Elenco Electronics SUPERHET AM-550TK Assembly And Instruction Manual

AM RADIO KIT

MODEL

SUPERHET AM-550TK

7 TRANSISTORS

Assembly and Instruction Manual

ElencoTMElectronics, Inc.

Copyright © 2003, 1999 ElencoTMElectronics, Inc. Revised 2003 REV-D 752550T

PARTS LIST

If you are a student, and any parts are missing or damaged, please see instructor or bookstore.
If you purchased this AM radio kit from a distributor, catalog, etc., please contact Elenco
(address/phone/e-mail is at the back of this manual) for additional assistance, if needed.

RESISTORS

Qty. Symbol Value Color Code Part #

1 R19 1W 1/4W 5% brown-black-gold-gold 111000
3 R8, R15, R17 100W 1/4W 5% brown-black-brown-gold 131000
1 R10 470W 1/4W 5% yellow-violet-brown-gold 134700
1 R18 820W 1/4W 5% gray-red-brown-gold 138200
2 R6, R16 1kW 1/4W 5% brown-black-red-gold 141000
1 R12 2.2kW 1/4W 5% red-red-red-gold 142200
2 R3, R11 3.3kW 1/4W 5% orange-orange-red-gold 143300
1 R9 10kW 1/4W 5% brown-black-orange-gold 151000
1 R2 12kW 1/4W 5% brown-red-orange-gold 151200
1 R5 27kW 1/4W 5% red-violet-orange-gold 152700
1 R7 39kW 1/4W 5% orange-white-orange-gold 153900
1 R14 47kW 1/4W 5% yellow-violet-orange-gold 154700
1 R1 56kW 1/4W 5% green-blue-orange-gold 155600
1 R13 82kW 1/4W 5% gray-red-orange-gold 158200
1 R4 1MW 1/4W 5% brown-black-green-gold 171000
1 Pot/SW1 50kW / SW Pot/SW with Nut and Washer 192522

CAPACITORS

Qty. Symbol Value Description Part #

1 C1 Variable Tuning 211677
1 C15 .001mF Discap (102) 231036
2 C3, C10 .01mF Discap (103) 241031
5 C2, C5, C7, C8, C9 .02mF or .022mF Discap (203) or (223) 242010
2 C4, C11 10mF Electrolytic 271045
1 C12 47mF Electrolytic 274742
1 C6 100mF Electrolytic 281044
2 C13, C14 470mF Electrolytic 284743

SEMICONDUCTORS

Qty. Symbol Description Part #

2 D1, D2 1N4148 Diode 314148
4 Q1, Q2, Q3, Q4 2N3904 Transistor NPN 323904
1 Q5 2N3906 Transistor PNP 323906
1 Q6 MPS6560 or 8050 Transistor NPN 328050
1 Q7 MPS6562 or 8550 Transistor PNP 328550

TM

Electronics

Qty. Symbol Value 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

Qty. Description Part #

1 PC Board 517040
1 Battery Holder 590096
1 Speaker 590102
1 Speaker Pad 780128
1 Knob (pot) 622017
1 Knob (dial) 622030
1 Earphone Jack w/ Nut 622130 or 622131
1 Radio Stand 626100

**** SAVE THE BOX THAT THIS KIT CAME IN. IT WILL BE USED ON PAGE 25. ****

COILS

MISCELLANEOUS

Qty. Description Part #

1 Earphone 629250
3 Screw 2-56 x 1/4” 641230
3 Screw M2.5 641310
3 Nut 2-56 644201
8 Test Pin 665008
1 Label, Dial Knob 720421
1 Wire 4” 814920
1 Solder 9ST4

-1-

PARTS IDENTIFICATION

RESISTORS CAPACITORS SEMICONDUCTORS

Resistor

Potentiometer/

Color Dot

50kW

Switch

with Nut and

Washer

Coil

Discap

Electrolytic

Radial

COILS

MISCELLANEOUS

Diode

Tuning

Transistor

Coil

Ferrite Core

Plastic Holders

Antenna Assembly

Knob (dial)

Speaker

Screw

M2.5

2-56 x 1/4”

Screw

OR

Earphone Jack with Nut

Speaker Pad

Knob (pot)

Nut

2-56

Earphone

Test Pin

Battery

Holder

Radio Stand

Label

-2-

IDENTIFYING RESISTOR VALUES

Use 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 +1%
Red +2%
Orange +
Green +.5%
Blue +.25%
Violet +

5%

3%

.1%

IDENTIFYING CAPACITOR VALUES

Capacitors will be identified by their capacitance value in pF (picofarads), nF (nanofarads), or mF (microfarads). Most
capacitors will have their actual value printed on them. Some capacitors may have their value printed in the following
manner. The maximum operating voltage may also be printed on the capacitor.

Multiplier

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

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

For the No.01234589
Multiply By 1 10 100 1k 10k 100k .01 0.1

First Digit
Second Digit
Multiplier

103K

100

Tolerance

Maximum Working Voltage

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

-3-

INTRODUCTION

The Elenco Superhet 550T AM Radio is a
“superheterodyne” receiver of the standard AM
(amplitude modulated) broadcast frequencies. The
unique design of the Superhet 550T allows you to
place the parts over its 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
assembly is attempted.

The actual assembly is broken down into five
sections. The theory of operation for each section, or

GENERAL DISCUSSION

The Superhet 550T 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 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 550T’s reception capabilities.

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.

Antenna

MIXER

LOCAL

OSCILLATOR

FIRST

IF AMPLIFIER

SECOND

IF AMPLIFIER

Figure 1

-4-

DETECTOR

AGC

Section 1Section 2Section 3Section 4Section 5

Speaker

AUDIO

AMPLIFIER

CONSTRUCTION

Introduction

The most important factor in assembling your Elenco Superhet 550T AM Transistor 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. The
solder should have flowed
smoothly and not lump
around the wire lead.

4.

Here is what a good solder
connection looks like.

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
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.

Rosin

Soldering iron positioned
incorrectly.

Solder

Component Lead

Solder

Soldering Iron

Foil

Gap

Drag

-5-

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 a NPN transistor. Refer
the Figure C (page 8) 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.

Refer to parts list and find a PNP transistor, refer to
Figure D (page 8) for locating the Emitter, Base and
Collector. Using an Ohmmeter, connect the transistor
as shown in Test C. Your meter should be reading a low
resistance. Switch the lead from the Emitter to the
Collector. Your meter should again be reading a low
resistance.

Using an Ohmmeter, connect the transistor as shown in
Test B. Your meter should be reading a high resistance.
Switch the lead from the Emitter to the Collector. Your
meter should again be reading a high resistance.
Typical results read approximately 1MW to infinity.

Low Resistance

W

COM

NPN

W

EBC

COM

High Resistance

W

W

NPN

EBC

TEST A TEST B TEST C TEST D

Using an Ohmmeter, connect the transistor as shown in
Test D. 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.

Low Resistance

W

COM

W

PNP

EBC

COM

High Resistance

W

PNP

W

EBC

DIODE TEST

Refer to the parts list and find a diode. Refer to Figure E
(page 8) for locating the Cathode and Anode.

The end
with the band is the cathode. Using an Ohmmeter,
connect the diode as shown in Test E. Your meter
should be reading a low resistance. Using an

Low Resistance

W

COM

W

Diode

TEST E TEST F

Ohmmeter, connect the diode as shown in Test F. Your
meter should be reading a high resistance. Typical
results read approximately 1MW to infinity for silicon
diodes (1N4148).

High Resistance

W

COM

W

Diode

-6-

SECTION 1

AUDIO AMPLIFIER

Theory of Operation - 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. 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
distortion is shown in Figure 2.
In order to eliminate crossover distortion and
maximize efficiency, the transistors (Q6 and Q7) 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.

Transistor Q4 is a Class A amplifier that drives the
base of transistor Q5 directly. Q5 is a current
amplifier that multiplies the collector current of Q4 by
the beta (current gain, B) of Q5. The current from Q5
drives the output transistors Q6 and Q7 through the
bias string R17, D2 and R18. Bias stability is
achieved by using 100% DC feedback from the output
stage to the emitter of Q4 through resistor R16. This
gives the Audio Amplifier a DC gain of one. The AC
gain is set by resistors R16, R15 and capacitor C12.
In this circuit, the value of R16 is 1000 ohms and R15
is 100 ohms. Their ratio is 10 to 1, therefore the AC
gain of the amplifier is 10 times. Resistors R13 and
R14 set the DC voltage at the base of Q4 to
approximately 5.2V. The emitter of Q4 is set at 4.5V,
which is the same voltage at this output to the
speaker. Note that this voltage is 1/2 the battery
voltage. Capacitor C11 AC 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 2

-7-

ASSEMBLY INSTRUCTIONS - AUDIO AMPLIFIER

We will begin by installing resistor R14. Identify the resistor by its color code and install as shown on page 3. 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

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

Figure A

Diode

Be sure that the
band is in the
correct direction.

R14 - 47kW Resistor

(yellow-violet-orange-gold)

Q4 -

2N3904 Transistor NPN

(see Figure C)

TP6 - Test Point Pin

(see Figure A)

C11 - 10mF Lytic

(see Figure B)

C14 - 470mF Lytic

(see Figure B)

R13 - 82kW Resistor

(gray-red-orange-gold)

Pot / SW1 with
Nut and Washer
Knob (pot)

Top Side

Solder 5 lugs
to PC board

C12 - 47mF Lytic

(see Figure B)

Lytic Capacitor

Polarity Mark

Figure B

Figure E

( )

Band

CathodeAnode

(+)

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.

PNP Transistor

Flat
Side

EBC

E

B

Mount so E lead is
in the arrow hole
and flat side is in
the same direction
as shown on the
top legend. Leave

C

1/4” between the
part and PC board.

Figure D

Q5 - 2N3906 Transistor PNP

(see Figure D)

TP7 - Test Point Pin

(see Figure A)

Q6 - MPS6560 (8050)

Transistor NPN

(see Figure C)

TP8 - Test Point Pin

(see Figure A)

C13 - 470mF Lytic

(see Figure B)

R17 - 100W Resistor

(brown-black-brown-gold)
R16 - 1kW Resistor

(brown-black-red-gold)

R19 - 1W Resistor

(brown-black-gold-gold)

D2 - 1N4148 Diode

(see Figure E)

Q7 - MPS6562 (8550)

Transistor PNP

(see Figure D)

R18 - 820W Resistor

(gray-red-brown-gold)

R15 - 100W Resistor
(brown-black-brown-gold)

-8-

ASSEMBLY INSTRUCTIONS

Figure F

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

J1 - Earphone Jack
with Nut

(see Figure F)

Speaker
Speaker Pad
4” Wire

(see Figures G & H)

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

Foil Side

2

3

Jack

Nut

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

Part #

622130

1

GND Pad

Foil Side

2

Jack

Nut

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

Part #

3

1

GND Pad

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.

622131

Figure G

Step 1: If the speaker pad has

center and outside pieces, then
remove them. Peel the backing off of
one side of the speaker pad and stick
the pad onto the speaker.

Step 2: Remove the other backing
from the speaker pad.

Step 3: Stick the speaker onto the
solder side of the PC board.

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.

Step 1 Step 3

Backing

Pad

Backing

Speaker

Figure H

From Terminal 3

1” Wire

Part # 622130 Part # 622131

1 ½”

Wires

1” Wire

Step 2

PC Board

(solder side)

1 ½”

Wires

-9-

You have completed wiring the Audio Amplifier. We shall proceed in testing this circuit. You will need a Volt-Ohm­Milliammeter, preferably a digital type.

STATIC MEASUREMENTS

RESISTANCE TEST

Adjust the Volt-Ohm-Milliammeter (VOM) to the highest
resistance scale available. Connect the VOM to the
circuit as shown in Figure 3. Do not connect the battery.
The VOM should indicate a low resistance first and then
as C14 charges, resistance should rise to
approximately 100kW. If you get a lower reading,

reverse multimeter leads. If you get a reading lower
than 20kW, check the circuit for shorts or parts inserted
incorrectly. Check C14 to see if it’s leaky or inserted
backwards. If you get a reading higher than 150kW,
check for open copper or bad solder connections on
resistors R13 and R14.

W

COM

Amps

V/W

GND

R15

Figure 3

POWER UP TEST

Set your VOM to read the highest possible current.
Connect the meter to the circuit as shown in Figure 4.
Make sure that the On/Off switch (SW1) is in the OFF
position.
While watching your VOM, flip switch SW1 to the ON
position. The VOM should indicate a very low current.

Adjust your meter for a more accurate reading if necessary.
If the current is greater than 25 milliamps, immediately turn
the power off. The current should be between 5 and 15
milliamps. If you circuit fails this test, check that all parts
have been installed correctly and check for shorts or poor
solder connections. Turn OFF SW1.

DC Amps

Figure 4

+

Amps COM V/W

-10-

OUTPUT BIAS TEST

Adjust your VOM to read 9 volts and connect it to test
point 8 (TP8) as shown in Figure 5.
Make sure that the battery, or a 9 volt power supply (if
available), is properly connected and turn the power
ON. The voltage at TP8 should be between 4 to 5 volts.
If you get this reading, go on to the next test. If your

V

Amps COM V/W

circuit fails this test, turn the power OFF and check that
all of the transistors are correctly inserted in the correct
locations. The E on the transistor indicates the emitter
lead and should always be in the hole with the arrow.
Check that resistors R13 and R14 are the correct
values and not interchanged.

Battery

GND

R15

Figure 5

TRANSISTOR BIAS TEST

Move the positive lead of your VOM to test point 7
(TP7). Make sure that the power is ON. The voltage
should be between .5 and .8V higher than the voltage
at TP8. All silicon transistors biased for conduction will

INPUT BIAS

Move the positive lead of the VOM to test point 6 (TP6).
Make sure that the power is ON. The voltage at TP6
should be very close to the voltage at TP7. This is true
because very little DC current flows through resistor
R16 making the voltage at the emitter of Q4 very close
to the voltage at the emitter of Q5. If your circuit passes

have approximately .7V from the base to the emitter. If
your circuit fails this test, turn off the power and check
that Q6 is properly inserted into the circuit board.

this test, leave the VOM connected and go to test 1 in
the Dynamic Measurements Section. If your circuit fails
this test, turn the power OFF and check transistors Q4,
Q7 and resistor R16. All static tests must pass before
proceeding to the Dynamic Tests or the next section.

-11-

DYNAMIC MEASUREMENTS

DC GAIN

Adjust your VOM to read 9 volts DC. Connect the
positive lead of the VOM to TP6 and the negative lead
to any ground. Turn the power ON and record the
voltage at TP6 here:

V1=________ volts.

Place resistor R4 across resistor R13 as shown in
Figure 6.
The voltage at TP6 should drop to a lower value.
Record that lower voltage here:

V2=__________ volts.

Remove R4 from the circuit and move the positive lead
of the VOM to TP8. Record the voltage at TP8 here:

V3=__________ volts.

Once again, parallel resistor R13 with resistor R4 as
shown in Figure 6. The voltage at TP8 should also drop
to a lower voltage. Record the new reading at TP8
here:

V4=__________ volts.

Remove R4 from the circuit but leave your VOM
connected to TP8 for the next test. Turn the power OFF.
Since the DC GAIN equals the DC change at the output
divided by the DC change at the input, the DC gain of
this amplifier is (V1-V2)/(V3-V4). Your calculated
answer should be very close to 1.

V

1MW

Amps COM V/W

Battery

GND

R15

Figure 6

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

-12-

AC GAIN

Connect the VOM and generator to TP6 as shown in
Figure 7.
Turn the power ON. Normally the AC gain is measured
at a frequency of 1 kilohertz (kHz). Your VOM, however,
may not be able to accurately read AC voltages at this
frequency. It is recommended, therefore, that this test
be performed at 400Hz. Set the generator at 400Hz and
minimum voltage output. Set your VOM to read an AC
voltage of 1 volt at the output of your Audio Amplifier.
Slowly increase the output of the generator until the
VOM reads 1 volt AC. Leave the audio at this setting

10mFGenerator

and move the positive lead of your VOM to TP6. Record
the AC voltage input 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.
Your calculated AC Gain should be approximately 10.

Amps COM V/W

GND

R15

V

Output Adjust

GND
R15

Battery

Figure 7

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

-13-

AC BANDWIDTH

Connect the oscilloscope and generator to your circuit
as shown in Figure 8.
Set the generator for a frequency of 1kHz and minimum
voltage output. Set the oscilloscope to read .5 volts per
division. Turn the power ON and slowly increase the
generator output until the oscilloscope displays 2 volts
peak to peak (Vpp) at TP8. Move the oscilloscope
probe to TP6 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 TP8 and slowly
increase the frequency from the generator until the

waveform on the oscilloscope drops to .7 of its original
reading, 1.4 Vpp or 2.8 divisions. Use the oscilloscope
probe to check TP6 to make sure the input voltage did
not change. The frequency of the generator when the
output drops to .7 of its original value is called the high
frequency 3 decibel (dB) corner.

Repeat this procedure by lowering the frequency from
the generator to obtain the low frequency 3dB corner.
Leave the oscilloscope connected to TP8 and turn the
power OFF. By subtracting the frequency of the low
corner from the frequency of the high corner, you
calculate the bandwidth of the Audio Amplifier. Your
bandwidth should be greater than 100kHz.

Generator

Output Adjust

GND

R15

10mF

Figure 8

Probe

Oscilloscope

GND
R15

-14-

DISTORTION

Connect the generator and oscilloscope as shown in
Figure 8. Set the generator at a frequency of 1kHz, turn
the power ON and adjust the generator output until the

peaks of the sinewave at TP8 are clipped as shown in
Figure 9A.

Clipped

A

Figure 9

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

Vclp = _______ Vpp.

Using a wire short out resistor R17 and diode D2 as
shown in Figure 10.

Crossover
Distortion

B

The waveform on your oscilloscope should resemble
Figure 9B. The “flat spots” near the center of each
sinewave demonstrate what is called crossover
distortion. This distortion should disappear when you
remove the shorting lead. Turn the power OFF

MAXIMUM POWER OUTPUT

The maximum power output before distortion due to
“clipping” can be calculated using the voltage Vclp
obtained in step 4 as follows:

Vpeak (Vp) = Vclp/2
Vroot mean squared (Vrms) = Vp x .7

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

Wire Lead

or Clip Lead

Figure 10

output. It is best to use a power supply to prevent
battery voltage from changing during this
measurement. Efficiency can then be calculated as
follows: Eff = Max audio power/Battery Power.

-15-

SECTION 2

AM DETECTOR AND AGC STAGES

THEORY OF OPERATION

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 11). 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 capacitors C9 and C10 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 R11, R12 and the volume control. The
discharge time constant for this circuit 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 at the volume control as shown in Figure 11.
The purpose of the automatic gain control (AGC) circuit

is to maintain a constant audio level at the detector,
regardless of the strength of the incoming signal.
Without AGC, the volume control would have to be
adjusted for each station and even moderately strong
stations would clip in the final IF amplifier causing audio
distortion. AGC is accomplished by adjusting the DC
bias of the first IF amplifier to lower its gain as the signal
strength increases. Figure 11 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 three (TP3), the audio is removed by a low
pass filter, R11 and C4, leaving only the DC
component. Resistor R5 is used to shift the voltage at
TP3 high enough to bias the base of transistor Q2 to
the full gain position when no signal is present.
Resistors R5 and R11 also forward bias diode D1 just
enough to minimize “On Condition” threshold voltage.

Figure 11

-16-

ASSEMBLY INSTRUCTIONS - DETECTOR

C6 - 100mF Lytic

(see Figure B)

R5 - 27kW Resistor

(red-violet-orange-gold)

T1 - IF Coil (yellow)

TP3 - Test Point Pin

(see Figure A)

C4 - 10mF Lytic

(see Figure B)

R11 - 3.3kW Resistor

(orange-orange-red-gold)

C9 - .02mF or .022mF Discap

(marked 203 or 223)

R12 - 2.2kW Resistor

(red-red-red-gold)

STATIC MEASUREMENTS

R8 - 100W Resistor

(brown-black-brown-gold)

T3 - IF Coil (black)

TP5 - Test Point Pin

(see Figure A)

D1 - 1N4148 Diode

(see Figure E)

C15 - .001mF Discap

(marked 102)

C10 - .01mF Discap

(marked 103)

AGC ZERO SIGNAL BIAS

With the power turned OFF, connect the VOM to test
point three (TP3) as shown in Figure 12.
Check that the VOM is adjusted to read 9 volts DC and
turn the power ON. The voltmeter should read

V

Amps COM V/W

Figure 12

GND

R15

approximately 1.5 volts DC. If your reading varies more
than .5 volts from this value, turn the power OFF and
check the polarity of D1, and resistors R11 and R5.
Also check that transformer T1 is properly installed.

T3 TEST

With the power turned OFF, connect the positive lead of
the VOM to TP5 and the negative lead to any ground.
Make sure that the VOM is set to read 9 volts DC and
turn the power ON. The voltage on the VOM should be

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

the same as your battery voltage or power supply
voltage. If not, turn OFF the power and check that T3
is properly installed.

-17-

DYNAMIC MEASUREMENTS

DETECTOR AND ACG TEST

Turn the power OFF and connect the VOM and RF
generator as shown in Figure 13.
accurately read 2 volts DC and set the output of the RF
generator for 455kHz, no modulaton, and minimum
amplitude. Turn the power ON and slowly increase the
amplitude of the 455kHz signal from the RF generator

Set the VOM to

V

Amps COM V/W

until the voltage at TP3 just starts to drop. This point is
called the AGC threshold with no IF gain. Make a note
of the amplitude setting on the RF generator here:

____________.

Turn the power OFF.

Generator

.02mF

GND

R15

Figure 13

GND

R15

If your RF generator does not have amplitude modulation or you do not have an oscilloscope, go to Section 3.

SYSTEM CHECK

Connect equipment as shown in Figure 14.
Set the RF generator at 455kHz, 1kHz at 80%
modulation and minimum output. Turn the power ON
and put the volume control at full clockwise position.

Oscilloscope

GND
R15

Slowly adjust the amplitude of the RF generator output
until you hear the 1kHz on the speaker. If this test fails,
turn the power OFF and check C11, R12, volume
control, D1 and TP3.

Generator

.02mF

GND

R15

Figure 14

DETECTOR BANDWIDTH TEST

Connect equipment as shown in Figure 14. Set the RF
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 put
the volume control at minimum. Increase the
amplitude of the RF generator until the signal on the
oscilloscope is 4 divisions peak to peak. Check the
signal to make sure it is free of all distortion. Leave the

frequency of the RF output at 455kHz, but increase the
modulation frequency until the output drops to 0.28
Vpp. Record the modulation frequency on the RF
generator here:

_________.

This frequency should be greater than 5kHz. Turn the
power OFF.

-18-

SECTION 3

SECOND IF AMPLIFIER

THEORY OF OPERATION

The purpose of the SECOND IF AMPLIFIER is to
increase the amplitude of the intermediate frequency (IF)
and at the same time provide SELECTIVITY. Selectivity
is the ability to “pick out” one radio station while rejecting
all others. The second IF transformer (T3) 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 15.

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 separate the radio stations.

.707

The gain at 455kHz in the second IF amplifier is fixed
by the AC impedance of the primary side of transformer
T3, and the DC current in Q3. The current in Q3 is set
by resistors R7, R9 and R10. Both C7 and C8 bypass
the 455kHz signal to ground, making Q3 a common
emitter amplifier. The signal is coupled from the first IF
amplifier to the second IF amplifier through transformer
T2. The IF transformers not only supply coupling and
selectivity, they also provide an impedance match
between the collector of one stage and the base of the
next stage. This match allows maximum power to
transfer from one stage to the next.

452kHz 458kHz

455kHz

Figure 15

ASSEMBLY INSTRUCTIONS - SECOND IF AMPLIFIER

TP4 - Test Point Pin

(see Figure A)

T2 - IF Coil

(White)

Q3 - 2N3904 Transistor NPN

(see Figure C)

R10 - 470W Resistor

(yellow-violet-brown-gold)

R7 - 39kW Resistor
(orange-white-orange-gold)

R9 - 10kW Resistor

(brown-black-orange-gold)

C7 - .02mF or .022mF Discap

(marked 203 or 223)

C8 - .02mF or .022mF Discap

(marked 203 or 223)

-19-

STATIC MEASUREMENTS

Q3 BIAS

With the power OFF, connect the negative lead of your
VOM to any ground and the positive lead to the emitter
of Q3 as shown in Figure 16. Set the VOM to read 9
volts DC and turn ON the power. The voltage at the

V

Amps COM V/W

GND

Figure 16

R10

emitter of Q3 should be approximately 1 volt. If your
reading is different by more than 0.5 volts, turn off the
power and check your battery of power supply voltage.
Also check components R7, R9, R10 and Q3.

If you do not have an RF generator or oscilloscope, skip the following test and go to Section 4.

DYNAMIC MEASUREMENTS

AC GAIN

With the power turned OFF, connect the oscilloscope and
the RF generator to the circuit as shown in Figure 17.

Set
the RF generator at a frequency of 455kHz, no
modulation and minimum amplitude output. Set the
oscilloscope vertical sensitivity at 1 volt/division. The
scope probe must have an input capacitance of less
than 50pF or it will detune transformer T3. Turn the
power ON and slowly increase the amplitude of the RF
signal until you have 4 volts peak to peak on the
oscilloscope. Tune transformer T3 for a maximum
output while readjusting the RF generator amplitude to

Generator

.02mF

keep 4Vpp at the oscilloscope. After T3 is aligned,
move the scope probe tip to the base of Q3 and record
the peak to peak amplitude of the signal here:

Vb=__________Vpp.

Turn the power OFF. The AC gain of the second IF
amplifier at 455kHz is equal to 4/Vb, and should be
greater than 100. If your gain is less than 100, check
components C7, C8, R7, R9 and R10. Also, make sure
that transistor Q3 is properly installed.

Oscilloscope

Probe

Output

Adjust

GND

R10

Figure 17

-20-

GND

R10

BANDWIDTH TEST

With the power OFF, connect your equipment as shown
in Figure 17A. Turn the power ON and adjust the RF
generator for .4Vpp at the cathode of D1. If necessary,
realign transformer T3 for maximum output while
adjusting the output of the RF generator to maintain
.4Vpp. Slowly decrease the frequency of the RF
generator until the signal drops to .707 of its peaked
value or .28Vpp. Record the frequency of the RF
generator here:

FL=___________kHz.

Now increase the frequency of the RF generator past
the peak to a point where the signal drops to .707 of its
peak value. Record that frequency point here:

FH=___________kHz.

By subtracting the frequency of the lower 3dB corner
from the frequency of the higher 3dB corner you get the
BANDWIDTH of the second IF amplifier. Your results
should be similar to the values shown in Figure 15.

Generator

.02mF

Output

Adjust

GND

R10

Figure 17A

SECTION 4

FIRST IF AMPLIFIER

THEORY OF OPERATION

The operation of the first IF amplifier is the same as for
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 Q2, lower as the
signal strength increases. Since the voltage from base
to emitter is fairly constant, the drop in voltage at the
base produces a similar drop in voltage at the emitter of

Oscilloscope

Probe

GND

R10

Q2. This drop lowers the voltage across R6 and thus
reduces the DC current through R6. Since all of the DC
current from the emitter of Q2 must go through R6, the
DC current in Q2 is therefore lowered. When the DC
current in a transistor is lowered, its effective emitter
resistance increases. The AC gain of transistor Q2 is
equal to the AC collector load of Q2 divided by its
effective emitter resistance. Raising the value of the
effective emitter resistance thus lowers the AC gain of
Q2.

ASSEMBLY INSTRUCTIONS - FIRST IF AMPLIFIER

R4 - 1MW Resistor

(brown-black-green-gold)

TP2 - Test Point Pin

(see Figure A)

Q2 - 2N3904 Transistor NPN

(see Figure C)

R6 - 1kW Resistor

(brown-black-red-gold)

C5 - .02mF or .022mF Discap

(marked 203 or 223)

-21-

STATIC MEASUREMENTS

Q2 BASE BIAS

With the power turned OFF, reconnect your VOM to test
point 3 (TP3) as shown in Figure 12. Set the VOM to read
2 volts DC accurately and turn the power ON. The voltage

should be approximately 1.5 volts. If your circuit fails this
test, turn the power OFF and check Q2 and R6.

Q2 CURRENT

With the power turned OFF, connect the positive lead of
the VOM to the emitter of Q2. Connect the negative
lead of the VOM to any DC ground and turn the power

If you do not have an RF generator or oscilloscope, skip the following test and go to Section 5.

ON. The voltage should be approximately .8 volts.
Since the current in Q2 is equal to the current in R6,
I(Q2)=.8/R6 or approximately .8 milliamps.

DYNAMIC MEASUREMENTS

AC GAIN

With the power turned OFF, connect the RF generator
and the oscilloscope to your circuit as shown in Figure

18. Using a clip lead, short TP5 to R8 as shown in
Figure 18. This short prevents the AGC from lowering
the gain of the first IF ampifier. Set the RF generator to
455kHz, no modulation, and minimum amplitude
output. Set the oscilloscope for a vertical sensitivity of
1 volt/division and turn the power ON. Increase the
amplitude output from the RF generator until
approximately 4Vpp registers on the oscilloscope. Tune
the IF transformer (T2) to maximize the 455kHz at TP4.

After tuning T2, adjust the RF generator amplitude in
order to keep 4Vpp at TP4. Now move the oscilloscope
probe to the base of Q2 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 of this amplifier should be greater than

100. DO NOT TURN THE POWER OFF. GO TO THE
NEXT TEST.

AGC ACTION

Move the oscilloscope probe back to TP4 and adjust the
RF generator for 4Vpp if necessary. Remove the clip

Generator

.02mF

Output

Adjust

GND

R6

Figure 18

lead shorting TP5 to R8. The AGC should reduce the
signal level at TP4 to approximately .8 volts.

Clip Lead

Oscilloscope

GND

R6

-22-

SECTION 5

MIXER AND OSCILLATOR

THEORY OF OPERATION

In a superheterodyne type receiver the radio wave at the
antenna is amplified and then mixed with the local
oscillator to produce the intermediate frequency (IF).
Transistor Q1 not only amplifies the RF signal but also
simultaneously oscillates at a frequency 455kHz above
the desired radio station frequency. Positive feedback
from the collector to the emitter of Q1 is provided by coil
L2 and capacitor C3. During the heterodyne process,
the following four frequencies are present at the collector
of Q1.

1. The local oscillator frequency, LO.

2. The RF carrier or radio station frequency.

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

4. The difference of these two frequencies, LO - RF.

The “difference frequency” is used as the intermediate
frequency in AM radios. The collector of Q1 also
contains an IF transformer (T1) tuned only to the
difference frequency. This transformer rejects all

frequencies except those near 455kHz. T1 also
couples the 455kHz signal to the base of Q2 to be
processed by the IF amplifiers.

The antenna and the oscillator coils are the only two
resonant circuits that change when the radio is tuned
for different stations. Since a radio station may exist
455kHz above the oscillator frequency, it is important
that the antenna rejects this station and selects only the
station 455kHz below the oscillator frequency. The
frequency of the undesired station 455kHz above the
oscillator is called the image frequency. If the selectivity
of the antenna (Q factor) is high, the image will be
reduced sufficiently.

The oscillator circuit must also change when the radio
is tuned in order to remain 455kHz above the tuning of
the desired radio station. The degree of accuracy in
keeping the oscillator frequency exactly 455kHz above
the tuning of the antenna is called tracking accuracy.

-23-

ASSEMBLY INSTRUCTIONS - ANTENNA, MIXER AND OSCILLATOR

R1 - 56kW Resistor

L1 - Antenna with Holders

(see Figures I & J)

C2 - .02mF or .022mF Discap

(marked 203 or 223)

TP1 - Test Point Pin

(see Figure A)

R2 - 12kW Resistor

(brown-red-orange-gold)

R3 - 3.3kW Resistor

(orange-orange-red-gold)

(green-blue-orange-gold)

L2 - Oscillator Coil (red)

Q1 - 2N3904 Transistor NPN

(see Figure C)

C3 - .01mF Capacitor

(marked 103)

C1 - Tuning Gang Capacitor
3 Screws M2.5
Knob (dial)
Label (dial knob)

(See Figure K)

Figure I

Determine if you have a three wire or four wire
coil. Resistance measurements will be used to
check the configuration of the coil. Slide one
holder off the ferrite core of the antenna
assembly. Then slide the coil off the the ferrite
core. Measure the resistance of the coil. Your
readings should match the approximate values
as shown.

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.

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.
Slide the ferrite core through the antenna coil and the holder. Cut the second holder as shown.
Put the tab of the second holder into the left hole and twist the tab 90
Slide the ferrite core through the holder. Make sure that the ferrite core sticks out 1/2” on each side.

1/2”

C (white)

B (black)

A (red)

1/2”

3 Wire

White

Black

Red

C (white) B

R=9 - 11W

}

R=1 - 1.5W

}

O

.

Twisted Together

Black

4 Wire

White

}

Black

Red

}

Green

B Twisted Together

1/2”

R=9 - 11W

R=1 - 1.5W

C (white)

Black

Tabs 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”.

OR

Red

A (green)

Figure J

-24-

Your kit may contain a 3 lead or a 4 lead
capacitor. Bend the leads as shown. Fasten C1
into place on the top side of the PC board with
two M2.5 screws.

Fasten the knob to the
shaft of the capacitor
with a M2.5 screw.

Screw

Knob

Knob Post

Screw Holes

Solder leads

to pads

C1

Turn the dial fully clockwise.
Remove the protective
backing from the label and
align the 1600 with the
arrow on the PC board.

3 Leads

4 Leads

Figure K

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.

Cut Holder

Coil

Figure L

PC Board Stand

Insert the PC board into the stand as shown.

-25-

STATIC MEASUREMENTS

Q1 BIAS

With the power turned OFF, connect the VOM to your
circuit as shown in Figure 19. Connect a clip lead from
test point two (TP2) to the collector of Q1. This short
prevents Q1 from oscillating. Set the VOM to read 2 volts
DC accurately and turn the power ON. The DC voltage

V

Amps COM V/W

at TP1 should be 1.6 volts. If the voltage in your circuit
differs by more than 0.5 volts, leave the power ON and
check the battery voltage. If the battery voltage is
greater than 8.5 volts, turn the power OFF and check
components R1, R2, R3 and Q1.

GND

R2

Clip Lead

Figure 19

If you do not have an oscilloscope, go to the Final Alignments With No Test Equipment Section.

DYNAMIC MEASUREMENTS

OSCILLATOR CIRCUIT

With the power turned OFF, connect the oscilloscope to
the circuit as shown in Figure 20.

Set the oscilloscope
for a vertical sensitivity of 1 volt/division and turn the
power ON. The oscilloscope should display a low

voltage sine wave. The frequency of the sine wave
should change when capacitor C1 is turned. If your
circuit fails this test, turn the power OFF and check
components Q1, C1, C2, C3, L1 and L2.

Oscilloscope

Figure 20

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

go to the Final Alignments with No Test Equipment Section.

-26-

GND

R2

FINAL ALIGNMENTS

IF BANDWIDTH

With the power turned OFF, connect the RF generator
and the oscilloscope to your circuit as shown in Figure 21.
Short TP2 to the collector of Q1 with a clip lead to “kill”
the local oscillator. Set the RF generator at a frequency
of 455kHz, modulation of 400Hz 80%, minimum
amplitude output. Set the oscilloscope to read 0.1Vpp
and turn the power ON. Increase the amplitude of the
RF signal until the oscilloscope registers 0.5Vpp. Align
transformers T3, T2 and T1 for the maximum AC
reading on the oscilloscope. Decrease the amplitude of
the signal from the RF generator to restore 0.5Vpp on
the oscilloscope. Repeat the last two steps until no
change in the peak at the oscilloscope is noticed.

Generator

.02mF

After IF alignment, lower the frequency from the RF
generator until the reading on the VOM drops to 0.707
of its peaked value. Record the frequency of this lower
3dB corner here:

Fl=____________kHz.

Increase the RF generator frequency past the peak to
the upper 3dB corner and record that frequency here:

Fh=____________kHz.

The bandwidth of the IF amplifiers is BW=Fh - Fl. IF
bandwidth should be between 1 to 2kHz. This
bandwidth will widen as the AGC is approached.

Oscilloscope

Output

Adjust

GND

R2

Clip Lead

Figure 21

SETTING OSCILLATOR RANGE

With the power turned OFF, connect the equipment to
the circuit as shown in Figure 21. DO NOT connect the
clip lead from TP2 to Q1. Set the RF generator at
540kHz, 400Hz 80% modulation, and a low level of
output. Turn the tuning capacitor fully counter­clockwise. Turn the power ON and a 400Hz tone should
be heard coming from the speaker. Tune the oscillator
coil (L2) for a peak on the oscilloscope. Adjust the RF

Antenna Trimmer

Oscillator Trimmer

Probe

GND

R6

generator output during this process to maintain a peak
at 0.5Vpp or less. After peaking L2, set the RF
generator frequency to 1600kHz and turn the tuning
capacitor (C1) fully clockwise. A 400Hz tone should be
heard coming from the speaker. Tune the oscillator
trimmer capacitor on the back of C1 for a peak on the
oscilloscope (see Figure 22).

Antenna Trimmer

Figure 22

3 Leads 4 Leads

-27-

After peaking the oscillator trimmer capacitor, return the
RF generator to 540kHz, and capacitor C1 to the fully
counter-clockwise position and readjust L2. Repeat the
last few steps until both settings of the oscillator are
correct. This process sets the oscillator range at
995kHz to 2055kHz. If a frequency counter is available,

ANTENNA ALIGNMENT

With the power turned OFF, connect test equipment to
your circuit as shown in Figure 23. Set the RF generator
at 600kHz, 400Hz 80% modulation, moderate signal
strength. Set the oscilloscope to read .5Vpp and turn the
power ON. Turn C1 fully counter-clockwise, then slowly
turn C1 clockwise until a 400Hz tone can be heard
coming from the speaker. Slowly slide the antenna coil
back and forth on the ferrite rod to obtain a peak on the
oscilloscope. For maximum signal, your location of the
antenna coil may have to be on the end of the ferrite rod
(as shown in Figure 24). Change the frequency of the RF

Close to
Antenna

Wire Loop

you may verify this alignment by measuring the
frequency at the emitter of Q1 for both ends of the
tuning capacitor (C1). Be careful not to mistune the
oscillator during this measurement. A coupling
capacitor of 82 picofarads or less to the frequency
counter is recommended.

generator to 1400kHz and adjust C1 until a 400Hz tone
can be heard coming from the speaker. Carefully peak
the reading on the oscilloscope by adjusting the
frequency of the RF generator. Now tune the antenna coil
to this frequency by adjusting the antenna trimmer on the
back of C1 (see Figure 22). This process should be
repeated until both settings of the antenna track the
oscillator tuning. Once the antenna is properly aligned,
carefully apply candle wax or glue to the antenna coil and
ferrite rod (as shown in Figure 24).

Oscilloscope

Generator

Output

Adjust

Figure 23

Wax

Probe

GND

R6

HolderCoil

Figure 24

-28-

AM ALIGNMENT WITH NO TEST EQUIPMENT

It is best to use an earphone for this alignment
procedure. Rotate the tuning knob fully counter­clockwise and place the label on the knob with the
white arrow pointing at the 540kHz marking.

With an alignment tool or screwdriver, turn coils L2, T1,
T2 and T3 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 antenna coil
about 1/8” from the end of its ferrite rod. Refer to
Figure J on page 24.

Turn the power ON and adjust the volume to a
comfortable level. Tune the dial until a weak station is
heard. If no stations are present, carefully slide the
antenna back and forth on its ferrite rod and retune the
dial if necessary. With an alignment tool or screwdriver,
adjust T1 until the station is at its loudest. Reduce the
volume control if necessary. Adjust T2 until the station
is at its loudest and reduce the volume control if
necessary. Adjust T3 until the station is at its loudest
and reduce the volume if necessary. Retune the radio
for another weak station and repeat this procedure until
there is no more improvement noticed on the weakest
possible station. This procedure peaked the IF
amplifiers to their maximum gain.

Tune the radio until a known station around 600kHz is
found. It may be necessary to listen to the station until

their broadcast frequency is announced. If no stations
are present at the low side of the AM band, adjust L2
until a station is heard. Once a station is found and its
broadcast frequency is known, rotate the dial until the
white pointer is aligned with that station’s frequency
marking on the dial. Adjust L2 until the station is heard.
Tune the radio until a 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 at the high end of the AM band, adjust the
oscillator trimmer on the back of the gang. Once a
station is found and its broadcast frequency is known,
rotate the dial until the white pointer is aligned with that
station’s frequency marking on the dial. Adjust the
oscillator trimmer located on the back of the gang until
a station is heard. Repeat these steps until the
oscillator alignment is optimized. This procedure set
the oscillator range at 995kHz to 2055kHz.

Tune the radio for a station around 600kHz. Carefully
slide the antenna back and forth until the station is at its
loudest. Tune the radio for a station around 1400kHz.
Adjust the antenna trimmer located on the back of the
gang until the station is at its loudest. Repeat these
steps until the antenna alignment is optimized. This
procedure set the antenna to “track” the oscillator.
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 24).

DC Voltages

The voltage readings below should be used in troubleshooting the AM radio.

Q1 B 1.5V Q5 B 8.3V

E 1.0V E 9.0V
C 8.9V C 5.8V

Q2 B 1.4V Q6 B 5.8V

E 0.7V E 5.2V
C 8.9V C 9.0V

Test Conditions

1. Volume control set to minimum.

2. Connect a jumper wire between capacitor C2 (side
that goes to red lead of coil L1) to negative battery.

Q3 B 1.7V Q7 B 4.6V

E 1.0V E 5.2V
C 9.0V C 0.0V

3. Battery voltage - 9.0V

4. All voltages are referenced to circuit common.

5. Voltage reading can vary +

Q4 B 5.7V

E 5.2V
C 8.3V

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10%.

SCHEMATIC DIAGRAM

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ElencoTMElectronics, Inc.

150 W. Carpenter Avenue

Wheeling, IL 60090

[phone]

http://www.elenco.com

e-mail: [e-mail]