AM RADIO KIT
MODEL AM-550CK
DUAL AUDIO SUPERHET
INTEGRATED CIRCUIT, 7 TRANSISTORS, 2 DIODES
Assembly and Instruction Manual
Copyright © 2016, 2011 by ELENCO®Electronics, Inc. All rights reserved. REV-A Revised 2016 753008
No part of this book shall be reproduced by any means; electronic, photocopying, or otherwise without written permission from the publisher.
®
ELENCO
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
additional assistance, if needed. DO NOT contact your place of purchase as they will not be able to help you.
RESISTORS (see page 3 “Identifying Resistor Values”)
Qty. Symbol Value Color Code Part #
r 1 R19 1W 1/4W 5% brown-black-gold-gold 111000
r 1 R21 10W 1/4W 5% brown-black-black-gold 121000
r 3 R8, R15, R17 100W 1/4W 5% brown-black-brown-gold 131000
r 1 R10 470W 1/4W 5% yellow-violet-brown-gold 134700
r 1 R18 820W 1/4W 5% gray-red-brown-gold 138200
r 2 R6, R16 1kW 1/4W 5% brown-black-red-gold 141000
r 1 R20 1.2kW 1/4W 5% brown-red-red-gold 141200
r 1 R12 2.2kW 1/4W 5% red-red-red-gold 142200
r 2 R3, R11 3.3kW 1/4W 5% orange-orange-red-gold 143300
r 1 R9 10kW 1/4W 5% brown-black-orange-gold 151000
r 1 R2 12kW 1/4W 5% brown-red-orange-gold 151200
r 1 R5 27kW 1/4W 5% red-violet-orange-gold 152700
r 1 R7 39kW 1/4W 5% orange-white-orange-gold 153900
r 1 R14 47kW 1/4W 5% yellow-violet-orange-gold 154700
r 1 R1 56kW 1/4W 5% green-blue-orange-gold 155600
r 1 R13 82kW 1/4W 5% gray-red-orange-gold 158200
r 1 R4 1MW 1/4W 5% brown-black-green-gold 171000
r 1 Pot/SW1 50kW / SW Pot/SW with nut and washer 192522
CAPACITORS (see page 3 “Identifying Capacitor Values”)
Qty. Symbol Value Description Part #
r 1 C1 Variable Tuning 211677
r 1 C15 0.001mF Discap (102) 231036
r 2 C3, C10 0.01mF Discap (103) 241031
r 5 C2, C5, C7, C8, C9 0.02mF or 0.022mF Discap (203) or (223) 242010
r 1 C20 0.047mF Discap (473) 244780
r 1 C19 0.1mF Discap (104) 251010
r 5 C4,C11,C16,C17,C18 10mF Electrolytic (Lytic) 271045
r 1 C12 47mF Electrolytic (Lytic) 274744
r 1 C6 100mF Electrolytic (Lytic) 281044
r 2 C13, C14 470mF Electrolytic (Lytic) 284744
®
(address/phone/e-mail is at the back of this manual) for
SEMICONDUCTORS
Qty. Symbol Description Part #
r 2 D1, D2 1N4148 Diode 314148
r 4 Q1, Q2, Q3, Q4 2N3904 Transistor NPN 323904
r 1 Q5 2N3906 Transistor PNP 323906
r 1 Q6 MPS8050 or 6560 Transistor NPN 328050
r 1 Q7 MPS8550 or 6562 Transistor PNP 328550
r 1 U1 LM386 Integrated circuit 330386
COILS
Qty. Symbol Value Description Part #
r 1 L2 Oscillator (red dot) 430057
r 1 T1 IF (yellow dot) 430260
r 1 T2 IF (white dot) 430262
r 1 T3 Detector (black dot) 430264
r 1 L1 AM Antenna with holders 484004
MISCELLANEOUS
Qty. Description Part #
r 1 PC board 517039
r 1 Switch 541023
r 1 Battery holder 590096
r 1 Speaker 590102
r 1 Knob (dial) 622040
r 1 Knob (pot) 622050
r 1 Earphone jack with nut 622130
r 1 Radio stand 626100
r 1 Earphone 629250
r 4 Screw M1.8 x 7mm (battery holder) 641100
Qty. Description Part #
r 1 Screw M2.5 x 8mm (gang) 641107
r 2 Screw 2.5 x 4mm (gang) 641310
r 4 Nut M1.8 644210
r 1 Socket 8-pin 664008
r 10 Test point pin 665008
r 1 Label, dial knob 720422
r 1 Speaker pad 780128
r 1 Wire 4” 814920
r 1 Solder lead-free 9LF99
**** SAVE THE BOX THAT THIS KIT CAME IN. IT WILL BE USED ON PAGES 29 AND 34. ****
-1-
PARTS IDENTIFICATION
RESISTORS CAPACITORS SEMICONDUCTORS
Resistor
Potentiometer/
Color dot
50kW
Switch
with Nut and
Washer
Coil
Discap
Electrolytic
Radial
COILS
Coil
MISCELLANEOUS
Tuning
Antenna Assembly
Diode
Transistor
Plastic holders
LM386 IC
8-pin Socket
Ferrite core
Knob (dial)
Speaker
Screw
M2.5 x 4mm
M1.8 x 7mm
Screw
Test Point Pin
Screw
M2.5 x 8mm
Slide Switch
Nut
M1.8
Battery
Holder
Label
Speaker Pad
Radio Stand
Earphone
Knob (pot)
Earphone Jack
with Nut
-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
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9
1
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%
BANDS
2 Multiplier Tolerance
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.
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. Also, the
negative lead of a radial electrolytic is
shorter than the positive one.
Warning:
If the capacitor is
connected with
incorrect polarity, it
may heat up and
either leak, or
cause the capacitor
to explode.
(+)
Axial
(–)
(+)
Polarity
marking
(–)
Radial
Multiplier
Second digit
First digit
The value is 10 x 10 =
100pF, +
The letter M indicates a tolerance of +20%
*
The letter K indicates a tolerance of +10%
The letter J indicates a tolerance of +
For the No. 0 1 2 3 4 5 8 9
Multiply By 1 10 100 1k 10k 100k .01 0.1
CERAMIC DISC MYLAR
Multiplier
101K
50V
Tolerance*
Maximum working voltage
(may or may not appear
on the cap)
10%, 50V
5%
Note: The letter “R” may be used at times
to signify a decimal point; as in 3R3 = 3.3
Tolerance*
Multiplier
Second digit
First digit
The value is 22 x 100 =
2,200pF or .0022mF, +
5%, 100V
2A222J
100V
METRIC UNITS AND CONVERSIONS
Abbreviation Means Multiply Unit By Or
p Pico .000000000001 10
n nano .000000001 10
m micro .000001 10
m milli .001 10
– unit 1 10
k kilo 1,000 10
M mega 1,000,000 10
-12
-9
-6
-3
0
3
6
-3-
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
INTRODUCTION
The Elenco®Dual Audio Superhet 550C AM Radio is
a “superheterodyne” receiver of the standard AM
(amplitude modulated) broadcast frequencies. The
unique design of the Superhet 550C 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
stage, should be read before the assembly is started.
GENERAL DISCUSSION
The Dual Audio Superhet 550C 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. The audio amplifier is switchable
between transistor or IC function.
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
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 Dual
Audio Superhet 550C’s reception capabilities.
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.
Antenna
MIXER
LOCAL
OSCILLATOR
FIRST
IF AMPLIFIER
SECOND
IF AMPLIFIER
Figure 1
-4-
DETECTOR
AGC
Section 1Section 2Section 3Section 4Section 5
Speaker
TRANSISTOR
AUDIO
AMPLIFIER
IC AUDIO
AMPLIFIER
CONSTRUCTION
Introduction
The most important factor in assembling your Elenco®Dual Audio
Superhet 550C AM Radio Kit is good soldering techniques. Using the
proper soldering iron is of prime importance. A small pencil type soldering
iron of 25 watts is recommended. The tip of the iron must be kept
clean at all times and well-tinned.
● 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
Cleaner (Elenco
®
#TTC1). If you use a sponge to clean your tip, then use
®
#SH-1025) or Tip
distilled water (tap water has impurities that accelerate corrosion).
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 lead-free
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.
What Good Soldering Looks Like
A good solder connection should be bright, shiny, smooth, and uniformly
flowed over all surfaces.
Soldering Iron
1. Solder all components from the
copper foil side only. Push the
soldering iron tip against both the
lead and the circuit board foil.
Component Lead
Foil
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!
Types of Poor Soldering Connections
Rosin
1. Insufficient heat - the solder will
not flow onto the lead as shown.
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.
Solder
Foil
Solder
Foil
Circuit Board
Soldering Iron
Soldering Iron
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.
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 A (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 B (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
NPN
W
COM
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.
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 Ohmmeter,
Low Resistance
W
COM
W
Diode
TEST E TEST F
The end
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
W
COM
Diode
-6-
SECTION 1A
TRANSISTOR 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 0.5 or 50%,
but in a Class B amplifier (transistor on for 1/2 cycle) the
maximum theoretical efficiency is 0.785 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.
NPN Transistor
Mount so E lead is in
the arrow hole and flat
side is in the same
direction as shown on
the top legend. Leave
Flat side
1/4”
1/4” between the part
and PC board.
Figure A
PNP Transistor
Flat side
1/4”
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 B
Jumper Wire
Use an excess lead to form a jumper
wire. Bend the wire to the correct
length and mount it to the PC board.
1/8”
Figure C
R14 - 47kW Resistor
(yellow-violet-orange-gold)
Q4 -
Solder 5 lugs
to PC board
2N3904 Transistor NPN
(see Figure A)
TP6 - Test Point Pin
(see Figure F)
J1 - Jumper Wire
(see Figure C)
C11 - 10mF Lytic
(see Figure Da)
R13 - 82kW Resistor
(gray-red-orange-gold)
C14 - 470mF Lytic
(see Figure Db)
Pot / SW1 with
Nut and Washer
Knob (pot)
Top Side
C12 - 47mF Lytic
(see Figure Da)
Electrolytics have a polarity
marking indicating the (–) lead.
The PC board is marked to
show the lead position.
Polarity mark
(–) (+)
Figure Da Figure Db
Diode
Be sure that the band is in
the correct direction.
Band
CathodeAnode
Figure E
Warning: If the capacitor is connected with incorrect polarity,
or if it is subjected to voltage exceeding its working voltage,
it may heat up and either leak or cause the capacitor to
explode.
Capacitor C14
For safety, solder
capacitor C14 on the
copper side as shown.
Bend the leads 90° and
insert into holes. Check
that the polarity is correct,
then solder in place. Trim
the excess leads on
legend side.
Test Point Pin
Legend side
of PC board
Figure F
Q5 - 2N3906 Transistor PNP
(see Figure B)
TP7 - Test Point Pin
(see Figure F)
Q6 - MPS8050 (6560)
Transistor NPN (see Figure A)
R19 - 1W Resistor
(brown-black-gold-gold)
R17 - 100W Resistor
(brown-black-brown-gold)
TP8 - Test Point Pin
(see Figure F)
C13 - 470mF Lytic
(see Figure Da)
SW2 - Slide Switch
Q7 - MPS8550 (6562)
Transistor PNP (see Figure B)
R18 - 820W Resistor
(gray-red-brown-gold)
D2 - 1N4148 Diode
(see Figure E)
R16 - 1kW Resistor
(brown-black-red-gold)
TP10 - Test Point Pin
(see Figure F)
R15 - 100W Resistor
(brown-black-brown-gold)
+
–
-8-
ASSEMBLY INSTRUCTIONS
Figure G
Figure H
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.
J1 - Earphone Jack
with Nut
(see Figure G)
Speaker
Speaker Pad
4” Wire
(see Figures H & I)
Battery Holder
3 Screws M1.8 x 7mm
3 Nuts M1.8
Solder and cut off
excess leads.
Step 1 Step 3Step 2
Pad
Backing
Backing
Speaker
Your kit may contain a different type of
earphone jack. Before installing the jack,
determine which one you have.
GND pad
Jack
2
3
1 - GND
2 - Tip
3 - N.C. Tip
Nut
Foil side
1
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.
PC Board
(solder side)
Step 3: Stick the speaker onto the
solder side of the PC board.
Cut two 1” wires and one 1½” wire and strip ¼” of insulation
off of both ends. Solder the wires in the locations shown.
Figure I
1½” Wire
1” Wires
-9-
You have completed wiring the Transistor Audio Amplifier. We shall proceed in testing this circuit. You will need a VoltOhm-Milliammeter, preferably a digital type.
STATIC MEASUREMENTS - TRANSISTOR AUDIO AMPLIFIER
(SW2 on the top [TR] position)
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.
GND
TP10
Figure 3
POWER UP TEST
Set your VOM to read the highest possible DC 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.
+
–
Figure 4
-10-
OUTPUT BIAS TEST
Adjust your VOM to read 9 volts DC 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 4.5 to 5.5 volts.
If you get this reading, go on to the next test. If your
GND
TP10
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.
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 0.5 and 0.8V higher than the voltage at TP8.
All silicon transistors biased for conduction will have
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 Q6. If your circuit passes this
approximately 0.7V 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.
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 TP10. 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.
1MW
GND
TP10
Figure 6
If you do not have a generator, skip the following test and go directly to Section 1B.
-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 and move
Generator
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.
10mF
GND
TP10
Output Adjust
GND
TP10
Figure 7
If an oscilloscope is not available, skip the following test and go directly to Section 2.
-13-
AC BANDWIDTH
Connect the oscilloscope (set to AC input measurement)
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 0.5 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 0.7 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 0.7 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
TP10
10mF
Figure 8
Probe
Oscilloscope
GND
TP10
-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
output. It is best to use a power supply to prevent battery
Wire lead
or clip lead
Figure 10
voltage from changing during this measurement.
Efficiency can then be calculated as follows:
Eff =
Max audio power
Battery power
/
-15-
SECTION 1B
INTEGRATED CIRCUIT (IC) AUDIO AMPLIFIER
For the IC Audio Amplifier, we use the integrated circuit
(IC) LM-386. In Figure 11, you can see equivalent
schematic and connection diagrams.
Dual In-Line and Small Outline Packages
Figure 12a
Top View
Figure 11
To make the LM-386 a more versatile amplifier, two
pins (1 and 8) are provided for gain control. With pins
1 and 8 open, the 1.35kW resistor sets the gain at 20
(see Figure 12a). The gain will go up to 200 (see Figure
12b) 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 in our kit with a gain of 50 is shown in
Figure 13. 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 12b
Figure 13
-16-
ASSEMBLY INSTRUCTIONS
C20 - .047mF (473) Discap
C18 - 10mF Lytic Capacitor
C16 - 10mF Lytic Capacitor
(see Figure Da)
R20 - 1.2kW 5% 1/4W Resistor
(brown-red-red-gold)
TP9 - Test Point Pin
(see Figure F)
C17 - 10mF Lytic Capacitor
(see Figure Da)
U1 - IC Socket 8-Pin
U1 - Integrated Circuit LM-386
(see Figure J)
C19 - 0.1mF Discap (104)
R21 - 10W 5% 1/4W Resistor
(brown-black-black-gold)
Integrated Circuit
Insert the IC socket into the PC
board with the notch in the direction
shown on the top legend. Solder
the IC socket into place. Insert the
IC into the socket with the notch in
the same direction as the notch on
the socket.
IC
Notch
PC board
Notch
marking
IC socket
Figure J
You have completed wiring the IC Audio Amplifier. We shall proceed in testing this circuit. You will need for static
measurements, a Volt-Ohm-Milliammeter, preferably a digital type.
STATIC MEASUREMENTS - IC AUDIO AMPLIFIER
(SW2 on the down [IC] position)
RESISTANCE TEST
Adjust the Volt-Ohm-Milliammeter (VOM) to the highest
resistance scale available. Connect the VOM to pin 6 of
the IC as shown in Figure 14. Do not connect the battery.
The VOM should indicate a low resistance first and then
as C14 charges, resistance should rise to approximately
4MW. If you get a lower reading, reverse multimeter
leads. If you get a reading lower than 100kW, check the
circuit for shorts or parts inserted incorrectly. If you get
a reading higher than 10MW, check for open copper or
bad solder connections on all components.
GND
TP10
Figure 14
-17-
POWER UP TEST
Set your VOM to read the highest possible DC current.
Connect the meter to the circuit as shown in Figure 15.
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 3 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.
+
–
Figure 15
OUTPUT BIAS TEST
Adjust your VOM to read 9 volts DC and connect it to
test point 8 (TP8) as shown in Figure 16.
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 circuit
fails this test, turn the power OFF and check that the
integrated circuit is correctly inserted in the correct
locations.
INPUT BIAS
Move the positive lead of the VOM to test point 9 (TP9).
Make sure that the power is ON. The voltage at TP9
should be close to the voltage at test point 10 (TP10). If
your circuit passes 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 the IC. All static tests must pass before
proceeding to the Dynamic Tests or the next section.
GND
TP10
Figure 16
If you do not have an audio generator, skip the following test and go directly to Section 2.
-18-
DYNAMIC MEASUREMENTS
AC GAIN
Connect the VOM and audio generator as shown in
Figure 17. 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 audio
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 (TP8). Slowly increase the output
Generator
of the audio generator until the VOM reads 1 volt AC.
Leave the audio at this setting and move the positive
lead of your VOM to TP9. 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 30 - 50.
10mF
GND
TP10
Output Adjust
Figure 17
If an oscilloscope is not available, skip the following test and go directly to Section 2.
AC BANDWIDTH
Connect the oscilloscope and audio generator to your
circuit as shown in Figure 18.
Set the audio generator for a frequency of 1kHz and
minimum voltage output. Set the oscilloscope to read
0.5 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 TP9 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
audio generator until the waveform on the oscilloscope
drops to 0.7 of its original reading, 1.4 Vpp or 2.8
GND
TP10
divisions Use the oscilloscope probe to check TP9 to
make sure the input voltage did not change. The
frequency of the generator when the output drops to 0.7
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.
-19-
Generator
10mF
Probe
Oscilloscope
GND
TP10
Output Adjust
Figure 18
GND
TP10
DISTORTION
Connect the generator and oscilloscope as shown in
Figure 18. Set the generator at a frequency of 1kHz, turn
the power ON and adjust the generator output until the
Measure the maximum voltage peak to peak when
clipping first occurs and record that value here:
Vclp = _______ Vpp.
MAXIMUM POWER OUTPUT
The maximum power output before distortion due to
“clipping” can be calculated using the voltage Vclp
obtained in step 3 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
peaks of the sinewave at TP8 are clipped as shown in
Figure 9A.
Maximum power output should be greater than 200
milliwatts.
EFFICIENCY
By measuring the DC power taken from the battery at
the maximum power output level, the efficiency to the
Audio Amplifier can be calculated. Power from the
battery is equal to the current taken from the battery
times the voltage of the battery during maximum power
output. 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 power output
Battery power
/
-20-
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 19). 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 19.
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 19 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 19
-21-
ASSEMBLY INSTRUCTIONS - DETECTOR
C6 - 100mF Lytic
(see Figure Da)
R5 - 27kW Resistor
(red-violet-orange-gold)
T1 - IF Coil (yellow)
TP3 - Test Point Pin
(see Figure F)
C4 - 10mF Lytic
(see Figure Da)
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)
R8 - 100W Resistor
(brown-black-brown-gold)
T3 - IF Coil (black)
TP5 - Test Point Pin
(see Figure F)
C15 - .001mF Discap
(marked 102)
D1 - 1N4148 Diode
(see Figure E)
C10 - .01mF Discap
(marked 103)
STATIC MEASUREMENTS
(SW2 on the top [TR] position)
AGC ZERO SIGNAL BIAS
With the power turned OFF, connect the VOM to test
point three (TP3) as shown in Figure 20.
Check that the VOM is adjusted to read 9 volts DC and
turn the power ON. The voltmeter should read
GND
TP10
approximately 1.5 volts DC. If your reading varies more
than 0.5 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.
Figure 20
T3 TEST
With the power turned OFF, connect the positive lead of
the VOM to TP5 and the negative lead to TP10. 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
If you do not have an RF generator, go to Section 3.
as your battery voltage or power supply voltage. If not,
turn OFF the power and check that T3 is properly
installed.
-22-
DYNAMIC MEASUREMENTS
DETECTOR AND ACG TEST
Turn the power OFF and connect the VOM and RF
generator as shown in Figure 21. Set the VOM to
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
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
Output Adjust
GND
TP10
Figure 21
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 22.
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
Probe
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
Output Adjust
GND
TP10
GND
TP10
GND
TP10
DETECTOR BANDWIDTH TEST
Connect equipment as shown in Figure 22. Set the RF
generator at 455kHz with 80% modulation at a
modulation frequency of 1kHz. Set the oscilloscope to
read 0.1 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
Figure 22
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.
-23-
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 23.
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.
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.
.707
452kHz 458kHz
455kHz
Figure 23
ASSEMBLY INSTRUCTIONS - SECOND IF AMPLIFIER
T2 - IF Coil
(White)
TP4 - Test Point Pin
(see Figure F)
R7 - 39kW Resistor
(orange-white-orange-gold)
R9 - 10kW Resistor
(brown-black-orange-gold)
Q3 - 2N3904 Transistor NPN
(see Figure A)
R10 - 470W Resistor
(yellow-violet-brown-gold)
C7 - .02mF or .022mF Discap
(marked 203 or 223)
C8 - .02mF or .022mF Discap
(marked 203 or 223)
-24-
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 24. Set the VOM to read 9
volts DC and turn ON the power. The voltage at the
GND
TP10
Figure 24
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 25.
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 keep
Generator
.02mF
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
TP10
Figure 25
-25-
GND
TP10
BANDWIDTH TEST
With the power OFF, connect your equipment as shown
in Figure 26. Turn the power ON and adjust the RF
generator for 0.4Vpp at the cathode of D1. If necessary,
realign transformer T3 for maximum output while
adjusting the output of the RF generator to maintain
0.4Vpp. Slowly decrease the frequency of the RF
generator until the signal drops to 0.707 of its peaked
value or 0.28Vpp. 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 0.707 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 23.
Generator
.02mF
Output Adjust
GND
TP10
Figure 26
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 Q2. This drop
Oscilloscope
Probe
GND
TP10
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 F)
Q2 - 2N3904 Transistor NPN
(see Figure A)
R6 - 1kW Resistor
(brown-black-red-gold)
C5 - .02mF or .022mF Discap
(marked 203 or 223)
-26-
STATIC MEASUREMENTS
Q2 BASE BIAS
With the power turned OFF, reconnect your VOM to test
point 3 (TP3) as shown in Figure 20. 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 TP10 and turn the power ON. The voltage
If you do not have an RF generator or oscilloscope, skip the following test and go to Section 5.
should be approximately 0.8 volts. Since the current in
Q2 is equal to the current in R6, I(Q2)=0.8/R6 or
approximately 0.8 milliamps.
DYNAMIC MEASUREMENTS
AC GAIN
With the power turned OFF, connect the RF generator and
the oscilloscope to your circuit as shown in Figure 27.
Using a clip lead, short TP5 to R8 as shown in Figure 27.
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
Probe
Generator
.02mF
Output Adjust
GND
TP10
Figure 27
lead shorting TP5 to R8. The AGC should reduce the
signal level at TP4 to approximately 0.3 volts.
Oscilloscope
Wire lead
or clip lead
GND
TP10
-27-
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.
ASSEMBLY INSTRUCTIONS - ANTENNA, MIXER AND OSCILLATOR
R1 - 56kW Resistor
L1 - Antenna with Holders
(see Figures K & L)
C2 - .02mF or .022mF Discap
(marked 203 or 223)
TP1 - Test Point Pin
(see Figure F)
R2 - 12kW Resistor
(brown-red-orange-gold)
R3 - 3.3kW Resistor
(orange-orange-red-gold)
Figure K
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.
3 Wire
White
Black
Red
R=9 - 11W
}
R=1 - 1.5W
}
(green-blue-orange-gold)
L2 - Oscillator Coil (red)
Q1 - 2N3904 Transistor NPN
(see Figure A)
C3 - .01mF Capacitor
(marked 103)
C1 - Tuning Gang Capacitor
2 Screws M2.5 x 3.8mm
Knob (dial)
Screw M2.5 x 8mm
Label (dial knob)
(see Figure M)
4 Wire
White
Black
Red
Green
R=9 - 11W
}
R=1 - 1.5W
}
-28-
Assemble it to the PC board as shown below.
Mount the antenna assembly to the PC board.
r Put the tab of the first holder into the right hole
and twist the tab 90°.
r Put the tab of the second holder into the left
hole and twist the tab 90°.
r Slide the ferrite core through the holders.
r Slide the antenna coil through the ferrite core.
Note: If the end of a wire from the antenna should
break off, strip the insulation off the end with a hot
soldering iron. Lay the wire down on a hard
surface and stroke the wire with your iron. The
insulation should come off very easily.
CAUTION: The soldering iron will burn the hard
surface that you are working on.
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.
3 Wire Type Antenna
Solder the 3 colored wires to the PC board.
r Wire A (red) to the hole marked “3”.
r Wire B (black) to the hole marked “2”.
r Wire C (white) to the hole marked “1”.
C (white)
B (black)
A (red)
Tabs
4 Wire Type Antenna
Solder the 4 colored wires to the PC board.
r Wire A (green) to the hole marked “3”.
r Wire B (red and black twisted together) to the
hole marked “2”.
r Wire C (white) to the hole marked “1”.
Twisted together
C (white)
Red
B
Black
Figure L
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 x 4mm screws.
Knob post
C1
Screw holes
3 Leads
OR
B Twisted together
Red
Fasten the knob to the
shaft of the capacitor
with one M2.5 x 8mm
screw.
Turn the dial fully
clockwise. Remove the
protective backing from
the label and align the
1600 with the arrow on the
PC board.
A (green)
Black
A (green)
Tabs
C (white)
M2.5 x 8mm
Screw
Knob
Solder leads
to pads
Figure M
4 Leads
-29-
PC Board Stand
Insert the PC board into the stand as shown.
STATIC MEASUREMENTS
Figure N
Q1 BIAS
With the power turned OFF, connect the VOM to your
circuit as shown in Figure 28. 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 at
GND
TP10
Figure 28
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.
Wire lead
or clip lead
If you do not have an oscilloscope, go to the Final Alignments With No Test Equipment Section.
-30-
DYNAMIC MEASUREMENTS
OSCILLATOR CIRCUIT
With the power turned OFF, connect the oscilloscope to
the circuit as shown in Figure 29.
a vertical sensitivity of 1 volt/division and turn the power
ON. The oscilloscope should display a low voltage sine
Set the oscilloscope for
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 29
If you do not have an RF generator, go to the Final Alignments with No Test Equipment Section.
FINAL ALIGNMENTS
IF BANDWIDTH
With the power turned OFF, connect the RF generator and
the oscilloscope to your circuit as shown in Figure 30.
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
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.
GND TP10
Oscilloscope
Output Adjust
GND
TP10
Figure 30
.02mF
Wire lead
or clip lead
-31-
Probe
GND
TP10
SETTING OSCILLATOR RANGE
With the power turned OFF, connect the equipment to the
circuit as shown in Figure 30. 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 counter-clockwise. 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 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 31).
Antenna trimmer
Oscillator trimmer
Figure 31
Oscillator trimmer
3 Leads
Antenna trimmer
4 Leads
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, you may
ANTENNA ALIGNMENT
With the power turned OFF, connect test equipment to
your circuit as shown in Figure 32. Set the RF generator
at 600kHz, 400Hz 80% modulation, moderate signal
strength. Set the oscilloscope to read 0.5Vpp 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 33). Change the frequency of the RF
Generator
Close to antenna
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 31). 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 33).
Probe
Oscilloscope
Output Adjust
GND
TP10
Figure 32
Figure 33
Coil
Wax
Wax
-32-
Holders
GND
TP10
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 1¼ to 1½ turns. Set the antenna coil about
1/8” from the end of its ferrite rod. Refer to Figure L on
page 29.
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 33).
DC Voltages
The voltage readings below should be used in troubleshooting the AM radio. Measure the voltage on transistors
Q4 - Q6 with switch SW2 in the top position. When measuring the voltage on the IC, make sure the switch SW2
is in the down position.
Q1 B 1.5V Q6 B 5.8V
E 1.0V E 5.2V
C 8.9V C 9.0V
Q2 B 1.4V Q7 B 4.6V
E 0.7V E 5.2V
C 8.9V C 0.0V
Q3 B 1.7V U1 1 - 1.3V
E 1.0V 2 - 0
C 9.0V 3 - 0
4 - 0
Q4 B 5.7V 5 - 4.5V
E 5.2V 6 - 9V
C 8.3V 7 - 4.6V
8 - 1.3V
Q5 B 8.3V
E 9.0V
C 5.8V
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.
3. Battery voltage: 9.0V
4. All voltages are referenced to circuit common.
5. Voltage reading can vary +
10%.
-33-
AM-550CK RADIO BAFFLE
NOTICE:
The final step in the radio kit will be to assemble and attach a baffle to the speaker. You will need to remove the baffle located in the
bottom of the box. If it does not want to come out easily, use a knife to cut the holding tabs.
When a speaker is not enclosed, sound waves can travel in all directions. As a speaker moves outward, it creates positive pressure on
the air in front of it and negative pressure on the rear. At low frequencies, out of phase front and rear waves mix causing partial or total
cancellation of the sound wave. The end result is a speaker less efficient and distorted.
To eliminate the low frequency cancellation, a speaker is placed inside an enclosure. Now the front sound waves are prevented from
traveling to the back. The speaker will now compress and decompress air inside, increasing its resonant frequency and Q relative to the
free air values. This type of effectively air-tight box is called an Acoustic
Nut M1.8
Baffle
Keep the box the kit came in. After you have completed the radio and it operates satisfactorily, you may want to install a baffle
to improve the sound.
Suspension.
Screw M1.8 x 7mm
1. Start at one edge and carefully remove the baffle from
the bottom of the kit box.
AM-550C Kit Carton
brown
side
2. Bend the four flaps upward as shown.
3. Bend the top side upward as shown. 4. Bend the two sides upward. Attach the
6.
Back View
M1.8 Nut
r Remove the nut from the top M1.8 x 7mm screw. Insert the baffle as shown in Step 6. Insert an M1.8 x 7mm screw and fasten down
the baffle with two M1.8 nuts as shown in Step 6.
Optional: To make an air tight seal, place a bead of seal between the PC board and the baffle.
three sides using scotch tape or glue
(Elmer’s, Duco Cement, or other).
M1.8 x 7mm
Screw
Seal
M1.8 x 8mm Screw
(from battery holder)
-34-
5. Bend the bottom side upward and
attach it to the other sides using scotch
tape or glue. Bend the two mounting
flaps as shown.
SCHEMATIC DIAGRAM
(847) 541-3800 ● Website: www.elenco.com ● e-mail: elenco@elenco.com
ELENCO
150 Carpenter Avenue ● Wheeling, IL 60090
®
Loading...