Elenco AMFM108CK User Manual

AM/FM RADIO KIT

MODEL AM/FM-108K

INTEGRAL CIRCUIT, 9 TRANSISTORS, 4 DIODES

Elenco®Electronics, Inc.

Copyright © 2009, 1989 by Elenco®Electronics, Inc. All rights reserved. Revised 2009 REV-Z 753508

No part of this book shall be reproduced by any means; electronic, photocopying, or otherwise without written permission from the publisher.

Assembly and Instruction Manual

-1-

The AM/FM Radio project is divided into two parts, the AM Radio Section and the FM Radio Section. At this time, only
identify the parts that you will need for the AM radio as listed below. DO NOT OPEN the bags listed for the FM radio. A
separate parts list will be shown for the FM radio after you have completed the AM radio.

PARTS LIST FOR THE AM RADIO SECTION

If you are a student, and any parts are missing or damaged, please see instructor or bookstore.
If you purchased this kit from a distributor, catalog, etc., please contact Elenco®Electronics (address/phone/e­mail is at the back of this manual) for additional assistance, if needed. DO NOT contact your place of purchase
as they will not be able to help you.

RESISTORS

Qty. Symbol Value Color Code Part #

r 1 R45 10Ω 5% 1/4W brown-black-black-gold 121000
r 1 R44 47Ω 5% 1/4W yellow-violet-black-gold 124700
r 2 R38, 43 100Ω 5% 1/4W brown-black-brown-gold 131000
r 1 R41 470Ω 5% 1/4W yellow-violet-brown-gold 134700
r 1 R37 1kΩ 5% 1/4W brown-black-red-gold 141000
r 1 R42 2.2kΩ 5% 1/4W red-red-red-gold 142200
r 2 R33, 36 3.3kΩ 5% 1/4W orange-orange-red-gold 143300
r 1 R40 10kΩ 5% 1/4W brown-black-orange-gold 151000
r 1 R32 12kΩ 5% 1/4W brown-red-orange-gold 151200
r 1 R35 27kΩ 5% 1/4W red-violet-orange-gold 152700
r 1 R39 39kΩ 5% 1/4W orange-white-orange-gold 153900
r 1 R31 56kΩ 5% 1/4W green-blue-orange-gold 155600
r 1 R34 1MΩ 5% 1/4W brown-black-green-gold 171000
r 1 Volume/S2 50kΩ / SW Pot/SW with nut and washer 192522

CAPACITORS

Qty. Symbol Value Description Part #

r 1 C30 150pF Discap (151) 221510
r 2 C31, 38 .01μF Discap (103) 241031
r 5 C29, 33, 35, 36, 37 .02μF or .022μF Discap (203) or (223) 242010
r 1 C44 .047μF Discap (473) 244780
r 2 C28, C45 .1μF Discap (104) 251010
r 4 C32, 40, 41, 42 10μF Electrolytic Radial (Lytic) 271045
r 1 C34 100μF Electrolytic Radial (Lytic) 281044
r 2 C39, 43 470μF Electrolytic Radial (Lytic) 284744
r 1 C1 Variable Tuning Gang AM/FM 299904

SEMICONDUCTORS

Qty. Symbol Value Description Part #

r 1 D4 1N4148 Diode 314148
r 3 Q7, 8, 9 2N3904 Transistor NPN 323904
r 1 U1 LM386 Integrated Circuit 330386

COILS MAGIC WAND

Qty. Symbol Color Description Part # Qty. Description Part #

r 1 L5 Red AM Oscillator 430057 r 1 Iron Core 461000
r 1 T6 Yellow AM IF 430260 r 1 Brass Core 661150
r 1 T7 White AM IF 430262 r 4” Shrink Tubing 890120
r 1 T8 Black AM IF 430264
r 1 L4 AM Antenna with Holders 484004

MISCELLANEOUS

**** SAVE THE BOX THAT THIS KIT CAME IN. IT WILL BE USED ON PAGES 23 & 52. ****

Qty. Description Part #

r 1 PC Board 517055
r 1 Switch 541023
r 1 Battery Holder 590096
r 1 Speaker 590102
r 1 Speaker Pad 780128
r 1 Knob (dial) 622040
r 1 Knob (pot) 622050
r 1 Earphone Jack with Nut 622130 or 622131
r 1 Radio Stand 626100
r 1 Earphone 629250

Qty. Description Part #

r 3 Screw M1.8 x 7.5mm (battery holder) 641100
r 1 Screw M2.5 x 7.5mm (dial) 641107
r 2 Screw M2.5 x 3.8mm (gang) 641310
r 3 Nut M1.8 644210
r 1 Plastic Washer 645108
r 1 Socket 8-pin 664008
r 8 Test Point Pin 665008
r 1 Label AM/FM 723059
r 8” Wire 22AWG insulated 814520
r 1 Solder Lead-Free 9LF99

-2-

IDENTIFYING RESISTOR VALUES

Use the following infor mation as a guide in proper ly identifying the value of resistors.

BANDS

METRIC UNITS AND CONVERSIONS

Abbreviation Means Multiply Unit By Or

p Pico .000000000001 10

-12

n nano .000000001 10

-9

μ micro .000001 10

-6

m milli .001 10

-3

– unit 1 10

0

k kilo 1,000 10

3

M mega 1,000,000 10

6

1. 1,000 pico units = 1 nano unit

2. 1,000 nano units = 1 micro unit

3. 1,000 micro units = 1 milli unit

4. 1,000 milli units = 1 unit

5. 1,000 units = 1 kilo unit

6. 1,000 kilo units = 1 mega unit

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.

Second Digit

First Digit

Multiplier
Tolerance*

Note: The letter “R”
may be used at times
to signify a decimal
point; as in 3R3 = 3.3

103K

100V

The letter M indicates a tolerance of +20%
The letter K indicates a tolerance of +

10%

The letter J indicates a tolerance of +5%

Maximum Working Voltage

The value is 10 x 1,000 =
10,000pF or .01μF 100V

*

Electrolytic capacitors have a positive
and a negative electrode. The
negative lead is indicated on the
packaging by a stripe with minus
signs and possibly arrowheads.

Warning:

If the capacitor
is connected
with incorrect
polarity, it may
heat up and
either leak, or
cause the
capacitor to
explode.

Polarity
Marking

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

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

Resistance

Tolerance

Color Tolerance

Silver ±10%
Gold ±5%
Brown ±1%
Red ±2%
Orange ±3%
Green ±0.5%
Blue ±0.25%
Violet ±0.1%

1

2 Multiplier Tolerance

Multiplier

For the No. 0 1 2 3 4 5 8 9
Multiply By 1 10 100 1k 10k 100k .01 0.1

Section 9

-3-

FM RF

AMPLIFIER

FM

OSCILLATOR

1ST FM IF

AMPLIFIER

AFC

Figure 1

Section 8

Section 7 Section 6

Section 1Section 5 Section 4

Section 3 Section 2

FM MIXER

2ND FM IF

AMPLIFIER

FM

DETECTOR

AM MIXER

AM

OSCILLATOR

1ST AM IF

AMPLIFIER

2ND AM IF

AMPLIFIER

AM

DETECTOR

AGC

AUDIO

AMPLIFIER

Speaker

FM RADIO

AM RADIO

The purpose of Section 1, the Audio Amplifier Stage, is to
increase the power of the audio signal received from either
detector to a power level capable of driving the speaker.
Section 2 includes the AM detector circuit and the AGC
(automatic gain control) stage. The AM detector converts the
amplitude modulated IF (intermediate frequency) signal to a
low level audio signal. The AGC stage feeds back a DC
voltage to the first AM IF amplifier in order to maintain a near
constant level of audio at the detector . Section 3 is the second
AM IF amplifier. The second AM IF amplifier is tuned to
455kHz (Kilohertz) and has a fixed gain at this frequency of

50. Section 4 is the first AM IF 2 amplifier which has a
variable gain that depends on the AGC voltage received from
the AGC stage. The first AM IF amplifier is also tuned to
455kHz. Section 5 includes the AM mixer, AM oscillator and
AM antenna stages. When the radio w a ve passes through the
antenna, it induces a small voltage across the antenna coil.
This voltage is coupled to the mixer, or converter, stage to be
changed to a frequency of 455kHz. This change is
accomplished by mixing (heterodyning) the radio frequency
signal with the oscillator signal. Section 6 is the FM ratio

detector circuit. The FM ratio detector has a fixed gain of
about 20. Section 7 is the second FM IF amplifier. The second
FM IF amplifier is tuned to 10.7MHz (Megahertz) and has a
set gain of approximately 20. The 3dB bandwidth of this stage
should be approximately 350kHz. Section 8 is the first FM IF
amplifier. The first FM IF amplifier is also tuned to 10.7MHz
and has a set gain of approximately 10. It also has a 3dB
bandwidth of 350kHz. Section 9 includes the FM mixer, FM
oscillator, FM RF (Radio Frequency) amplifier, AFC
(Automatic Frequency Control) stage, and the FM antenna.
The incoming radio waves are amplified by the FM RF
amplifier, which is tuned to a desired radio station in the FM
frequency bandwidth of 88MHz to 108MHz. These amplified
signals are then coupled to the FM mixer stage to be
changed to a frequency of 10.7MHz. This change, as in AM,
is accomplished by heterodyning the radio frequency signal
with the oscillator signal. The AFC stage feeds back a DC
voltage to the FM oscillator to prevent the oscillator from
drifting. Each of these blocks will be explained in detail in the
Theory of Operation given before the assembly instructions
for that stage.

GENERAL DISCUSSION

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 ver y important, however, that good soldering
practices are used to prevent bad connections . The Soldering
Guide should be reviewed before any soldering is attempted.

The actual assembly is broken down into 9 sections. The
theory of operation for each section, or stage, should be read
before the assembly is started. This will provide 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.

-4-

CONSTRUCTION

Solder

Soldering Iron

Foil

Solder

Soldering Iron

Foil

Component Lead

Soldering Iron

Circuit Board

Foil

Rosin

Soldering iron positioned
incorrectly.

Solder

Gap

Component Lead

Solder

Soldering Iron

Drag

Foil

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 allo ws 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.

1. Insufficient heat - the solder will
not flow onto the lead as shown.

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

4.

Here is what a good solder
connection looks like.

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 terminals.

4. Solder bridges - occur when
solder runs between circuit paths
and creates a short circuit. This is
usually caused by using too much
solder.
To correct this, simply drag your
soldering iron across the solder
bridge as shown.

What Good Soldering Looks Like

A good solder connection should be bright, shiny, smooth, and uniformly
flowed over all surfaces.

Types of Poor Soldering Connections

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.

Solder

For many years leaded solder was the most common type of solder
used by the electronics industry, but it is now being replaced by lead­free solder for health reasons. This kit contains lead-free solder, which
contains 99.3% tin, 0.7% copper, and has a rosin-flux core.

Lead-free solder is different from lead solder: It has a higher melting
point than lead solder, so you need higher temperature for the solder to
flow properly. Recommended tip temperature is approximately 700

O

F;
higher temperatures improve solder flow but accelerate tip decay. An
increase in soldering time may be required to achieve good results.
Soldering iron tips wear out faster since lead-free solders are more
corrosive and the higher soldering temperatures accelerate corrosion,
so proper tip care is important. The solder joint finish will look slightly
duller with lead-free solders.

Use these procedures to increase the life of your soldering iron tip when
using lead-free solder:

• Keep the iron tinned at all times.

• Use the correct tip size for best heat transfer. The conical tip is the
most commonly used.

• Turn off iron when not in use or reduce temperature setting when
using a soldering station.

•

Tips should be cleaned frequently to remove oxidation before it becomes
impossible to remove. Use Dry Tip Cleaner (Elenco

®

#SH-1025) or Tip
Cleaner (Elenco®#TTC1). If you use a sponge to clean your tip, then use
distilled water (tap water has impurities that accelerate corrosion).

Safety Procedures

• Always wear safety glasses or safety goggles to

protect your eyes when working with tools or
soldering iron, and during all phases of testing.

• Be sure there is adequate ventilation when soldering.

•

Locate soldering iron in an area where you do not have to go around
it or reach over it. Keep it in a safe area away from the reach of
children.

• Do not hold solder in your mouth. Solder is a toxic substance.
Wash hands thoroughly after handling solder.

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.

DO NOT USE ACID CORE SOLDER!

'

-5-

SEMICONDUCTOR PARTS FAMILIARIZATION

This section will familiarize you with the proper method used to test the transistors and the diode.

TRANSISTOR TEST

TEST C

TEST D

High Resistance

Diode

Low Resistance

Diode

Ω

Ω

COM

Ω

Ω

COM

TEST A

TEST B

Low Resistance

NPN

EBC

High Resistance

NPN

EBC

Ω

Ω

COM

Ω

Ω

COM

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 sho wn
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.

DIODE TEST

Refer to the parts list and find a diode. This is a
silicon 1N4148 diode. Refer to Test C for locating the
Cathode and Anode. The end with the band is the
cathode. Using an Ohmmeter, connect the diode as
shown in Test C. Your meter should be reading a low

resistance. Using an Ohmmeter , connect the diode as
shown in Test D. Your meter should be reading a high
resistance. Typical results read approximately 1MΩ to
infinity.

-6-

SECTION 1

AUDIO AMPLIFIER

Figure 3

The purpose of the Audio Amplifier is to increase the
audio power to a lev el sufficient to drive an 8 ohm speak er .
To do this, DC (direct current) from the battery is
converted by the amplifier to an AC (alternating current) in
the speaker. The ratio of the power delivered to the
speaker and the power taken from the battery is the
efficiency of the amplifier. For the Audio Amplifier, we use
the integrated circuit (IC) LM-386. In Figure 2, you can
see equivalent schematic and connection diagrams.
In a Class A amplifier (transistor on over entire cycle), the
maximum theoretical efficiency is .5 or 50%. But, in a
Class B amplifier (transistor on for 1/2 cycle), the
maximum theoretical efficiency is .785 or 78.5%. Since
transistor characteristics are not ideal in a pure Class B
amplifier, the transistors will introduce crossover
distortion. This is due to the non-linear transf er 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 lev els of pow er to the speaker , b ut
they slide toward Class B operation at larger po w er 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 1.35kΩ 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.

The amplifier in our kit with a gain of 150 is shown in
Figure 5. Capacitor C40 couples the audio signal from the
volume control to the input of the audio amplifier.
Capacitor C43 blocks the DC to the speaker, while
allowing the AC to pass.

Figure 2

Figure 4a

Figure 4c

Figure 4b

Figure 5

Typical Applications

Amplifier with Gain = 20

Minimum Parts

VIN

VS

2

6

1

8

5

7

4

LM386

+

+

–

.05μF

10Ω

10kΩ

Amplifier with Gain = 150

Amplifier with Gain = 200

3

V

IN

VS

2

6

1

8

5

7

4

LM386

+

–

10kΩ

3

+

.05μF

10Ω

BYPASS

+

10μF

VIN

VS

2

6

1

8

5

7

4

LM386

+

–

10kΩ

3

.05μF

10Ω

BYPASS

47Ω

10μF

+

+

Equivalent Schematic and Connection Diagrams

VOUT

VS

6

5

7

4

15kΩ

BYPASS

GND

15kΩ

2

– INPUT

150Ω

1.35kΩ

8

GAIN

1

GAIN

15kΩ

50kΩ

50kΩ

+ INPUT

Dual-In-Line and Small Outline Packages

Top Vie w

GAIN

– INPUT

+ INPUT

GND

GAIN

BYPASS

V

S

VOUT

4

1

2

3

5

8

7

6

3

Diode

Test Point Pin

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.

Wear safety goggles during all assembly stages in this manual.

Foil side of PC board

Figure A

NPN T ransistor

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.

Figure D

-7-

EBC

E

B

C

Flat side

Band

CathodeAnode

Integrated Circuit

C39 - 470μF Lytic

(see Figure Ba)

Mount on copper side.

C40 - 10μF Lytic

(see Figure B)

C41 - 10μF Lytic

(see Figure B)

C43 - 470μF Lytic

(see Figure B)
C44 - .047μF Discap (473)
TP1 - Test Point Pin

(see Figure A)
R45 - 10Ω Resistor

(brown-black-black-gold)

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

TP2 - Test Point Pin

(see Figure A)

C42 - 10μF Lytic

(see Figure B)

R44 - 47Ω Resistor

(yellow-violet-black-gold)

U1 - IC Socket 8-pin
U1 -

LM386 Integrated Circuit

(see Figure E)

1/8”

Notch

J3 - Jumper Wire

(use a discarded lead)

TP-15 - Test Point Pin

(see Figure A)

Lytic Capacitor

Figure B

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

Warning:

If the capacitor is connected with
incorrect polarity, it may heat up
and either leak, or cause the
capacitor to explode.

Figure Ba

Polarity
Mark

+

–

Be sure that the band is in
the same direction as
marked on the PC board.

Figure E

r Inser t the IC socket into the

PC board with the notch in
the direction shown on the
top legend. Solder the IC
socket into place.

r Inser t the IC into the socket

with the notch in the same
direction as the notch on the
socket.

'

Polarity
mark

(–)

(+)

0.1μF Capacitor

Pin 2

r C45 - Solder the 0.1μF

capacitor across pins 2 & 6
of IC U1 as shown. The
capacitor prevents the IC
from oscillating.

Pin 6

-8-

ASSEMBLY INSTRUCTIONS

Figure F

Knob
Nut

Washer

Cut off
locating pin

Solder all 5 tabs to PC board

Plastic Washer

Figure H

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.

Part # 622131

1 - GND
2 - Tip
3 - N.C. Tip

1

3

2

Your kit may contain a different type of earphone jack. Before installing
the jack, determine which one you have.

Nut

Figure I

GND

Pad

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.

***

Part # 622130

Part # 622131

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.

Pad

Speaker

Backing

Remove

Battery Holder
3 Screws M1.8 x 7.5
3 Nuts M1.8
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 #22AWG Insulated

(see Figures G & I)

Backing

Foil Side

Foil Side

1

Part # 622130

1 - GND
2 - Tip
3 - N.C. Tip

2
3

Nut

GND

Pad

Note: Mount the Pot/SW, earphone
jack, and speaker to the foil side of the
PC board.

2” Wire

1.5” Wire

1” Wire

1” Wire

From Terminal 3

2” Wire

1.5” Wire

-9-

OUTPUT BIAS TEST

Put the battery into the holder.

STATIC MEASUREMENTS

POWER TEST

TP15

COM

V

V

For all measurements, connect y our equipment GND
to circuit GND TP15. Set your VOM (Volt-Ohm­Millimeter) to read 2 amps DC. Connect the meter to
the circuit as shown in Figure 6. Make sure that the
volume control is in the OFF position (turned fully
counter-clockwise). While watching your VOM, turn
the volume to the ON position (rotate clockwise until
a “click” is heard). The VOM should indicate a very
low current. Adjust your meter for a more accurate

reading if necessary. If the current is greater than 20
milliamps, immediately turn the power off. The
current should be less than 10 milliamps. This is the
current drawn by the battery when no input signal is
present (the “idle current”). Turn OFF the power. If
your circuit fails this test, check that all of the parts
have been installed correctly, and check for shorts or
poor solder connections.

Amps

Amps

COM

–

+

+

–

Figure 6

Figure 7

Adjust your VOM to read 9 volts and connect it as
shown in Figure 7. Make sure that the batter y, or a 9
volt power supply (if a v ailable), is properly connected
and turn the power ON. The voltage at TP1 should be
between 3 to 6 volts. If you get this reading, go on to
the next test. If your circuit fails this test, turn the
power OFF and check that the integrated circuit is

correctly inserted in the correct location. The notch of
the IC must be in the same direction as marked on
the PC board. Check that all resistor values are the
correct value and not interchanged. All static tests
must pass before proceeding to the Dynamic Tests or
the next section.

DYNAMIC MEASUREMENTS

GAIN

-10-

Figure 8

TP15

COM

V

V

TP15

Hz

Generator

If you do not have an audio generator, skip the following test and go directly to Section 2.

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 generator at this setting and move the positive

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 A C
output voltage divided by the AC input voltage, or
1/Vin.

Calculate the gain. The gain should be 100–180.

Gain = _________

-11-

Figure 9

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 wavefor m on the oscilloscope drops to .7 of its
original reading 1.4Vpp or 2.8 divisions. The
frequency of the generator when the output drops to
.7 of its original value is called the high frequency 3
decibel (dB) corner. Record this frequency here:

(f high 3dB) = __________ kHz.

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

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 sine w av e ma y clip bef ore
the other depending on the DC centering at TP1. If
oscillations are seen, connect a clip lead from the
GND of your generator to the GND of the circuit.

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

Vclp = _______ Vpp.

Turn the power OFF.

Battery

Generator

Hz

TP15

TP15

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

MAXIMUM POWER OUTPUT

The 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
Max power out = (Vrms)

2

/8 ohms = (Vclp x .35)2/8
Maximum power output should be greater than 200
milliwatts.

EFFICIENCY

-12-

Figure 10

A

B

Clipped

Crossover
Distortion

By measuring the DC power taken from the battery
at the maximum power output level, the efficiency to
the audio amplifier can be calculated. Power from
the battery is equal to the current taken from the
battery times the voltage of the battery during
maximum power output. Efficiency can then be
calculated as follows: Eff = Max audio power/Battery
power. It is best to use a power supply (if available)
to prevent supply v oltage 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. Tur n the
power ON and rotate the volume control to
maximum. Slowly increase the amplitude of the
audio generator until the output is clipped as shown
in Figure 10A.

Record Vclp here:

Vclp = _________ Vpp.

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

Current (I) max = ________ A.

Measure the supply voltage and record the V supply
here:

V supply = ________ volts.
Turn the power OFF.
Calculate the maximum power output as done in the

Maximum Power Output Step.

Record your answers on page 13.

-13-

Vp = Vclp/2 Vp = ______
Vrms = Vp x .7 Vr ms = ______
Max power out = (Vrms)

2

/8 Max power out = ______

Since the battery power equals the battery voltage times the current taken from the battery; calculate the battery
power:

Battery power = Imax x V supply Battery power = ______

Since the efficiency (N) is equal to the Max power out divided by the Battery power, we can now calculate the
efficiency of the audio amplifier.

N = Max power out/Battery power N = _______
N in % = N x 100 N = _______%

Your calculated answer should be around .6 or 60%.

Figure 11

Generator

Hz

TP15

Amps

Amps COM

Power Supply

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

TP15

SECTION 2

AM DETECTOR AND AGC STAGE

-14-

Figure 12

Switch
J2 - Jumper Wire

(use a discarded lead)

The purpose of the automatic gain control (AGC)
circuit is to maintain a constant level at the detector,
regardless of the strength of the incoming signal.
Without AGC, the volume control would have to be
adjusted for each station and ev en moderately strong
stations would clip in the final IF amplifier causing
audio distortion. AGC is accomplished by adjusting
the DC bias of the first IF amplifier to lower its gain
as the signal strength increases. Figure 12 shows
that the audio at the top of the volume control is
actually “riding” on a negative DC voltage when

strong signals are encountered. This negative DC
component corresponds to the strength of the
incoming signal. The larger the signal, the more
negative the component. At test point five (TP5), the
audio is removed by a low pass filter, R36 and C32,
leaving only the DC component. Resistor R35 is
used to shift the voltage at TP5 high enough to bias
the base of transistor Q8 to the full gain position
when no signal is present. Resistors R35 and R36
also forward bias diode D4 just enough to minimize
“On Condition” threshold voltage.

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.

ASSEMBLY INSTRUCTIONS

1/8”

STATIC MEASUREMENTS

AGC ZERO SIGNAL BIAS

With the power turned OFF, connect your V OM to TP5 as shown in Figure 13. Make sure that the AM/FM switch
is in the AM position.

Check that the VOM is adjusted to
read 9 volts DC and turn the power
ON. The voltmeter should read
approximately 1.5 volts DC. If your
reading varies by more than .5 volts
from this value, turn the power OFF
and check the polarity of D4. Also
check R36 and R35 and check that
transformer T6 is properly installed.

T8 TEST

-15-

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)

TP15

V

COM V

If you do not have an RF generator, skip to Section 3.

With the power turned OFF, connect the positive lead
of the VOM to TP3 and the negative lead to ground
pin TP15. Make sure that the VOM is set to read 9
volts DC and turn the power ON. The voltage on the

VOM should be the same as your battery voltage or
power supply voltage . If not, turn the power OFF and
check that T8 is properly installed. Tur n the power
OFF.

Figure 13

DYNAMIC MEASUREMENTS

AM DETECTOR AND AGC TEST

Connect your VOM and RF generator as shown in Figure 14.

-16-

Figure 15

TP15

GENERATOR

Hz

TP15

.001μF

If your RF generator does not have amplitude modulation and

you do not have an oscilloscope, skip to Section 3.

TP15

.001μF

TP15

COM

V

V

GENERATOR

Hz

Figure 14

Set the VOM to accurately read 2 volts DC and set
the output of the RF generator for 455kHz, no
modulation, and minimum voltage output. Turn the
power ON and slowly increase the amplitude of the

generator until the voltage 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: ____________.

Set the RF generator at 455kHz, 1kHz at 80%
modulation and minimum voltage output. Turn the
power ON and set the volume control at maximum.
Slowly adjust the amplitude of the RF generator

output until you hear the 1kHz tone on the speaker. If
this test fails, turn the power OFF and chec k R42 and
D4. Turn the power OFF.

SYSTEM CHECK

Connect your equipment as shown in Figure 15.