Elenco Electronics AM/FM-108K Assembly And Instruction Manual

AM/FM RADIO KIT

MODEL AM/FM-108K

INTEGRAL CIRCUIT, 9 TRANSISTORS, 4 DIODES

Assembly and Instruction Manual

Elenco®Electronics, Inc.

ight © 2003, 1989 b

yr

Cop

t of this book shall be reproduced b

No par

y Elenco

®

Electronics

y means;

y an

, Inc.

electronic

ights reser

All r

, photocopying, or otherwise without written permission from the publisher.

ved. Revised 2003 REV-X 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

eparate parts list will be shown for the FM radio after you have completed the AM radio.

s

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
mail 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.

RESISTORS

Qty. Symbol Value Color Code Part #

1 R45 10Ω 5% 1/4W brown-black-black-gold 121000
1 R44 47Ω 5% 1/4W yellow-violet-black-gold 124700
2 R38, 43 100Ω 5% 1/4W brown-black-brown-gold 131000
1 R41 470Ω 5% 1/4W yellow-violet-brown-gold 134700
1 R37 1kΩ 5% 1/4W brown-black-red-gold 141000
1 R42 2.2kΩ 5% 1/4W red-red-red-gold 142200
2 R33, 36 3.3kΩ 5% 1/4W orange-orange-red-gold 143300
1 R40 10kΩ 5% 1/4W brown-black-orange-gold 151000
1 R32 12kΩ 5% 1/4W brown-red-orange-gold 151200
1 R35 27kΩ 5% 1/4W red-violet-orange-gold 152700
1 R39 39kΩ 5% 1/4W orange-white-orange-gold 153900
1 R31 56kΩ 5% 1/4W green-blue-orange-gold 155600
1 R34 1MΩ 5% 1/4W brown-black-green-gold 171000
1 Volume/S2 50k

Ω / SW Pot/SW with nut and washer 192522

®

Electronics (address/phone/e-

CAPACITORS

Qty. Symbol Value Description Part #

1 C1 Variable Tuning Gang AM/FM 299904
1 C30 150pF Discap (151) 221510
2 C31, 38 .01µF Discap (103) 241031
5 C29, 33, 35, 36, 37 .02µF or .022µF Discap (203) or (223) 242010
1 C44 .047µF Discap (473) 244780
1 C28 .1µF Discap (104) 251010
4 C32, 40, 41, 42 10µF Electrolytic Radial (Lytic) 271045
1 C34 100µF Electrolytic Radial (Lytic) 281044
2 C39, 43 470µF Electrolytic Radial (Lytic) 284744

SEMICONDUCTORS

Qty. Symbol Value Description Part #

1 D4 1N4148 Diode 314148
3 Q7, 8, 9 2N3904 Transistor NPN 323904
1 U1 LM-386 Integrated Circuit 330386

COILS MAGIC WAND

. Symbol Color Description Part # Qty. Description Part #

Qty

L5 Red AM Oscillator 430057 1 Iron Core 461000

1

T6 Yellow AM IF 430260 1 Brass Core 661150

1

T7 White AM IF 430262 4” Shrink Tubing 890120

1

T8 Black AM IF 430264

1

L4 AM Antenna with Holders 484004

1

MISCELLANEOUS

Qty. Description Part # Qty. Description Part #

1 PC Board 517055
1 Switch 541023
1 Battery Holder 590096
1 Speaker 590102
1 Speaker Pad 780128
1 Knob (pot) 622017
1 Knob (dial) 622030
1 Earphone Jack with Nut 622130 or 622131
1 Radio Stand 626100
1 Earphone 629250

3 Screw 2-56 x 1/4” 641230
3 Screw #3 641310
3 Nut 2-56 644201
1 Plastic Washer 645108
1 Socket 8-Pin 664008
8 Test Point Pin 665008
1 Label AM/FM 723508
1 Manual 753508
8” Wire 22 insulated 814520
1 Solder 9ST4

T THIS KIT CAME IN.

THE BOX

**** SA

VE

THA

-1-

WILL BE USED ON PAGES 23 & 52. ****

IT

IDENTIFYING RESISTOR VALUES

U

se the following information as a guide in properly identifying the value of resistors.

BAND 1

1st Digit

Color Digit
Black 0
Brown 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9

BAND 2

2nd Digit

Color Digit
Black 0
Brown 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9

2 Multiplier Tolerance

1

Multiplier

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

BANDS

Resistance

Tolerance

Color Tolerance
Silver +10%
Gold +
Brown +
Red +
Orange +
Green +.5%
Blue +
Violet +.1%

5%
1%
2%
3%

.25%

IDENTIFYING CAPACITOR VALUES

Capacitors will be identified by their capacitance value in pF (picofarads), nF (nanofarads), or µF (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

Note:
to signify a decimal point; as in 3R3 = 3.3

The letter “R” may be used at times

alue is 10 x 1,000 = 10,000pF or .01

The v

For the No.01234589
Multiply By

103K

1 10 100 1k 10k 100k .01 0.1

First Digit
Second Digit
Multiplier

The letter M indicates a tolerance of +20%
The letter K indicates a tolerance of +10%
The letter J indicates a tolerance of +5%

orking Voltage

W

100

Tolerance

um

Maxim

µF 100V

-2-

INTRODUCTION

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.

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

Section 8

Section 7 Section 6

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.

FM RF

AMPLIFIER

FM

OSCILLATOR

AM MIXER

OSCILLATOR

FM MIXER

1ST FM IF

AMPLIFIER

AFC

1ST AM IF

AMPLIFIER

AM

Section 3 Section 2

AM RADIO

The purpose of Section 1, the Audio Amplifier Stage, is to
increase the po
detector to a power level capable of driving the speaker.
Section 2 includes the AM detector circuit and the AGC
(automatic gain control) stage
amplitude modulated IF (intermediate frequency) signal to a
low level audio signal. The AGC stage feeds back a DC

oltage to the first AM IF amplifier in order to maintain a near

v
constant level of audio at the detector. Section 3 is the
second AM IF amplifier. The second AM IF amplifier is tuned
to 455kHz (Kiloher
of 50. Section 4 is the first AM IF 2 amplifier which has a
variable gain that depends on the AGC voltage received from

GC stage. The first AM IF amplifier is also tuned to

the A
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 v
coil. This voltage is coupled to the mixer, or converter, stage
to be changed to a frequency of 455kHz. This change is
accomplished b
signal with the oscillator signal. Section 6 is the FM ratio

wer of the audio signal received from either

The AM detector converts the

.

tz) and has a fix

ed gain at this frequency

oltage across the antenna

y mixing (heterodyning) the radio frequency

2ND FM IF

AMPLIFIER

2ND AM IF

AMPLIFIER

Figure 1

FM

DETECTOR

Speaker

AUDIO

AMPLIFIER

AM

DETECTOR

AGC

Section 1Section 5 Section 4

detector circuit. The FM ratio detector has a fixed gain of
about 20.
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 appro
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.
mixer, FM oscillator, FM RF (Radio Frequency) amplifier, AFC
(Automatic Frequency Control) stage, and the FM antenna.
The incoming r
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 mix
changed to a frequency of 10.7MHz. This change, as in AM,
is accomplished by heterodyning the radio frequency signal
with the oscillator signal.
voltage to the FM oscillator to prevent the oscillator from
drifting. Each of these blocks will be explained in detail in the
Theor
for that stage.

Section 7 is the second FM IF amplifier. The

ximately 350kHz.

Section 8 is the

Section 9 includes the FM

adio w

aves are amplified by the FM RF

er stage to be

The AFC stage feeds back a DC

y of Operation given before the assembly instructions

-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.

Use only rosin core solder of 63/37 alloy.

Foil Side

DO NOT USE ACID CORE SOLDER!

What Good Soldering Looks Like

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.

2. 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.

3. Allow the solder to flow
around the connection.
Then, remove the solder
and the iron and let the
connection cool.
solder should have flowed
smoothly and not lump
around the wire lead.

4.

Here is what a good solder
connection looks like.

The

Component Lead

Foil

Solder

Foil

Solder

Foil

Soldering Iron

Circuit Board

Soldering Iron

Soldering Iron

Mount Part

Bend Leads to

Hold Part

Solder and

Cut Off Leads

Types of Poor Soldering Connections

1. Insufficient heat - the

solder will not flow onto the
lead as shown.

2. Insufficient solder - let the

solder flow over the
connection until it is
covered. Use just enough
solder to cover the
connection.

3. Excessive solder - could

make connections that you
did not intend to between
adjacent foil areas or

.

minals

ter

4. Solder bridges - occur

een

ing

ag y

uns betw

our solder

when solder r
circuit paths and creates a
short circuit. This is usually
caused by using too much
solder. To correct this,
simply dr
iron across the solder
bridge as shown.

Rosin

Soldering iron positioned
incorrectly.

Solder

Component Lead

Solder

Soldering Iron

Foil

Dr

Gap

ag

-4-

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 1MΩ
to infinity.

Low Resistance

Ω

NPN

Ω

COM

EBC

TEST A

DIODE TEST

Refer to the parts list and find a diode. This is a
silicon 1N4148 diode. Ref
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 lo

er to Test C for locating the

w

High Resistance

Ω

Ω

COM

NPN

BC

E

TEST B

resistance. Using an Ohmmeter, connect the diode
as shown in Test D. Your meter should be reading a
high resistance. Typical results read approximately

Ω to infinity.

1M

Low Resistance

Ω

Ω

COM

TEST C

Diode

High Resistance

COM

TEST D

-5-

Ω

Ω

Diode

AUDIO AMPLIFIER

SECTION 1

The purpose of the Audio Amplifier is to increase the
audio power to a level sufficient to drive an 8 ohm speaker.

o do this, DC (direct current) from the battery is

T
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.

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.

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 1.35k

Ω 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.

Equivalent Schematic and Connection Diagrams

VS

6

5

UT

VO

4

GND

– INPUT

50kΩ

5kΩ

1

7

YPASS

B

15kΩ

150Ω

2

AIN

G

AIN

G

1

8

1.35kΩ

15kΩ

3

50kΩ

+ INPUT

Dual-In-Line and Small Outline Packages

GAIN

– INPUT

+ INPUT

GND

1

2

3

4

Top View

8

7

6

5

GAIN

BYPASS

V

S

UT

VO

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.

ypical Applications

T

Amplifier with Gain = 20

Minimum Parts

S

V

6

2

–

VIN

10kΩ

LM386

3

+

Figure 4a

Figure 4c

1

8

+

5

7

VIN

10kΩ

10Ω

.05µF

VS

2

3

6

–

LM386

+

4

47Ω

1

4

Amplifier with Gain = 200

VS

6

2

–

VIN

10kΩ

LM386

3

+

Figure 4b

Amplifier with Gain = 150

+

10µF

8

+

5

7

10Ω

BYPASS

.05µF

10µF
+

1

8

+

5

7

BYPASS

.05µF

10Ω

4

Figure 3

Figure 5

-6-

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

Foil Side
of PC Board

Figure A

Be sure that the band is
in the correct direction.

Diode

Figure D

Lytic Capacitor

Polarity Mark

(–)

(+)

Electrolytics have a polarity marking
indicating the (–) lead. The PC board
is marked to show the lead position.

Figure B

Band

CathodeAnode

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.

Integrated Circuit

Figure E

NPN Transistor

Flat
Side

EBC

B

E

C

Figure 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.

Notch

R43 - 100Ω Resistor
(brown-black-brown-gold)

TP2 - Test Point Pin

(see Figure A)

C42 - 10µF Lytic

(see Figure B)

R44 - 47Ω Resistor

ellow-violet-black-gold)

(y

U1 - IC Socket 8-pin
U1 -

LM386 Integrated Circuit

(see Figure E)

R45 - 10Ω Resistor

(brown-black-black-gold)

J1 - Jumper Wire

(use lytic lead)

1/8”

C39 - 470µF Lytic

(see Figure B)

C40 - 10µF Lytic

(see Figure B)

C41 - 10µF Lytic

(see Figure B)

C43 - 470µF L

ytic

(see Figure B)

C44 - .047µF Discap (473)

TP1 - Test Point Pin

(see Figure A)

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.

Knob
Nut

Washer

Cut off
locating pin

Plastic Washer

Solder all 5 tabs to PC board

Figure F

Battery Holder
3 Screws 2-56 x 1/4”
3 Nuts 2-56
Solder and cut off
excess leads.

Volume/S2
(50kΩ Pot / SW)
with Nut & Washer
Plastic Washer
Knob (pot)

(see Figure F)

Earphone Jack
with Nut

(see Figure H)

Speaker
Speaker Pad
Wire #22 Insulated

(see Figures G & I)

Figure G

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.

Remove

Figure H

our kit ma

Y
the jack, determine which one you have.

oil Side

F

y contain a diff

2

3

1

erent type of earphone jack. Before installing

oil Side

F

2

Nut

GND

Pad

3

Nut

GND

1

Pad

Backing

Speaker

Pad

Backing

Figure I

Wire

2”

1.5” Wire

Wire

2”

1.5” Wire

1 - GND

Tip

2 ­3 - N.C. Tip

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.

t # 622130

ar

P

1 - GND

Tip

2 ­3 - N.C. Tip

Part # 622131

1” Wire

1”

Part # 622130

From Terminal 3

Part # 622131

Wire

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.

+

–

+

–

mps

A

Amps

COM

OUTPUT BIAS TEST

Put the battery into the holder.

Figure 6

Figure 7

TP15

COM

V

V

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

er ON. The voltage at TP1 should

and tur
be betw

n the po

een 3 to 6 volts. If you get this reading, go on

w

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 v

ust pass before proceeding to the Dynamic Tests or

m

alue and not interchanged.

All static tests

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.

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 gener

ator at this setting and move the positive

Generator

lead of your VOM to the Jumper J3. Record the AC
input voltage to the amplifier here:

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 = _________

Hz

TP15

Figure 8

-10-

COM

TP15

V

V

If an oscilloscope is not available, skip the following test and go directly to Section 2.

AC BANDWIDTH

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:

Vin = _______ Vpp

(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

. Record this frequency here:

decibel (dB) cor

Slowly decrease the frequency of the generator until
the output drops to .7 of its original reading, 1.4Vpp

ner

(f high 3dB) = __________ kHz.

or 2.8 divisions. This frequency is called the low
frequency 3dB corner. Record your answer.

(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

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 y

our gener

Measure the maximum voltage peak to peak when
clipping first occurs and record that value here:

Vclp = _______ Vpp.

Turn the power OFF.

ator to the GND of the circuit.

Generator

Hz

TP15

Battery

TP15

Figure 9

-11-

MAXIMUM POWER OUTPUT

T

he maximum power output before distortion due to

“

clipping” can be calculated using the voltage Vclp

obtained in the Distortion Step as follows:

Vpeak (Vp) = Vclp/2

Vroot mean squared (Vrms) = Vp x .7

Clipped

Crossover
D

istortion

Max power out = (Vrms)
Maximum power output should be greater than 200
milliwatts.

2

/8 ohms = (Vclp x .35)2/8

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

Eff = Max audio power/Batter

calculated as follo
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
power ON and rotate the volume control to
maxim
audio generator until the output is clipped as shown
in Figure 10A.

um. Slowly increase the amplitude of the

ws:

.

n the

. Tur

A

Record Vclp here:

This should be equal to Vclp in the Distortion Step.
Record the DC current drawn from the 9 volt supply

y

here:

Current (I) max = ________ A.

Measure the supply voltage and record the V supply
here:

Turn the power OFF.

Calculate the maximum power output as done in the
Maximum P

ower Output Step.

Figure 10

Vclp = _________ Vpp.

V supply = ________ volts.

B

Record your answers on page 13.

-12-

Generator

Hz

TP15

If you do not have a power supply, use a 9
volt battery instead.

ower Supply

P

mps

A

mps COM

A

Figure 11

Vclp/2 Vp = ______

Vp =
Vrms = Vp x .7 Vrms = ______
Max power out = (Vrms)

er equals the battery voltage times the current taken from the battery; calculate the battery

w

Since the batter

wer:

po

y po

Battery power = Imax x V supply Battery power = ______

Since the efficiency (N) is equal to the Max po
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%.

2

/8 Max power out = ______

er out divided by the Battery power, we can now calculate the

w

TP15

-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 pur
circuit is to maintain a constant level at the detector,
regardless of the strength of the incoming signal.
Without AGC
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 v
actually

pose of the automatic gain control (AGC)

, the volume control w

iding”

“r

on a negativ

ould have to be

olume control is

e DC voltage when

ASSEMBLY INSTRUCTIONS

Switch

J2 - Jumper Wire

(use lytic lead)

1/8”

strong signals are encountered.
component corresponds to the strength of the
incoming signal.
negative the component.
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

orward bias diode D4 just enough to minimize

also f
“On Condition”

The larger the signal, the more

At test point five (TP5), the

threshold v

oltage

This negative DC

.

-14-

ASSEMBLY INSTRUCTIONS

C34 - 100µF Lytic

see Figure B)

(

T6 - AM IF Coil

(Yellow Dot)

R35 -

27kΩ Resistor

(red-violet-orange-gold)

TP5 - Test Point Pin

(see Figure A)

C32 - 10µF Lytic

(see Figure B)

R36 - 3.3kΩ Resistor
(orange-orange-red-gold)

C33 - .02µF Discap (203)

or .022µF Discap (223)

R38 - 100Ω Resistor

brown-black-brown-gold)

(

TP3 - Test Point Pin

(see Figure A)

T8 - AM IF Coil

(Black Dot)

D4 - 1N4148 Diode

(see Figure D)

C38 - .01µF Discap (103)
R42 - 2.2kΩ Resistor

(red-red-red-gold)

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 po

wer
ON. The voltmeter should read
approximately 1.5 volts DC. If your
reading v

aries b

y more than .5 volts
from this value, turn the power OFF
and chec
chec

k the polarity of D4. Also

k R36 and R35 and check that

transformer T6 is properly installed.

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

V

V

COM

TP15

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.

V

COM

V

TP15

.001µF

GENERATOR

Hz

Figure 14

Set the

VOM to accurately read 2 v
the output of the RF generator for 455kHz, no
modulation, and minimum voltage output. Turn the
power ON and slo

wly increase the amplitude of the

If your RF generator does not have amplitude modulation and

you do not ha

olts DC and set

ve an oscilloscope, skip to Section 3.

SYSTEM CHECK

Connect your equipment as shown in Figure 15.

.001µF

GENERATOR

TP15

ator until the v

gener

oltage at TP5 just star

ts 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:

____________.

TP15

Hz

TP15

Set the RF gener
modulation and minim

wer ON and set the volume control at maximum.

po

ator at 455kHz, 1kHz at 80%

oltage output.

um v

Slowly adjust the amplitude of the RF generator

T

ur

n the

Figure 15

output until y
If this test f
and D4.

-16-

ou hear the 1kHz tone on the speaker.

er OFF and chec

ails

, tur

n the po

w

Turn the power OFF.

k R42

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

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

mer (T8)

rejecting all others
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.

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

. The second IF transf

or

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.

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 b
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,

y also provide an impedance match between the

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.

y resistors R39, R40 and R41.

kHz

Figure 16

kHz

kHz

-17-

ASSEMBLY INSTRUCTIONS

R39 - 39kΩ Resistor

(orange-white-orange-gold)

T7 - AM IF Coil

(White Dot)

TP4 - Test Point Pin

(see Figure A)

R40 - 10kΩ Resistor

(brown-black-orange-gold)

R41 - 470Ω Resistor

(yellow-violet-brown-gold)

STATIC MEASUREMENTS

Q9 BIAS

Connect y

our VOM as sho
VOM to read 9 volts DC and turn the power ON. The
voltage at the emitter of Q9 should be approximately

wn in Figure 17. Set the

C36 - .02µF Discap (203)

or .022µF Discap (223)

Q9 - 2N3904 transistor

(see Figure C)

C37 - .02µF Discap (203)

or .022µF Discap (223)

1 volt. If y

our reading is diff

erent by more than .5
volts, turn the power OFF and check components
R39, R40, R41 and Q9.

If you do not have an RF generator and oscilloscope, skip to Section 4.

V

COM V

TP15

Figure 17

-18-

DYNAMIC MEASUREMENTS

AC GAIN

Connect your test equipment as shown in Figure 18.

.001µF

GENERATOR

Hz

TP15

TP15

Figure 18

Set the generator at 455kHz, no modulation and

um voltage output. Set the oscilloscope at 1

minim
volt per division. The scope probe must have an
input capacitance of 12pF or less or it will detune T8.
Turn the po
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 gener
Vpp at the oscilloscope
the scope probe to the base of Q9 and record the
peak to peak amplitude of the signal here:

urn the power OFF. The AC gain of the second IF

T
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

er OFF

w

po

wer ON and slowly increase the

ator’s amplitude to maintain 4

. After T8 is aligned, move

Vb=________ Vpp.

.

BANDWIDTH

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 necessar
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 v

alue. Record that frequency here:

Fh = __________kHz.

By subtr

acting 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)

y,

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

ASSEMBLY INSTRUCTIONS

R34 - 1MΩ Resistor
(brown-black-green-gold)

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.

TP6 - Test Point Pin

(see Figure A)

CAUTION: Test point must
not touch can of IF Coil.

STATIC MEASUREMENTS

ASE BIAS

Q8 B

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.

n the po

ur

T

wer OFF.

Q8 - 2N3904 Transistor

(see Figure C)

C35 - .02µF Discap (203)

or .022µF Discap (223)

R37 - 1kΩ Resistor

(brown-black-red-gold)

Q8 CURRENT

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

ximately .8 milliamps. Turn the power OFF.

appro

ou do not ha

If y

ve an RF g

enerator and oscilloscope

-20-

, skip to Section 5.

DYNAMIC MEASUREMENTS

.001µF

Generator

Hz

TP15

S

hort TP3 to R38 as shown below.

Figure 19

TP15

AC GAIN

Connect your test equipment as shown in Figure 19.

The scope probe m
12pF or less, otherwise it will detune transformer T7.
Using a clip lead, short TP3 to R38 as shown. This
short pre
first IF amplifier. Set the generator to 455kHz, no
modulation, and minimum voltage output. Set the
scope to read 1 v
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

vents the AGC from lowering the gain of the

ust have an input capacitance of

olt per division and turn the power

SECTION 5

AM MIXER, AM OSCILLATOR, AND AM ANTENNA

In a superheterodyne type receiv
the antenna is amplified and then mix
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.

e f

ositiv

P
Q7 is pro

eedbac
vided b

k from the collector to the emitter of

y coil L5 and capacitor C31.

er

adio w

, the r

ave at

ed with the

ing

Dur

probe to the base of Q8 and record the peak to peak
level of the 455kHz signal here:

Vb=___________Vpp.

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.

wing f

the heterodyning process the f
frequencies are present at the collector of Q7.

1. The local oscillator frequency, OF.

2. The RF carrier or radio station frequency.

3. The sum of these two frequencies, OF + RF.

The diff

4.

erence of these tw

o frequencies

ollo

, OF – RF.

our

-21-

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

ASSEMBLY INSTRUCTIONS

C28 - .1µF Discap (104)

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.

R31 - 56kΩ Resistor
(green-blue-orange-gold)

C30 - 150pF Discap (151)

L5 - AM Oscillator Coil

(Red Dot)

J1 - Jumper Wire

(use lytic lead)

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 f
the PC

board.

oil side of

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

errite core. Measure the

f

our

resistance of the coil.
readings should match the
appro

ximate v

alues as sho

Y

wn.

White

Black

Red

R=9 - 11Ω

}

}

R=1 - 1.5Ω

1/8”

TP7 - Test Point Pin

(see Figure A)

C31 - .01µF Discap (103)

Q7 - 2N3904 Transistor

(see Figure C)

R32 - 12kΩ Resistor

(brown-red-orange-gold)

R33 - 3.3kΩ Resistor

(orange-orange-red-gold)

C29 - .02µF Discap (203)

or .022µF Discap (223)

Wire3 Wire

4

White

R=9 - 11Ω

}

Black

Red

R=1 - 1.5Ω

}

Green

-22-

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.
Slide the ferrite core through the holders.

Punch out one antenna shim from the front flap of the box.
Insert the cardboard antenna shim between the ferrite core and the

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 (black)
A (red)

C (white)

B

Twisted Together

Black

B Twisted Together

C (white)

Black

OR

Red

A (green)

Tabs

3 Wire Type Antenna: Solder the 3 colored wires to
the PC board: Wire A (red) to the hole marked
“RED”, Wire B (black) to the hole marked “BLK” and

Wire C (white) to the hole marked “WHT”.

Red

A (green)

4 Wire Type Antenna: Solder the 4 colored wires to the PC board: Wire A (green) to the
hole marked “RED”, Wire B (red and black twisted together) to the hole marked “BLK” and

Wire C (white) to the hole marked “WHT”.

Tabs

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.

FM Antenna

Trimmer

FM Oscillator

Trimmer

FM SIDE

Locator Lead

AM SIDE

AM Antenna

Trimmer

AM Oscillator

Trimmer

AM Antenna

Trimmer

FM Antenna

Trimmer

FM SIDE

AM SIDE

AM Oscillator

Trimmer

FM Oscillator

Trimmer

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 P

w Holes

Scre

ost

ASSEMBLY INSTRUCTIONS

Figure M

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 arro
on the PC board.

Screw

Knob

ws

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.

Figure 20

COM

TP15

V

V

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

leave the po
the battery voltage is greater than 8.5 volts, check
components R31, R32, R33 and Q7.
Turn the power OFF.

voltage in your circuit differs by more than .5 volts,

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.

wer ON and check the battery voltage. If

Set the scope to read 1 v

olt 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

Figure 21

circuit f
capacitor, C28, C29, C30, C31, L4 and L5. Turn the
power OFF.

-25-

TP15

ails this test, check components Q7, gang

AM FINAL ALIGNMENTS

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.

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 w
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.

as selectiv

e. When aligning the antenna

AM ALIGNMENT WITHOUT TEST
EQUIPMENT

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.

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.

or another weak station and repeat

Retune the r
this procedure until there is no more improvement
noticed on the weakest possible station. This
process peaks the IF amplifiers to their maximum
gain.

adio f

Soldering Iron Tip

Shrink Tubing

Iron Slug

Brass Slug

Magic Wand Assembly

Place the piece of br
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.

ass inside the end of the

OSCILLATOR ALIGNMENT

Tune the radio until a known AM station around
600kHz is heard.
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.
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 f

wn, rotate the dial until the white pointer is

kno
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 r

It may be necessar

ound and its broadcast frequency is

ange at 955kHz to 2055kHz.

y to listen to the

Once a

-26-

ANTENNA ALIGNMENT

T

une 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 w
antenna trimmer and antenna coil will effect the
antenna alignment, it is advisable to repeat the entire
procedure until the antenna alignment is optimiz
This process sets the tracking of the AM radio section.

and”. Since the adjustment of both the

ed.

O

nce 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

If the antenna needs:

• More inductance, slide the coil

• Less inductance, slide the coil

Antenna Coil
Ferrite Core
Antenna Holder

Figure 22

Wax

Wax

Figure 23

AM ALIGNMENT WITH TEST EQUIPMENT

IF ALIGNMENT

Connect y

wn in Figure 24. Make sure that the switch is in

sho

Figure 24

our RF generator and oscilloscope as

.001µF

GENERATOR

Hz

TP15

-27-

the AM position. Place a shor
of Q7 to

TP6.

This shor

t “kills” the AM oscillator.

t from the collector

TP15

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.

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 v
of its peak

oltage seen on the scope drops .707

ed value or .35Vpp. Record the frequency

of the high 3dB corner here:

Fh = __________kHz.

Tur

The

n the

The bandwidth of the IF is equal to BW = Fh - Fl.

s bandwidth should be around 6kHz.

IF’
power OFF and remove the short from the

collector of Q7 to TP6.

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
optimiz

e the oscillator alignment. This process sets

the oscillator range at 955kHz to 2055kHz.

ANTENNA ALIGNMENT

With the power turned OFF, connect your test
equipment as shown in Figure 25.

Generator

Hz

Wire loop
close to
antenna

Battery

TP15

Figure 25

-28-

Set the generator at 600kHz, 400Hz 80%
modulation, moderate signal strength. Set the
oscilloscope to read .1 volts per division. Turn the
t

uning 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

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
f

or 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.

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

1. The number of vibrations (or cycles) per second
produced by a sound is called the frequency, and
is measured in hertz.
The distance between peaks of sound waves is

2.
called the wavelength.

3. Sound waves are produced as a certain number

ations per second.

of vibr
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.

The more vibrations per

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 w
performed in an AM radio by the detector.

7. The amount of signal picked up by the antenna
will depend on the po
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).

ave is called demodulation, which is

wer of the signal transmitted

DC VOLTAGES

The voltage readings below should be used in troubleshooting the AM section. (Switch at AM position).

Q7 B 1.5 U1 1 1.3

E 1.0 2 0
C 8.8 3 0

40

TP5 (AGC) 1.4 5 4.5

6 9.0

Q8 B

Q9 B

1.4
E .8 8 1.3
C 8.8

1.7
E 1.0
C 9.0

TP1 4.5

7

4.6

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.

5. Voltage readings can vary +

Test Conditions

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.

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.

3. The process of removing the audio wave from
the radio wave is called ...

A) demodulation.
B) frequency reduction.
C) modulation.
D) vibrating.

4. When an electromagnetic wave (modulated radio
wave) passes an antenna, it ...

induces a v

A)

antenna.

B) changes an audio wave into a radio

wave.
C) changes the carrier frequency.
D) produces sidebands.

5. The power of the signal transmitted by the
broadcast station and the distance, the signal

avelled from the transmitter to the receiver

tr
determine the ...

frequency of the modulation.

A)
B) wavelength of the audio waves.

amount of signal picked up by the

C)

antenna.

D) type of filter that is used.

oltage and current in the

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.

7. The process of mixing two signals to produce a
third signal is called ...

A) filtering.
B) detecting.
C) rectification.
D) heterodyning.

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.

whether the oscillator is functioning.

D)

9. The IF frequency of your AM radio is ...

1600kHz.

A)
B) 455kHz.
C) 550kHz.

910kHz.

D)

The purpose of the AGC circuit is to ...

10.

automatically control the frequency of

A)

the oscillator circuit.

B) control the band width of the IF stages

reduce distortion in the audio circuit.

,

C)
D)

maintain a constant audio level at the
detector
the incoming signal.

, regardless of the strength of

.

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. Symbol Value Color Code Part #

2 R9, 23 100Ω 5% 1/4W brown-black-brown-gold 131000
1 R25 220Ω 5% 1/4W red-red-brown-gold 132200

R3 470Ω 5% 1/4W yellow-violet-brown-gold 134700

1
5 R18, 22, 24, 26, 27 1kΩ 5% 1/4W brown-black-red-gold 141000
1 R11 1.8kΩ 5% 1/4W brown-gray-red-gold 141800
2 R15, 6 2.2kΩ 5% 1/4W red-red-red-gold 142200
2 R2, 7 6.8kΩ 5% 1/4W blue-gray-red-gold 146800
7 R10,12,14,16,19,20,28 10kΩ 5% 1/4W brown-black-orange-gold 151000
2 R1, 8 22kΩ 5% 1/4W red-red-orange-gold 152200
2 R4, 5 33kΩ 5% 1/4W orange-orange-orange-gold 153300
3 R13, 17, 21 47kΩ 5% 1/4W yellow-violet-orange-gold 154700
2 R29, 30 390kΩ 5% 1/4W orange-white-yellow-gold 163900

CAPACITORS

Qty. Symbol Value Description Part #

1 C9 15pF Discap (15) 211510
1 C10 30pF Discap (30) 213010
1 C6 33pF Discap (33) 213317
1 C11 220pF Discap (221) 222210
2 C4, 5 470pF Discap (471) 224717
3 C3, 7, 27 .001µF Discap (102) 231036
3 C2, 8, 12 .005µF Discap (502) 235018
10 C13 - 22 .01µF Discap (103) 241031
1 C23 .02µF or .022µF Discap (203) or (223) 242010
1 C26 .1µF Discap (104) 251010
1 C25 10µF Electrolytic Radial (Lytic) 271045
1 C24 470µF Electrolytic Radial (Lytic) 284744

SEMICONDUCTORS

Qty. Symbol Value Description Part #

1 D1 Varactor Diode 310176
2 D2, 3 1N60 Diode 311065
6 Q1 - Q6 2N3904 Transistor 323904

COILS

Qty. Symbol Value Description Part #

1 T5 Blue FM Detector 430110
1 T4 Pink FM Detector 430120
2 T2, 3 Green FM IF 430130
1 T1 Orange FM Mixer 430140
1 L1 6 Turns FM RF Amp 430160
1 L2 2 Turns FM RF Amp 430170
1 L3 5 Turns FM Oscillator 430180

MISCELLANEOUS

Qty. Symbol Description Part #

1 Antenna FM 484005
2 Screw 2-56 x 1/4” 641230
2 Antenna Screw M2 643148
2 Nut 2-56 644201
7 TP8 - 14 Test Point Pin 665008
1 Coil Spacer 669108

-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

FM RADIO

Section 9

Section 8

Section 7 Section 6

Detector and work back to the FM Antenna. Each
stage will be tested before proceeding to the next
stage.

Section 1

FM RF

AMPLIFIER

FM

OSCILLATOR

FM MIXER

1ST FM IF

AMPLIFIER

AFC

FM RATIO DETECTOR

In the AM DETECTOR section w

e obser
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.
DETECTOR is redrawn below for ease of
explanation.
When an incoming signal is present at
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.

ved that the

The RA

T4 and

T5, a

TIO

2ND FM IF

AMPLIFIER

Speaker

FM

DETECTOR

AUDIO

AMPLIFIER

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 flo
in C23 which will produce an audio voltage across
C23. If the modulation is of opposite direction than
before, more current will flo

w 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 dr

awn from
the audio which causes the battery voltage to vary.
The ratio detector is decoupled further by the resistor
R23 and capacitor C21.

w

,

.01µF

µF

.02µF

µF

Figure 26

-32-

ASSEMBLY INSTRUCTIONS

R24 - 1kΩ Resistor

(brown-black-red-gold)

C21 - .01µF Discap (103)
R21 - 47kΩ Resistor

(yellow-violet-orange-gold)

C19 - .01µF Discap (103)

T3 - FM IF Coil

(Green Dot)

TP9 - Test Point Pin

(see Figure A)

TP8 - Test Point Pin

(see Figure A)

Q6 - 2N3904 Transistor

(see Figure C)

R20 - 10kΩ Resistor

(brown-black-orange-gold)

R22 - 1kΩ Resistor

(brown-black-red-gold)

C22 - .01µF Discap (103)

T5 - FM Detector Coil

(Blue Dot)

R23 - 100Ω Resistor

(brown-black-brown-gold)

C23 - .02µF Discap (203)

or .022µF Discap (223)

R25 - 220Ω Resistor

(red-red-brown-gold)

C24 - 470µF Lytic

(See Figure B)

D2 - 1N60 Diode

(see Figure D)

R26 - 1kΩ Resistor

(brown-black-red-gold)

R28 - 10kΩ Resistor

(brown-black-orange-gold)

C25 - 10µF Lytic

(see Figure B)

R27 - 1kΩ Resistor

(brown-black-red-gold)

D3 - 1N60 Diode

(see Figure D)

T4 - FM Detector Coil

(Pink Dot)

(see Figure 27)

Test Point Pin

Foil Side
of PC Board

Figure A

Test Point Pin

Lytic Capacitor

Flat
Side

Polarity Mark

( )

Be sure that the negative lead is in
the correct hole on the PC board.

(+)

E

Figure B

Notch

Note: Notch m
PC board.
line or part number in place of the notch.

ust be pointing to the top of the

Some FM detector coils have a red

NPN Transistor

Mount so E lead is
in the arrow hole
and flat side is in
the same direction

EBC

B

as shown on the
top legend. Leave
1/4” between the
part and PC board.

C

Figure C

Diode

Band

CathodeAnode

Be sure that the band is in the correct
direction.

Figure D

Figure 27

-33-

STATIC MEASUREMENTS

V

COM

V

TP15

Figure 28

FM VOLTAGE

Connect your
the AM/FM switch to the FM position. Set your VOM
to read 9 volts DC. Turn the power ON. The voltage

VOM as shown in Figure 28. Switch

TRANSISTOR CURRENT TEST

Battery

.Turn

at this point should be between 7 and 9 v

olts
the power OFF. If you do not get this reading, check
R25, C24 and the battery voltage.

Figure 29

Connect your VOM to the circuit as shown in Figure 29.

er ON.

n the po

ur

T

w

should be about .7 v

urn the power OFF. If your answer is greater than 2

T

k R20, R21, R22, R24, Q6 and the batter

, chec

olts

v

The voltage at the emitter of Q6

olts. Record the voltage here:

V(Q6) = ________.

y

.

-34-

V

V

COM

TP15

Since the current through resistor R22 is equal to the
current through tr
through Q6 as f

ansistor Q6, calculate the current

ollows:

Current (I) = V(Q6) / R22

our calculated answer should be between .0005

Y
amps (.5 milliamps) and .0011 amps (1.1 milliamps).

Current (I) = __________.

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 = __________

Generator

Hz

TP15

.001µF

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

.001µF

TP15

stop. DO NOT FORCE THE COILS ANY FURTHER.
Now turn both coils in about 1 1/4 to 1 1/2 turns.

Hz

TP15

Figure 31

-35-

TP15

METHOD #2
ALIGNMENT OF RATIO DETECTOR USING A
RF GENERATOR AND OSCILLOSCOPE

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

or 10.7MHz and minimum voltage out.

ator f

gener

Turn the power ON and set the volume control to a
minimum. Increase the amplitude of the sweep
g

enerator 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.

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 pro
“pick out”
acts as a bandpass filter that only passes signals
around 10.7MHz. The resistor R19 is used to widen

viding Selectivity. Selectivity is the ability to

one station while rejecting all others. T3

.707

the 3dB bandwidth of the 2nd FM IF amplifier.
The gain at 10.7MHz is fixed by the AC impedance of

imary side of T3 and the current in Q5. The

the pr
current is fixed b
C18 and C17 bypass the AC signal to ground. C20
is a bypass capacitor from

y R16, R17 and R18. Capacitors

V+ to ground.

10.625MHz

10.775MHz

10.7MHz

Figure 33

-36-

ASSEMBLY INSTRUCTIONS

TP10 - Test Point Pin

(see Figure A)

T2 - FM IF Coil

(Green Dot)

C17 - .01µF Discap (103)
R16 - 10kΩ Resistor

(brown-black-orange-gold)

R18 - 1kΩ Resistor

(brown-black-red-gold)

R17 - 47kΩ Resistor

(yellow-violet-orange-gold)

R19 - 10kΩ Resistor

(brown-black-orange-gold)

5 - 2N3904 Transistor

Q

(see Figure C)

C20 - .01µF Discap (103)
C18 - .01µF Discap (103)

STATIC TESTS

Q5 BIAS

Connect your VOM to the circuit as shown in Figure 34.

n the power ON.

Tur
should be approximately 1.4 volts. Turn the power

The voltage at the base of Q5

V

COM

V

TP15

OFF. If you do not get this reading, check R17, R16,
R18, Q5 and

T2.

ou don’t ha

If y

ve an RF g

Figure 34

enerator and oscilloscope, skip to Section 8.

-37-

GENERATOR

Hz

.001µF

TP15

Figure 35

AC GAIN

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 minim
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
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:

Vb = __________ mVpp.

Turn the power OFF. The AC gain can be calculated
as follows:

C Gain = 150mV /

A

Your calculated answer should be about 20.

um voltage output. Set the

. With an alignment

Vb

TP15

BANDWIDTH

With the power turned OFF, connect your test
equipment as shown in Figure 35. Set your
generator at 10.7MHz no modulation and minim
voltage output. Set the scope to read 50mV per
division. Turn the power ON and slowly adjust the

ator amplitude until 150mVpp is seen on the

gener
scope. Realign T3, if necessary, for maximum output
while adjusting the generator to maintain 150mVpp.

wly decrease the frequency of the generator until

Slo
the voltage drops .707 of its original value, 2.1
divisions or 106mVpp. Record the frequency of the

er 3dB drop-off point here:

low

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:

Fh = ___________MHz.

The bandwidth of the 2nd IF can be calculated as
follows:

um

Record your calculation:

C Gain = __________

A

Bandwidth = Fh - Fl

our results should be betw

Y
Record your calculation:

-38-

een 300 - 500kHz.

Bandwidth = __________

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.

ASSEMBLY INSTRUCTIONS

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.

C13 - .01µF Discap (103)
C14 - .01µF Discap (103)

T1 - FM Mixer Coil

(Orange Dot)

R12 - 10kΩ Resistor

(brown-black-orange-gold)

R15 - 2.2kΩ Resistor

(red-red-red-gold)

STATIC TESTS

Q4 BIAS

Connect y

our VOM as sho

power ON. The voltage at the base of Q4 should

wn in Figure 36. Turn the

R13 - 47kΩ Resistor

(yellow-violet-orange-gold)

R14 - 10kΩ Resistor

(brown-black-orange-gold)

TP11 - Test Point Pin

(see Figure A)

Q4 - 2N3904 Transistor

(see Figure C)

C16 - .01µF Discap (103)
C15 - .01µF Discap (103)

ximately be 1.4 volts

appro

. 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.

.001µF

GENERATOR

Hz

TP15

TP15

Figure 37

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 A

C 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 screwdriv

er, 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:

Vb = __________mVpp.

n the power OFF. The AC gain can be calculated

ur

T

ws:

ollo

as f

AC Gain = 60mV / Vb

Your calculated answer should be about 10.

BANDWIDTH

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
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
low 3dB drop-off point here:

Fl = ___________MHz.

The bandwidth of the first IF can be calculated as

ws:

ollo

f

. Increase the frequency of the

Record the frequency of the

.

Record y

our calculation:

AC Gain = __________

Bandwidth = Fh - Fl

Your calculated answer should be between 300 ­500kHz.

Record y

our 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

MIXER ASSEMBLY INSTRUCTIONS

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.

R10 - 10kΩ Resistor

(brown-black-orange-gold)

C12 - .005µF Discap (502)
R8 - 22kΩ Resistor

(red-red-orange-gold)

R7 - 6.8kΩ Resistor

(blue-gray-red-gold)

TP13 - Test Point Pin

(see Figure A)

R9 - 100Ω Resistor

(brown-black-brown-gold)

TP12 - Test Point Pin

(see Figure A)
CAUTION: Test Point must
not touch can of T1 FM Mixer
Coil.

Q3 - 2N3904 Transistor

(see Figure C)

R11 - 1.8kΩ Resistor

(brown-gray-red-gold)

-41-

STATIC MEASUREMENTS

Q3 BIAS

Figure 38

COM

TP15

V

V

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.

our answer varies by more than 2 volts, turn the

If y
power OFF and check components R7, R8, R11 and
Q3.

If you don’t have an RF generator and oscilloscope,

skip to the FM Oscillator Assembly Procedure.

AC GAIN

C gain of the mixer is set by the impedance of

The A

imary side of T1 and by the current flowing in

the pr
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 slo

wly increase the amplitude of the

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 v

oltage

here:

Vb = __________mVpp

.

Turn the power OFF. The gain can be calculated as

ollows:

f

AC Gain = 40mV / Vb.

our calculated ans

Y

wer 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 b

ypassed
to ground. This is why the gain of the mixer is low
compared to the other IF stages.

-42-

GENERATOR

.001µF

Hz

TP15

Figure 39

BANDWIDTH TEST

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 f

or 10mV per division.
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 v
or 28mVpp. Record the frequency of the low 3dB
drop-off point here:

Fl = _________MHz.

Turn the power

alue, 2.8 divisions

TP15

Turn the power OFF. The bandwidth can be
calculated as follows:

Bandwidth = Fh - Fl

Your calculated answer should be between 300 ­500kHz.

Record your calculation:

Bandwidth = _________kHz.

FM OSCILLATOR ASSEMBLY INSTRUCTIONS

C7 - .001µF Discap (102)
R4 - 33kΩ Resistor

ange-orange-orange-gold)

(or

ansistor

r

Q2 - 2N3904

(see Figure C)

R5 - 33kΩ Resistor

ange-or

(or

R6 - 2.2kΩ Resistor

(red-red-red-gold)

C8 - .005µF Discap (502)

T

ange-or

ange-gold)

L3 - FM Oscillator Coil

(5 Turns)

C9 - 15pF Discap (15)

C10 - 30pF Discap (30)

C11 - 220pF Discap (221)

-43-

STATIC MEASUREMENTS

Q2 BIAS

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

volts. Turn the power OFF. If you do not get this
measurement, check R4, R5 and Q2.

Figure 40

AFC

When a r
desirable for the radio to “lock” in on the station. Due
to changes in temperature, voltage and other effects,
the local oscillator ma
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 v
netw
voltage is produced. This voltage is fed to a special

adio is tuned to a station, it would be

y change its frequency of

oltage. This signal is fed to a filter

ork which removes the audio so that a pure DC

AFC ASSEMBLY INSTRUCTIONS

diode called a v

aractor. 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 v

oltage
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.

,

R30 - 390kΩ Resistor

w-gold)

ange-white-y

(or

Varactor Diode

D1 -

(see Figure D)

C27 - .001µF Discap (102)

ello

R29 - 390kΩ Resistor

(orange-white-yellow-gold)

C26 - .1µF Discap (104)

-44-

If you don’t have an RF generator, skip to the RF Amplifier Assembly Procedure.

Generator

Hz

TP15

.001µF

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 moder
ON. Record the voltage of D1 here:

While watching your VOM, slowly decrease the
frequency of y
decreases, the voltage at D1 should increase.

ate signal strength output. Turn the power

V(D1) = ________.

our generator. As the frequency

V

COM

TP15

V

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 y

our generator and repeat the same

steps again. Turn the power OFF.

RF AMPLIFIER ASSEMBLY INSTRUCTIONS

R1 - 22kΩ Resistor

(red-red-orange-gold)

C3 - .001µF Discap (102)
C2 - .005µF Discap (502)

Q1 - 2N3904 Transistor

(see Figure C)

R2 - 6.8kΩ Resistor

(blue-gray-red-gold)

C4 - 470pF Discap (471)

-45-

L1 - FM RF Amp Coil

(6 Turns) see Figure N

L2 - FM RF Amp Coil

(2 Turns) see Figure N

C5 - 470pF Discap (471)

C6 - 33pF Discap (33)
R3 - 470Ω Resistor

w-violet-bro

ello

(y

TP14 -

(see Figure A)

Test Point Pin

wn-gold)

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.

Figure 42

COM

TP15

V

V

ANTENNA FM ASSEMBLY

Antenna FM
2 Antenna Screws M2

3.5” Wire #22 Insulated

(extra wire in AM Section)

(see Figure O)

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 f
wire to the pads as sho

or the wire in this location, so tack solder the

wn.

FM FINAL ALIGNMENTS

There are two procedures for the final alignment
steps. The first alignment procedure is for those who

e test equipment and the second is f

do not ha
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

v

Threaded Holes

Figure O

or

Solder

umper Wire

J

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.

When aligning the oscillator and RF circuits, coils L1

er end of the band,

and L3 will be adjusted at the lo

w
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

W

ith an alignment tool or screwdriver turn coils T1,

T2 and T3 fully counter-clockwise. DO NOT FORCE

IF ALIGNMENT

Figure 43

T

HE COILS ANY FURTHER. Turn each coil in about

1 1/4 to 1 1/2 turns.

V

V

COM

TP15

With an alignment tool or screwdr

er turn coils

iv

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 tur

ns.
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 lo
voltage on y

w level. Adjust T1 for the minimum

our VOM.

Reduce the volume if

necessary. Adjust T2 for minimum voltage on your

OM and reduce the volume control if necessary.

V
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

. Repeat this procedure until the FM IF gain is

noise
optimiz

ed. This process peaked the FM IF amplifier

to their maximum gain.

DETECTOR ALIGNMENT

or minimum voltage on your VOM. Adjust

Adjust
T5 for minimum distortion. Repeat these 2 steps
until the ratio detector alignment is optimized.

T4 f

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 g

reat effect on the coils resonant
frequency. Repeat this step until the pointer is
aligned to the station’

s frequency

a station around 106MHz. Once a station is f

. Tune the radio to

ound
and its broadcast frequency is known, rotate the dial
until the white pointer is aligned with that station’
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 tr
page 23), on the bac

immer (as shown in Figure L,

k of the gang until the station

“Magic Wand”

RF Coil

.

s

OSCILLATOR ALIGNMENT

une the r

T

adio to a kno
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.

wn station around 90MHz.

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

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

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

Short the base of Q2 to the
emitter (as shown below).

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.

Top View

Figure 45

Hz

TP15

Turn the power ON. Slowly increase the amplitude of
the gener
scope

ator until a 1kHz signal is seen on the

eep the gener

K

.

ator at a lo

w le
prevent the IF sections from limiting. With an
alignment tool or screwdriver, adjust T1 for a peak on

Figure 46

vel of output to

TP15

the scope. Reduce the amplitude of the input signal
if necessar

y. Adjust T2 for a peak and reduce the
amplitude of the input signal if necessar
these steps until the IF alignment is optimized. This
procedure aligns the FM IF amplifiers to 10.7MHz.

-48-

.

y

Repeat

OSCILLATOR ALIGNMENT

R

emove the clip lead and set your generator at

8

8MHz 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 r
scope. Rotate the dial until the white pointer is
aligned to 108MHz. Place the brass end of your
“magic w
increases, it means that L3 needs more capacitance.
Adjust the FM oscillator trimmer on the gang (as

wn in Figure L on page 23) until the 1kHz signal

sho
is at a peak. Place the iron end of the “magic wand”
near L3.
needs less capacitance
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
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.

adio until a 1kHz signal is seen on the

and” near L3.

If the signal increases, it means that coil L3

If the signal on the scope

. Adjust the FM oscillator

“magic w

and”. Since

RF ALIGNMENT

S

et your generator at a frequency around 90MHz

m

odulated 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
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

ect the gain of th RF stage

eff
this procedure until the RF amplifier alignment is
optimiz
the FM oscillator stage

This concludes the alignment of the FM radio
section. If no stations are heard, v
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.

ed. This process sets the RF stage to “track”

, this means coil L1 needs less

, it is advisable to repeat

.

ify that FM

er

FM RADIO HIGHLIGHTS

1. The FM broadcast band covers the frequency
range from 88MHz to 108MHz.

2. FM signals are usually limited to line a sight.

udio signals up to 15kHz are tr

A

3.
FM carrier.

4.

The amount that the RF carrier changes frequency
is determined by the amplitude of the modulating
signal.

ansmitted on the

5. The number of times the carrier frequency
changes in a period of time is exactly equal to the
audio frequency.

6. The change in frequency is called the deviation
and is limited to 75kHz f

7. The bandwidth assigned for FM is 200kHz.

or monaur

al FM.

-49-

DC VOLTAGES

The voltage readings below should be used in troubleshooting the FM section. (Switch at FM position.)

Q1 B 1.6 Q4 B 1.3

E .9 E .7
C 7.0 C 7.5

Q2 B 3.3 Q5 B 1.3

E 3.0 E .6
C 7.1 C 7.5

Q3 B 1.6 Q6 B 1.2

E 1.3 E .6
C 7.0 C 6.6

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 +

Test Conditions

10%

SPECIFICATIONS

Audio:

Frequency response 3dB drop into 8 ohm resistive load.

Low end 800Hz - high end 120kHz

Maxim

Typical audio gain at 1000Hz: 150 times
Typical % distor

um po

wer out at 10% total har

500 MilliWatts

tion at 100 milliw

monic distortion.

atts output <2%.

AM Radio Specifications:

Tuning range - 520kHz to 1620kHz
IF frequency 455kHz
Tracking = +
10dB signal to noise at 200 micro

FM Radio Specifications:

Tuning range = 88MHz to 108MHz
IF frequency 10.7MHz

racking +

T
10dB signal to noise at 12 microvolts typical
Uses ratio detector and full time auto frequency control.

3dB from 700kHz to 1400kHz

volts typical

5dB from 90MHz to 106MHz

-50-

QUIZ - FM SECTION

INSTRUCTIONS: Complete the following examination, check your answers carefully.

6

1

. The FM broadcast band is . . .

A) 550 - 1,600kHz.
B) 10.7MHz.
C) 88 - 108MHz.
D) 98.7 - 118.7MHz.

. 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.

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

ier.

carr
D) amount of amplitude change of 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.

iations of the FM

vent amplitude v

pre

C)

carrier.

D) automatically control the audio

frequencies in the receiver.

5. The ratio detector transformer is tuned to ...
A) 10.7MHz.
B) 88MHz.
C) 455kHz.
D) 10.9MHz.

ar

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.

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.

the amount of resistance in the circuit.

D)

9. Limiting in FM receivers is the process of ...

removing interfering FM stations.

A)
B) providing greater station selectivity.
C) separating the FM stations from the AM

stations.

D) removing noise from the FM carrier.

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.

.

.

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

N

OTICE: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.

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.

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

i

t 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.

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.

1. Start at one edge and carefully remove the
baffle from the bottom the kit box.

2. Bend the four flaps upward as shown.

AM/FM-108K Kit Carton

brown
side

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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).

Bend the bottom side upward and attach it

5.
to the other sides using scotch tape or glue

.

Bend the two mounting flaps down.

er the speaker. Align the

Place the baffle o

6.

v
mounting holes and attach it using the tw
screws and two nuts supplied. The screws
should go through the X mark on the baffle flap.

Optional: To make an air tight seal, place a bead of
glue betw

een the PC board and the baffle edges

.

Side View

o

Baffle

Nut

PC board

Screw

Glue

Screw

Back View

Nut

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SCHEMATIC DIAGRAM - AM/FM RADIO

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Elenco®Electronics, Inc.

150 Carpenter Avenue

Wheeling, IL 60090

[phone]

Website: www.elenco.com

e-mail: [e-mail]