Elenco AM Radio Kit User Manual

AM RADIO KIT

MODEL AM-550K

INTEGRAL CIRCUIT, 3 TRANSISTORS, DIODE

Assembly and Instruction Manual

Copyright © 2007, 1999 Elenco®Electronics, Inc. Revised 2007 REV-S 752550

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

Elenco®Electronics, Inc.

-1-

PARTS LIST

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 #

1 R14 10Ω 5% 1/4W brown-black-black-gold 121000
1 R13 47Ω 5% 1/4W yellow-violet-black-gold 124700
1 R8 100Ω 5% 1/4W brown-black-brown-gold 131000
1 R10 470Ω 5% 1/4W yellow-violet-brown-gold 134700
1R6 1kΩ 5% 1/4W brown-black-red-gold 141000
1 R12 2.2kΩ 5% 1/4W red-red-red-gold 142200
2 R3, R11 3.3kΩ 5% 1/4W orange-orange-red-gold 143300
1 R9 10kΩ 5% 1/4W brown-black-orange-gold 151000
1 R2 12kΩ 5% 1/4W brown-red-orange-gold 151200
1 R5 27kΩ 5% 1/4W red-violet-orange-gold 152700
1 R7 39kΩ 5% 1/4W orange-white-orange-gold 153900
1 R1 56kΩ 5% 1/4W green-blue-orange-gold 155600
1R4 1MΩ 5% 1/4W brown-black-green-gold 171000
1 Pot/SW1 50kΩ (nut & washer) 192522

CAPACITORS

Qty. Symbol Value Description Part #

1 C1 Variable Tuning 211677
1 C15 .001μF Discap (102) 231036
2 C3, C10 .01μF Discap (103) 241031
5 C2, C5, C7, C8, C9 .02μF or .022μF Discap (203) or (223) 242010
1 C16 .047μF Discap (473) 244780
3 C4, C11, C12 10μF Electrolytic Radial (Lytic Capacitor) 271045
1 C6 100μF Electrolytic Radial (Lytic Capacitor) 281044
2 C13, C14 470μF Electrolytic Radial (Lytic Capacitor) 284743

SEMICONDUCTORS

Qty. Symbol Description Part #

1 D1 1N4148 Diode 314148
3 Q1, Q2, Q3 2N3904 Transistor NPN 323904
1 U1 LM-386 Integrated Circuit 330386

COILS

Qty. Symbol Description Part #

1 L2 Oscillator (red dot) 430057
1 T1 IF (yellow dot) 430260
1 T2 IF (white dot) 430262
1 T3 Detector (black dot) 430264
1 L1 Antenna with Holders 484004

MISCELLANEOUS

Qty. Description Part #

1 PC Board 517037
1 Battery Holder 590096
1 Speaker 590102
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
1 Screw 2-56 x 5/16” 641231

Qty. Description Part #

3 Screw M2.5 x 3.8mm 641310
4 Nut 2-56 644201
1 IC Socket 8-Pin 664008
8 Test Point Pin 665008
1 Label, Dial Knob 720421
1 Manual 752550
1 Speaker Pad 780128
1 Wire 4” 814920
1 Solder Lead-Free 9LF99

Punch out one antenna shim from the front flap of the box.

**** SAVE THE BOX THAT THIS KIT CAME IN. IT WILL BE USED ON PAGES 24 & 29. ****

PARTS IDENTIFICATION

-2-

RESISTORS CAPACITORS SEMICONDUCTORS

Resistor

50kΩ

Potentiometer

with Switch

Discap

Electrolytic

Radial

Tuning

Diode

Transistor

LM-386 IC

Antenna Assembly

Coil

Coil

Plastic Holders

Ferrite Core

Color Dot

Knob (pot)

Earphone Jack with Nut

Knob (dial)

Radio Stand

COILS

MISCELLANEOUS

Label, Dial Knob

Battery

Holder

Earphone

OR

Speaker PadSpeaker

IC Socket

Test

Point Pin

Screw

2-56 x 1/4”

Screw

M2.5 x 3.8mm

Nut

2-56

Screw

2-56 x 5/16”

-3-

IDENTIFYING RESISTOR VALUES

Use the following information as a guide in properly 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

-4-

Figure 1

Section 4

Section 3 Section 2 Section 1Section 5

FIRST

IF AMPLIFIER

SECOND

IF AMPLIFIER

DETECTOR

AUDIO

AMPLIFIER

MIXER

LOCAL

OSCILLATOR

AGC

Speaker

INTRODUCTION

The Elenco®Superhet 550 AM Radio Kit is a
“superheterodyne” receiver of the standard AM
(amplitude modulation) broadcast frequencies. The
unique design of the Superhet 550 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 five
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 550’s reception capabilities.

The Superhet 550 can best be understood by
analysis of the block diagram shown in Figure 1.
The purpose of section 1, the Audio Amplifier Stage,
is to increase the power of the audio signal received
from the detector to a power level capable of driving
the speaker. Section 2 includes the detector circuit
and the AGC (automatic gain control) circuit. The
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 IF amplifier in order to maintain a near
constant level of audio at the detector. Section 3 is
the second IF amplifier. The second IF amplifier is
tuned to 455kHz (Kilohertz) and has a fixed gain at
this frequency of 100. The 3dB bandwidth of this

stage should be approximately 6kHz. Section 4 is
the first IF amplifier which has a variable gain that
depends on the AGC voltage received from the AGC
stage. The first IF amplifier is also tuned to 455kHz
and has a 3dB bandwidth of approximately 6kHz.
Section 5 includes the mixer, oscillator and antenna
stages. When the radio wave passes through the
antenna, it induces a small voltage across the
antenna coil. This voltage is coupled to the mixer, or
converter, stage to be changed to a frequency of
455kHz. This change is accomplished by mixing
(heterodyning) the radio frequency signal with the
oscillator signal. Each of these blocks will be
explained in detail in the Theory of Operation given
before the assembly instructions for that stage.

GENERAL DISCUSSION

-5-

CONSTRUCTION

Introduction

Assembly of your AM-550 AM Radio Kit will prove to be an exciting project and give you much
satisfaction and personal achievement. If you have experience in soldering and wiring
techniques, then you should have no problem with the assembly of this kit. Care must be given
to identifying the proper components and in good soldering habits. Above all, take your time
and follow these easy step-by-step instructions. Remember, “An ounce of prevention is worth
a pound of cure”. Avoid making mistakes and no problems will occur.

Safety Procedures

• Wear eye protection when soldering and during all phases of construction.

• 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. 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 and the board is turned to solder the
component leads on the foil side. Solder immediately unless the pad is adjacent to another
hole which will interfere with the placement of the other component. Cut excessive leads with
a diagonal cutter. Then, place a check mark in the box provided next to each step to indicate
that the step is completed. Be sure to save the extra leads for use as jumper wires if needed.

Soldering

The most important factor in assembling your AM 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.
Many areas on the PC board are close together and care must be given not to form solder
shorts. Size and care of the tip will eliminate problems.
For a good soldering job, the areas being soldered must be heated sufficiently so that the
solder flows freely. Apply the solder simultaneously to the component lead and the component
pad on the PC board so that good solder flow will occur. Be sure that the lead extends through
the solder smoothly indicating a good solder joint. Use only rosin core solder.
DO NOT USE ACID CORE SOLDER! Do not blob the solder over the lead because this can
result in a cold solder joint.

Heat Sinking

Electronic components such as transistors,
IC’s, and diodes can be damaged by the heat
during soldering. Heat sinking is a way of
reducing the heat on the components while
soldering. Dissipating the heat can be
achieved by using long nose pliers, an alligator
clip, or a special heat dissipating clip. The heat
sink should be held on the component lead
between the part and the solder joint.

Mount Part

Bend Leads to Hold Part Solder and Cut Off Leads

Foil Side

Rx - 100Ω 5% 1/4W Resistor

(brown-black-brown-gold)

Heat Sink

(this can be ordered as part of Elenco’s Solder Ease

Kit Model SE-1).

Soldering Iron

Solder

Heat Sensitive
Component (Diode)

PC Board

-6-

What Good Soldering Looks Like

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

Soldering a PC board

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. The solder should have flowed
smoothly and not lump around the wire lead.

4.

Here is what a good solder connection looks like.

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 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.

A poorly soldered joint can greatly affect small current flow in circuits and can cause equipment failure. You can
damage a PC board or a component with too much heat or cause a cold solder joint with insufficient heat.
Sloppy soldering can cause bridges between two adjacent foils preventing the circuit from functioning.

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

-7-

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

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.

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.

-8-

SECTION 1

AUDIO AMPLIFIER

The purpose of the Audio Amplifier is to increase the audio power to a level sufficient to drive an 8 ohm speaker. To do this,
DC (direct current) from the battery is converted by the amplifier to an AC (alternating current) in the speaker. The ratio of
the power delivered to the speaker and the power taken from the battery is the efficiency of the amplifier. For the Audio
Amplifier, we use the integrated circuit (IC) LM-386. In Figure 2, you can see equivalent schematic and connection diagrams.
In a Class A amplifier (transistor on over entire cycle), the maximum theoretical efficiency is .5 or 50%. But, in a Class B

amplifier (transistor on for 1/2 cycle), the maximum theoretical efficiency is .785 or 78.5%. Since transistor characteristics are
not ideal in a pure Class B amplifier, the transistors will introduce crossover distortion. This is due to the non-linear transfer
curve near zero current or cutoff. This type of distortion is shown in Figure 3.

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 1.35kΩ resistor sets
the gain at 20 (see Figure 4a). The gain will go up to 200 (see Figure 4b)
if a resistor is placed in series with the capacitor. The gain can be set to
any value from 20 to 200. The amplifier with a gain of 50 is shown in
Figure 4c.

The amplifier in our kit with a gain of 150 is shown in Figure 5. Capacitor C11
couples the audio signal from the volume control to the input of the audio
amplifier. Capacitor C13 blocks the DC to the speaker, while allowing the AC to pass.

Figure 3

Figure 4a

Figure 4b

Figure 4c

Figure 2

Figure 5

from detector

C15

C11

C14

9V

R13

C12

C16

R14

C13

TP7

TP6

U1

6

1

8

5

7

4

2

3

-9-

ASSEMBLY INSTRUCTIONS

We will begin by installing the speaker. Be careful to properly mount and solder all components. Diodes 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.

Cut two 1 1/2” wires and one 1” wire and strip 1/4” of insulation off
of both ends. Solder the wires in the locations shown.

Figure D

1 ½”

Wires

Figure C

Foil Side

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.

1” Wire

1

Part # 622131

Part # 622130

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

1 ½”

Wires

1” Wire

1

3

2

Jack

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

Your kit may contain a different type of earphone jack. Before installing
the jack, determine which one you have. Solder the GND terminal to the
PC board pad.

GND Pad

Nut

Jack

2

3

Nut

GND

Pad

From Terminal 3

Figure B

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

Backing

J1 - Earphone Jack
with Nut

(see Figure C)

SP1 - 8Ω Speaker
Speaker Pad
Wire 4”

(see Figures B & D)

Battery Holder
1 Screw 2-56 x 5/16”
2 Screw 2-56 x 1/4”
3 Nuts 2-56
Solder and cut off
excess leads.

Part # 622131

Part # 622130