generator until 60mVpp
is seen on the scope. Increase the frequency of the
generator until the voltage drops .707 of its original
value, 2.1 divisions or 42mVpp.
Record the frequency of the high 3dB drop-off point
here:
Fh = ___________MHz.
Decrease the frequency of the generator until the
voltage drops to .707 of its original value, 2.1
divisions or 42mVpp. Record the frequency of the
low 3dB drop-off point here:
Vb = __________mVpp.
Turn the power OFF. The AC gain can be calculated
as follows:
Fl = ___________MHz.
AC Gain = 60mV / Vb
The bandwidth of the first IF can be calculated as
follows:
Your calculated answer should be about 10.
Record your calculation:
Bandwidth = Fh - Fl
Your calculated answer should be between 300 500kHz.
AC Gain = __________
Record your calculation:
Bandwidth = __________kHz.
-40-
SECTION 9
FM RF AMPLIFIER, MIXER, OSCILLATOR, AND AFC STAGE
In a superheterodyne receiver, the radio waves are
emitted and then mixed with the local oscillator to
produce the intermediate frequency (IF). The first
stage is the RF amplifier which selects a radio station
and amplifies it. The second stage is the local
oscillator which oscillates at a frequency 10.7MHz
above the desired radio station frequency. The third
stage is the mixer stage where the amplified radio
waves are heterodyned with the local oscillator.
During the mixing process, a difference frequency of
10.7MHz is produced. This difference frequency is
used as the IF in FM radios. The collector of
transistor Q3 contains an IF transformer (T1) which
is tuned only to the difference frequency. This
transformer rejects all frequencies except those near
10.7MHz. T1 also couples the 10.7MHz signal to the
first FM IF amplifier. The RF amplifier and the
oscillator are the only two resonant circuits that
change when the radio is tuned for different stations.
Since a radio station may exist 10.7MHz above the
oscillator frequency, it is important that the RF stage
rejects this station and selects only the station
10.7MHz below the oscillator frequency.
The frequency of the undesired station 10.7MHz
above the oscillator is called the image frequency.
Since this FM receiver has an RF amplifier, the
image frequency is reduced significantly. The
resistor R9 and capacitor C12 decouple the voltage
of the tuner from the voltage of the IF stages.
MIXER ASSEMBLY INSTRUCTIONS
R10 - 10kW Resistor
(brown-black-orange-gold)
R9 - 100W Resistor
(brown-black-brown-gold)
C12 - .005mF Discap (502)
TP12 - Test Point Pin
(see Figure A)
CAUTION: Test Point must
not touch can of T1 FM Mixer
Coil.
R8 - 22kW Resistor
(red-red-orange-gold)
R7 - 6.8kW Resistor
(blue-gray-red-gold)
Q3 - 2N3904 Transistor
(see Figure C)
TP13 - Test Point Pin
(see Figure A)
R11 - 1.8kW Resistor
(brown-gray-red-gold)
-41-
STATIC MEASUREMENTS
Q3 BIAS
V
COM
V
TP15
Figure 38
With the power turned OFF, connect your VOM to the
circuit as shown in Figure 38. Set your VOM to read
9 volts DC and turn the power ON. The DC voltage
at the base of Q3 should be approximately 1.8 volts.
If your answer varies by more than 2 volts, turn the
power OFF and check components R7, R8, R11 and
Q3.
generator until 4 divisions or 40mVpp are seen on
the scope. With an alignment tool or a screwdriver,
adjust T1 for peak. Reduce the generator amplitude
to maintain 4 divisions on the scope. Move the scope
probe to the base of Q3 and record the input voltage
here:
Vb = __________mVpp.
If you don’t have an RF generator and oscilloscope,
skip to the FM Oscillator Assembly Procedure.
Turn the power OFF. The gain can be calculated as
follows:
AC GAIN
AC Gain = 40mV / Vb.
The AC gain of the mixer is set by the impedance of
the primary side of T1 and by the current flowing in
Q3. The current in Q3 is set by the resistors R7, R8
and R11. Connect your test equipment to the circuit
as shown in Figure 39. Your scope probe must have
an input capacitance of 12pF or less, otherwise the
probe will detune T1 resulting in an incorrect
measurement. Set your scope to read 10mV per
division. Set your RF generator at 10.7MHz no
modulation minimum voltage output. Turn the power
ON and slowly increase the amplitude of the
Your calculated answer should be about 3.
Record your calculation:
AC Gain = __________
Because the signal from the oscillator is injected at
the emitter of Q3, the emitter resistor is not bypassed
to ground. This is why the gain of the mixer is low
compared to the other IF stages.
-42-
.001mF
GENERATOR
TP15
Hz
TP15
Figure 39
BANDWIDTH TEST
Turn the power OFF.
calculated as follows:
Connect your test equipment to the circuit as
shown in Figure 39. Set your generator at 10.7MHz
no modulation and minimum voltage output. Set
the scope for 10mV per division. Turn the power
ON and slowly increase the amplitude of the
generator until 40mVpp are seen on the scope.
Increase the frequency until the voltage drops .707
of its original value, 2.8 divisions or 28mVpp.
Record the frequency of the generator until the
voltage drops .707 of its original value, 2.8 divisions
or 28mVpp. Record the frequency of the low 3dB
drop-off point here:
The bandwidth can be
Bandwidth = Fh - Fl
Your calculated answer should be between 300 500kHz.
Record your calculation:
Bandwidth = _________kHz.
Fl = _________MHz.
FM OSCILLATOR ASSEMBLY INSTRUCTIONS
C7 - .001mF Discap (102)
L3 - FM Oscillator Coil
(5 Turns)
R4 - 33kW Resistor
(orange-orange-orange-gold)
C9 - 15pF Discap (15)
C10 - 30pF Discap (30)
Q2 - 2N3904 Transistor
(see Figure C)
C11 - 220pF Discap (221)
R5 - 33kW Resistor
(orange-orange-orange-gold)
R6 - 2.2kW Resistor
(red-red-red-gold)
C8 - .005mF Discap (502)
-43-
STATIC MEASUREMENTS
Q2 BIAS
volts. Turn the power OFF. If you do not get this
measurement, check R4, R5 and Q2.
Connect your VOM to the circuit as shown in Figure 40.
Set your VOM to read 9 volts and turn the power ON.
The voltage at the base of Q2 should be about 4
V
COM V
TP15
Figure 40
AFC
diode called a varactor. A varactor will change its
internal capacitance when a voltage is applied. The
ratio detector diodes are positioned in such a way
that when the 10.7MHz center frequency increases,
the DC correction voltage will decrease. Likewise,
when the 10.7MHz center frequency decreases, the
DC correction voltage will increase. This voltage
change causes the capacitance of the varactor to
change. The varactor is connected at the emitter of
Q2, so any capacitance change in the varactor is
seen at the emitter of the oscillator. A change in
capacitance at the emitter of Q2 will change the
frequency of oscillation of the local oscillator.
When a radio is tuned to a station, it would be
desirable for the radio to “lock” in on the station. Due
to changes in temperature, voltage and other effects,
the local oscillator may change its frequency of
oscillation. If this occurs, the center frequency of
10.7MHz will not be maintained.
Automatic
Frequency Control (AFC) is used to maintain the
10.7MHz center frequency. When the local oscillator
drifts, the ratio detector will produce a DC
“correction” voltage. The audio signal rides on this
DC correction voltage. This signal is fed to a filter
network which removes the audio so that a pure DC
voltage is produced. This voltage is fed to a special
AFC ASSEMBLY INSTRUCTIONS
R30 - 390kW Resistor
(orange-white-yellow-gold)
R29 - 390kW Resistor
(orange-white-yellow-gold)
D1 - Varactor Diode
(see Figure D)
C26 - .1mF Discap (104)
C27 - .001mF Discap (102)
-44-
If you don’t have an RF generator, skip to the RF Amplifier Assembly Procedure.
Generator
.001mF
V
Hz
COM
V
TP15
TP15
Figure 41
Connect the RF generator and VOM to the circuit as
shown in Figure 41. Set your VOM to read 9 volts
DC. Set your generator at 10.7MHz no modulation
and moderate signal strength output. Turn the power
ON. Record the voltage of D1 here:
Increase the frequency of the generator until the
voltage is equal to V(D1). While watching your VOM,
increase the frequency of your generator. As the
frequency increases, the voltage at D1 should
decrease. This correction voltage is what keeps the
oscillator from drifting. If the voltage at D1 still does
not change at D1, check D1, R29, R30, C26 and
C27. If these parts are inserted correctly and the
voltage at D1 still doesn’t change, then increase the
amplitude of your generator and repeat the same
steps again. Turn the power OFF.
V(D1) = ________.
While watching your VOM, slowly decrease the
frequency of your generator. As the frequency
decreases, the voltage at D1 should increase.
RF AMPLIFIER ASSEMBLY INSTRUCTIONS
R1 - 22kW Resistor
(red-red-orange-gold)
L1 - FM RF Amp Coil
(6 Turns) see Figure N
C3 - .001mF Discap (102)
L2 - FM RF Amp Coil
(2 Turns) see Figure N
C2 - .005mF Discap (502)
C5 - 470pF Discap (471)
Q1 - 2N3904 Transistor
(see Figure C)
C6 - 33pF Discap (33)
R2 - 6.8kW Resistor
(blue-gray-red-gold)
R3 - 470W Resistor
(yellow-violet-brown-gold)
C4 - 470pF Discap (471)
TP14 - Test Point Pin
(see Figure A)
-45-
Figure N
STATIC MEASUREMENTS
Q1 BIAS
Connect your VOM to the circuit as shown in
Figure 42. Set your VOM to read 9 volts and
turn the power ON. The voltage at the base
of Q1 should be about 1.6 volts. If you do
not get this reading, check R1, R2, R3 and
Q1. Turn the power OFF.
V
COM
V
TP15
Figure 42
ANTENNA FM ASSEMBLY
Antenna FM
2 Antenna Screws M2
3.5” Wire #22 Insulated
(extra wire in AM Section)
(see Figure O)
Threaded Holes
Mount the antenna to the PC board with two screws
as shown. NOTE: Some antennas have only one
threaded hole.
Cut a 2 1/4” wire and strip 1/4” of insulation off of both
ends of the remaining jumper wire. There are no
holes for the wire in this location, so tack solder the
wire to the pads as shown.
Solder
Jumper Wire
Figure O
FM FINAL ALIGNMENTS
a resonant circuit is changed, the resonant frequency
is changed also.
When aligning the oscillator, changing the resonant
frequency changes the frequency of oscillation.
Likewise, when aligning the RF amp, changing the
resonant frequency at which it was selective.
There are two procedures for the final alignment
steps. The first alignment procedure is for those who
do not have test equipment and the second is for
those who do have test equipment.
Your “magic wand” will be used to align the FM
oscillator circuit and the FM RF amplifier. When the
brass end of your “magic wand” is placed near the
FM oscillator coil L3, the coil reacts as if inductance
has been removed. Likewise, when the iron end of
the “magic wand” is placed near the coil L3, it reacts
as if inductance has been added. The same is true
for the RF coils L1 and L2. When the inductance of
When aligning the oscillator and RF circuits, coils L1
and L3 will be adjusted at the lower end of the band,
while the oscillator and RF trimmer capacitors are
adjusted at the higher end of the band. This is done
so that the RF amp tracks the oscillator properly.
-46-
ALIGNMENT WITH NO TEST EQUIPMENT
With an alignment tool or screwdriver turn coils T1,
T2 and T3 fully counter-clockwise. DO NOT FORCE
THE COILS ANY FURTHER. Turn each coil in about
1 1/4 to 1 1/2 turns.
IF ALIGNMENT
V
COM
V
TP15
Figure 43
With an alignment tool or screwdriver turn coils T1,
T2 and T3 fully counter-clockwise. DO NOT FORCE
THE COILS ANY FURTHER. Turn each coil in about
1 1/4 to 1 1/2 turns.
Use the earphone provided for best results. Switch
to the FM position. Connect your VOM to the circuit
as shown in Figure 43. Turn the radio ON and tune
the radio to a weak station. It is best to keep the
volume at a low level. Adjust T1 for the minimum
voltage on your VOM. Reduce the volume if
necessary. Adjust T2 for minimum voltage on your
VOM and reduce the volume control if necessary.
Adjust T3 for minimum voltage on your VOM and
reduce the volume control if necessary. As you
adjust the coils you should hear less distortion and
noise. Repeat this procedure until the FM IF gain is
optimized. This process peaked the FM IF amplifier
to their maximum gain.
If the station is heard, this means that L3 needs less
inductance. Carefully pull apart L3 until the station is
heard. Place the iron end near L3. If the station is
heard, this means that L3 needs more inductance.
Carefully press together L3 until the station is heard.
Pulling apart or pressing together L3 just a small
amount will have a great effect on the coils resonant
frequency. Repeat this step until the pointer is
aligned to the station’s frequency. Tune the radio to
a station around 106MHz. Once a station is found
and its broadcast frequency is known, rotate the dial
until the white pointer is aligned with that station’s
frequency on the dial. Place the brass end of the
“magic wand” near L3. If the station is heard, it
means that L3 needs more capacitance. Carefully
adjust the FM oscillator trimmer (as shown in Figure L,
page 23), on the back of the gang until the station
“Magic Wand”
DETECTOR ALIGNMENT
Adjust T4 for minimum voltage on your VOM. Adjust
T5 for minimum distortion. Repeat these 2 steps
until the ratio detector alignment is optimized.
RF Coil
OSCILLATOR ALIGNMENT
Tune the radio to a known station around 90MHz.
Once a station is found and its broadcast frequency
is known, rotate the dial until the white pointer is
aligned with that stations frequency on the dial.
Using the “magic wand”, place the brass end near
coil L3. Refer to Figure 44.
Spread apart the coil for less inductance
Press the coil together for more inductance
Figure 44
-47-
is heard. Place the iron end of the “magic wand” near
L3. If the station is heard, it means that L3 needs
less capacitance. Carefully adjust the FM oscillator
trimmer located on the back of the gang until the
station is heard. Repeat this step until the pointer is
aligned to the station’s frequency. Adjusting both the
oscillator coil L3 and the oscillator trimmer capacitor
will effect the oscillator’s frequency, so it is advisable
to repeat this procedure until the FM oscillator
alignment is optimized. This process sets the FM
oscillator range at 98.7MHz to 118.7MHz.
This concludes the alignment of the FM radio section.
If no stations are heard, verify that FM signals are
present in your location by listening to another FM
radio placed near the superhet 108. If the FM section
is still not receiving go back and check each stage for
incorrect values and for poor soldering.
Spacer
L1
L2
Approx.
1/16” gap
Approx.
1/16” gap
Top View
RF ALIGNMENT
Press together L1 and L2. Spread apart coil L1 so
that it resembles Figure 45. The gaps or spaces
should be between 1/32” and 1/16” wide. This
procedure sets the tracking of the RF section. Use the
special coil spacer provided to gap the coil as shown.
Carefully slide the coil spacer between the coils to get
the spacing shown in Figure 45.
L1
L2
Figure 45
ALIGNMENT WITH RF GENERATOR AND OSCILLOSCOPE
IF ALIGNMENT
Switch to the FM section. Connect your RF
generator and oscilloscope to the circuit as shown in
Figure 46. Set your RF generator at 10.7MHz
modulated at 1kHz deviation with minimum voltage
Generator
output. Set the scope to read 50mV per division.
With a clip lead, short the base emitter junction of
Q2. This short “kills” the local oscillator.
Short the base of Q2 to the
emitter (as shown below).
Hz
TP15
TP15
Figure 46
Turn the power ON. Slowly increase the amplitude of
the generator until a 1kHz signal is seen on the
scope. Keep the generator at a low level of output to
prevent the IF sections from limiting. With an
alignment tool or screwdriver, adjust T1 for a peak on
the scope. Reduce the amplitude of the input signal
if necessary. Adjust T2 for a peak and reduce the
amplitude of the input signal if necessary. Repeat
these steps until the IF alignment is optimized. This
procedure aligns the FM IF amplifiers to 10.7MHz.
-48-
OSCILLATOR ALIGNMENT
RF ALIGNMENT
Remove the clip lead and set your generator at
88MHz modulator at 1kHz, 22.5kHz deviation and
minimum voltage output. Tune the radio until a 1kHz
signal is seen on the scope. It may be necessary to
increase the amplitude of the generator. Rotate the
dial until the white pointer is aligned to 88MHz.
Using the “magic wand” place the brass end near L3
as shown in Figure 44. If the signal seen on the
scope increases, this means L3 needs less
inductance. To remove inductance, carefully spread
apart coil L3. Pulling apart or pressing together coil
L3, a small amount will have a great effect on the
coil’s resonant frequency. Place the iron end of the
“magic wand” near L3. If the signal seen on the
scope increases, it means L3 needs more
inductance. To add inductance carefully press
together coil L3. Repeat these steps until the signal
decreases for both ends of the “magic wand”.
Increase the frequency of your generator to 108MHz.
Tune the radio until a 1kHz signal is seen on the
scope. Rotate the dial until the white pointer is
aligned to 108MHz. Place the brass end of your
“magic wand” near L3. If the signal on the scope
increases, it means that L3 needs more capacitance.
Adjust the FM oscillator trimmer on the gang (as
shown in Figure L on page 23) until the 1kHz signal
is at a peak. Place the iron end of the “magic wand”
near L3. If the signal increases, it means that coil L3
needs less capacitance. Adjust the FM oscillator
trimmer on the gang until the 1kHz signal is at a
peak. Repeat these 2 steps until the signal
decreases for both ends of the “magic wand”. Since
adjusting both the oscillator coil L3 and the oscillator
trimmer will effect the frequency of oscillation, it is
advisable to repeat this procedure until the oscillator
alignment is optimized. This process sets the FM
oscillator range at 98.7MHz to 118.7MHz.
Set your generator at a frequency around 90MHz
modulated at 1kHz, 22.5kHz deviation and minimum
voltage out. Tune your radio until a 1kHz tone is
heard. Place the brass end of your “magic wand”
near RF coil L1. If the signal on the scope increases,
it means that coil L1 needs less inductance.
Carefully spread apart the coil L1 to reduce its
inductance. Place the iron end of the wand near L1.
If the signal increases, it means that coil L1 needs
more inductance. Carefully press together the coil
L1 to increase its inductance. Repeat these steps
until the signal on the scope decreases for both ends
of the “magic wand”. Increase your generator to a
frequency near 106MHz. Tune your radio until a
1kHz tone is heard. Place the brass end of your
“magic wand” near L1. If the signal increases, it
means that the coil L1 needs more capacitance.
With an alignment tool or screwdriver, adjust the FM
antenna trimmer (see Figure L on page 23). If the
signal increases, this means coil L1 needs less
capacitance. Carefully adjust the FM antenna
trimmer until a peak is seen on the scope. Repeat
these steps until the signal on the scope decreases
for both ends of the “magic wand”. Since adjusting
both the RF coil L1 and the antenna trimmer will
effect the gain of th RF stage, it is advisable to repeat
this procedure until the RF amplifier alignment is
optimized. This process sets the RF stage to “track”
the FM oscillator stage.
This concludes the alignment of the FM radio
section. If no stations are heard, verify that FM
signals are present in your location by listening to
another FM radio near the Superhet 108. If the FM
section is still not receiving, go back and check each
stage for incorrect values and for poor soldering.
FM RADIO HIGHLIGHTS
1. The FM broadcast band covers the frequency
range from 88MHz to 108MHz.
5. The number of times the carrier frequency
changes in a period of time is exactly equal to the
audio frequency.
2. FM signals are usually limited to line a sight.
6. The change in frequency is called the deviation
and is limited to 75kHz for monaural FM.
3. Audio signals up to 15kHz are transmitted on the
FM carrier.
7. The bandwidth assigned for FM is 200kHz.
4. The amount that the RF carrier changes frequency
is determined by the amplitude of the modulating
signal.
-49-
DC VOLTAGES
The voltage readings below should be used in troubleshooting the FM section. (Switch at FM position.)
Q1
B
E
C
1.6
.9
7.0
Q4
B
E
C
1.3
.7
7.5
Q2
B
E
C
3.3
3.0
7.1
Q5
B
E
C
1.3
.6
7.5
Q3
B
E
C
1.6
1.3
7.0
Q6
B
E
C
1.2
.6
6.6
Test Conditions
1. Volume set to minimum.
2. Connect TP14 to TP15 with a jumper
wire.
3. Battery voltage = 9V
4. All voltages are referenced to circuit
common.
5. Voltage readings can vary +10%
SPECIFICATIONS
Audio:
Frequency response 3dB drop into 8 ohm resistive load.
Low end 800Hz - high end 120kHz
Maximum power out at 10% total harmonic distortion.
500 MilliWatts
Typical audio gain at 1000Hz: 150 times
Typical % distortion at 100 milliwatts output <2%.
AM Radio Specifications:
Tuning range - 520kHz to 1620kHz
IF frequency 455kHz
Tracking = +3dB from 700kHz to 1400kHz
10dB signal to noise at 200 microvolts typical
FM Radio Specifications:
Tuning range = 88MHz to 108MHz
IF frequency 10.7MHz
Tracking +5dB from 90MHz to 106MHz
10dB signal to noise at 12 microvolts typical
Uses ratio detector and full time auto frequency control.
-50-
QUIZ - FM SECTION
INSTRUCTIONS: Complete the following examination, check your answers carefully.
1. The FM broadcast band is . . .
A) 550 - 1,600kHz.
B) 10.7MHz.
C) 88 - 108MHz.
D) 98.7 - 118.7MHz.
6. The ratio detector is used because ...
A) it is sensitive to noise.
B) it is insensitive to noise.
C) it provides amplification.
D) it doesn’t need a filter.
2. The maximum audio frequency used for FM is ...
A) 7.5kHz.
B) 15kHz.
C) 20kHz.
D) 10.7MHz.
7. The device most often used for changing the
local oscillator frequency with the AFC voltage is
a ...
A) feedthrough capacitor.
B) variable inductor.
C) varactor.
D) trimmer capacitor.
3. The frequency of the modulating signal
determines the . . .
A) number of times the frequency of the
carrier changes per second.
B) maximum deviation of the FM carrier.
C) maximum frequency swing of the FM
carrier.
D) amount of amplitude change of the FM
carrier.
8. The capacitance of a varactor is determined by ...
A) the voltage level.
B) the amount of current in the circuit.
C) the signal strength of the RF carrier.
D) the amount of resistance in the circuit.
9. Limiting in FM receivers is the process of ...
A) removing interfering FM stations.
B) providing greater station selectivity.
C) separating the FM stations from the AM
stations.
D) removing noise from the FM carrier.
4. The AFC circuit is used to ...
A) automatically hold the local oscillator on
frequency.
B) maintain constant gain in the receiver to
prevent such things as fading.
C) prevent amplitude variations of the FM
carrier.
D) automatically control the audio
frequencies in the receiver.
10. A detector circuit that does not require a limiter is
a ...
A) slope detector.
B) ratio detector.
C) Travis detector.
D) Foster-Seeley detector.
5. The ratio detector transformer is tuned to ...
A) 10.7MHz.
B) 88MHz.
C) 455kHz.
D) 10.9MHz.
Answers: 1. C, 2. B, 3. A, 4. A, 5. A, 6. B, 7. C, 8. A, 9. D, 10. B
-51-
AM/FM-108 Radio Baffle
NOTICE: Keep the box the kit came in. After you
have completed the radio and it operates
satisfactorily, you may want to install a
baffle to improve the sound.
it creates positive pressure on the air in front of it and
negative pressure on the rear. At low frequencies,
out of phase front and rear waves mix causing partial
or total cancellation of the sound wave. The end
result is a speaker less efficient and distorted.
The final step in the radio kit will be to assemble and
attach a baffle to the speaker. You will need to
remove the baffle located in the bottom of the box. If
it does not want to come out easily, use a knife to cut
the holding tabs.
To eliminate the low frequency cancellation, a
speaker is placed inside an enclosure. Now the front
sound wave are prevented from traveling to the back.
The speaker will now compress and decompress air
inside, increasing its resonant frequency and Q
relative to the free air values. This type of effectively
air-tight box is called an Acoustic Suspension.
When a speaker is not enclosed, sound waves can
travel in all directions. As a speaker moves outward,
2 Screws 2-56 x 1/4”
2 Nuts 2-56
Baffle
AM/FM-108K Kit Carton
1. Start at one edge and carefully remove the
baffle from the bottom the kit box.
2. Bend the four flaps upward as shown.
brown
side
-52-
3. Bend the top side upward as shown.
4. Bend the two sides upward. Attach the
three sides using scotch tape or glue
(Elmer’s, Duco Cement, or other).
5. Bend the bottom side upward and attach it
to the other sides using scotch tape or glue.
Bend the two mounting flaps down.
Side View
6. Place the baffle over the speaker. Align the
mounting holes and attach it using the two
screws and two nuts supplied. The screws
should go through the X mark on the baffle flap.
PC board
Screw
Baffle
Glue
Optional: To make an air tight seal, place a bead of
glue between the PC board and the baffle edges.
Screw
Nut
Back View
Nut
-53-
SCHEMATIC DIAGRAM - AM/FM RADIO
-54-
TM
Elenco Electronics, Inc.
150 W. Carpenter Avenue
Wheeling, IL 60090
(847) 541-3800
http://www.elenco.com
e-mail: elenco@elenco.com
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