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

AM/FM RADIO KIT

MODEL AM/FM-108TK

14 TRANSISTORS, 5 DIODES

Assembly and Instruction Manual

Elenco®Electronics, Inc.

ight © 2004, 2000 b

yr

Cop

t of this book shall be reproduced b

No par

y Elenco

®

Electronics

y means;

y an

, Inc.

electronic

ights reser

All r

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

ved. Revised 2004 REV-H 753508T

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

PARTS LIST FOR THE AM RADIO SECTION

If you are a student, and any parts are missing or damaged, please see instructor or bookstore. If you purchased this kit

rom a distributor, catalog, etc., please contact Elenco

f
additional assistance, if needed. DO NOT contact your place of purchase as they will not be able to help you.

Qty. Symbol Value Color Code Part #

1 R46 47Ω 5% 1/4W yellow-violet-black-gold 124700
4 R38, 43, 48, 49 100Ω 5% 1/4W brown-black-brown-gold 131000
1 R47 330Ω 5% 1/4W orange-orange-brown-gold 133300
1 R41 470Ω 5% 1/4W yellow-violet-brown-gold 134700
1 R37 1kΩ 5% 1/4W brown-black-red-gold 141000
1 R42 2.2kΩ 5% 1/4W red-red-red-gold 142200
3 R33, 36, 44 3.3kΩ 5% 1/4W orange-orange-red-gold 143300
1 R40 10kΩ 5% 1/4W brown-black-orange-gold 151000
1 R32 12kΩ 5% 1/4W brown-red-orange-gold 151200
1 R35 27kΩ 5% 1/4W red-violet-orange-gold 152700
1 R39 39kΩ 5% 1/4W orange-white-orange-gold 153900
1 R31 56kΩ 5% 1/4W green-blue-orange-gold 155600
1 R45 470kΩ 5% 1/4W yellow-violet-yellow-gold 164700
1 R34 1MΩ 5% 1/4W brown-black-green-gold 171000
1 Volume/S2 50kΩ / SW Pot/SW with nut and washer 192522

®

lectronics (address/phone/e-mail is at the back of this manual) for

E

RESISTORS

CAPACITORS

Qty. Symbol

1 C30 150pF Discap (151) 221510
1 C44 .001µF Discap (102) 231036
2 C31, 38 .01
5 C29, 33, 35, 36, 37 .02µF or .022µF Discap (203) or (223) 242010
1 C28 .1µF Discap (104) 251010
3 C32, 40, 41 10
1 C42 47µF Electrolytic Radial (Lytic) 274744
1 C34 100µF Electrolytic Radial (Lytic) 281044
2 C39, 43 470
1 C1 Variable Tuning Gang AM/FM 299904

Value

µF Discap (103) 241031

µF Electrolytic Radial (Lytic) 271045

µF Electrolytic Radial (Lytic) 284744

Description

Part #

SEMICONDUCTORS

Qty. Symbol Value Description Part #

2 D4, 5 1N4148 Diode 314148
5 Q7, 8, 9, 10, 11
1 Q12 2N3906 Transistor PNP 323906

Q14 MPS6560 or 8050 Transistor NPN 328050

1
1 Q13 MPS6562 or 8550 Transistor PNP 328550

2N3904 Transistor NPN 323904

COILS MAGIC WAND

Symbol

.

Qty

1 L5 Red AM Oscillator 430057 1 Iron Core 461000
1 T6 Yellow AM IF 430260 1 Brass Core 661150

T7

1
1 T8 Black AM IF 430264
1 L4 AM Antenna with Holders 484004

Color Description Part # Qty. Description Part #

White AM IF 430262 4” Shrink Tubing 890120

Description Part #

.

Qty

1 PC Board 517054
1 Switch 541023

Battery Holder 590096

1
1 Speaker 590102
1 Knob (pot) 622017

Knob (dial) 622030

1
1 Earphone Jack with Nut 622130 or 622131
1 Radio Stand 626100

Earphone 629250

1

T THIS KIT CAME IN. IT WILL BE USED ON PAGES 24 & 53. ****

THE BOX

**** SA

VE

THA

MISCELLANEOUS

.

Qty

3 Screw 2-56 x 1/4” 641230
3 Screw M2.5 641310
3
1 Plastic Washer 645108
8 Test Point Pin 665008
1
1 Speaker Pad 780128
6” Wire 22 insulated 814520
1

-1-

Description Part #

Nut 2-56 644201

Label AM/FM 723508

Solder 9ST4

IDENTIFYING RESISTOR VALUES

U

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

BAND 1

1st Digit

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

BAND 2

2nd Digit

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

2 Multiplier Tolerance

1

Multiplier

Color Multiplier
Black 1
Brown 10
Red 100
Orange 1,000
Yellow 10,000
Green 100,000
Blue 1,000,000
Silver 0.01
Gold 0.1

BANDS

Resistance

Tolerance

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

5%
1%
2%
3%

.25%

IDENTIFYING CAPACITOR VALUES

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

Multiplier

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

The letter “R” may be used at times

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

The v

For the No.01234589
Multiply By

103K

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

First Digit
Second Digit
Multiplier

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

orking Voltage

W

100

Tolerance

um

Maxim

µF 100V

-2-

INTRODUCTION

The Elenco®Superhet 108T AM/FM Radio Kit is a
“superheterodyne” receiver of the standard AM (amplitude
modulation) and FM (frequency modulation) broadcast
frequencies. The unique design of the Superhet 108
allows you to place the parts over their corresponding
symbol in the schematic drawing on the surface of the
printed circuit board during assembly. This technique
maximizes the learning process while keeping the
chances of an assembly error at a minimum. It is very
important, however, that good soldering practices are
used to prevent bad connections. The Soldering Guide
should be reviewed before any soldering is attempted.
The actual assembly is broken down into 9 sections. The
theory of operation for each section, or stage, should be

GENERAL DISCUSSION

FM RADIO

Section 9

Section 8

Section 7 Section 6

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 108T’s reception capabilities.

FM RF

AMPLIFIER

FM

OSCILLATOR

AM MIXER

OSCILLATOR

FM MIXER

1ST FM IF

AMPLIFIER

AFC

1ST AM IF

AMPLIFIER

AM

Section 3 Section 2

AM RADIO

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

k a DC v

bac
maintain a near constant level of audio at the detector.
Section 3 is the second AM IF amplifier. The second AM
IF amplifier is tuned to 455kHz (Kilohertz) and has a fix
gain at this frequency of 50. Section 4 is the first AM IF 2
amplifier which has a variable gain that depends on the

oltage received from the AGC stage. The first AM

GC v

A
IF amplifier is also tuned to 455kHz. Section 5 includes
the AM mixer, AM oscillator and AM antenna stages.
When the r
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.
accomplished by mixing (heterodyning) the radio

wer of the audio signal received from the

erts

oltage to the first AM IF amplifier in order to

ed

adio w

ave passes through the antenna, it

This change is

2ND FM IF

AMPLIFIER

2ND AM IF

AMPLIFIER

-3-

Figure 1

FM

DETECTOR

Speaker

AUDIO

AMPLIFIER

AM

DETECTOR

AFC

Section 1Section 5 Section 4

frequency signal with the oscillator signal. Section 6 is the

atio detector circuit. The FM ratio detector has a fixed

FM r
gain of about 20. Section 7 is the second FM IF amplifier.
The second FM IF amplifier is tuned to 10.7MHz
(Megahertz) and has a set gain of approximately 20. The
3dB bandwidth of this stage should be approximately
350kHz. Section 8 is the first FM IF amplifier. The first FM
IF amplifier is also tuned to 10.7MHz and has a set gain of
approximately 10. It also has a 3dB bandwidth of 350kHz.
Section 9 includes the FM mixer, FM oscillator, FM RF and
the AFC circuits.

The incoming r
by the FM RF amplifier, which is tuned to a desired radio
station in the FM frequency bandwidth of 88MHz to
108MHz.

These amplified signals are then coupled to the
FM mixer stage to be changed to a frequency of 10.7MHz.
This change, as in AM, is accomplished by heterodyning

adio frequency signal with the oscillator signal.

the r
AFC stage feeds back a DC voltage to the FM oscillator to
prevent the oscillator from drifting. Each of these blocks
will be e

xplained in detail in the

before the assembly instructions for that stage.

adio waves are amplified

The

Theory of Operation given

CONSTRUCTION

Introduction

The most important factor in assembling your Superhet 108 AM/FM 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 f
Immediately apply solder to
the opposite side of the
connection, away from the
iron. Allow the heated
component and the circuit

oil to melt the solder.

f

Allow the solder to flo

3.

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

4.

Here is what a good solder
connection looks like.

oil.

The

Component Lead

Foil

Solder

Foil

w

Solder

F

oil

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

vered. Use just enough

co
solder to co
connection.

3. Excessive solder - could

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

minals.

ter

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 dr
iron across the solder
bridge as shown.

ag y

ver the

our solder

ing

Rosin

Soldering iron positioned
incorrectly.

Solder

Component Lead

Solder

Solder

Foil

ing Iron

Dr

Gap

ag

-4-

SEMICONDUCTOR PARTS FAMILIARIZATION

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

TRANSISTOR TEST

Refer to the parts list and find a NPN transistor. Refer
the Figure C (page 7) 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 7) 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 1MΩ to infinity.

Low Resistance

Ω

OM

C

NPN

Ω

E

BC

COM

High Resistance

Ω

Ω

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

Ω

COM

Ω

PNP

E

BC

C

OM

High Resistance

Ω

PNP

Ω

E

BC

DIODE TEST

Refer to the parts list and find a diode. Refer to Figure E
(page 7) for locating the Cathode and Anode.

The end
with the band is the cathode. Using an Ohmmeter,
connect the diode as sho

wn in

Test E. Your meter

should be reading a low resistance. Using an

Low Resistance

Ω

COM

Ω

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 1MΩ to infinity for silicon
diodes (1N4148).

High Resistance

Ω

COM

Ω

Diode

-5-

SECTION 1

AUDIO AMPLIFIER

The purpose of the Audio Amplifier is to in-crease 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 (Q11 and Q12) of the audio
amplifier circuit are biased on for slightly more than 1/2
of the cycle, Class AB
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.

. In other words, the transistors

Figure 2

Transistor Q10 is a Class A amplifier that drives Q11
and Q12 through the bias string R44, D5 and R47. Q13
and Q14 are current amplifiers that amplify the current
of transistors Q11 and Q12. The AC and DC gain are
set by the DC current in transistor Q10 and the collector
resistor R44. The AC gain of the Audio Amplifier is
approximately equal to 100, while the DC gain equals
approximately 50. The transistors Q13 and Q14 self
bias so that the voltage at their emitters is
approximately 1/2 the supply voltage. R45 provides
feedback to the base of Q10 which is biased at
approximately .7 volts. Capacitor C40 couples the
audio signal from the v
audio amplifier. Capacitor C43 blocks the DC to the
speaker, while allowing the AC to pass.

olume control to the input of the

-6-

ASSEMBLY INSTRUCTIONS

We will begin by installing resistor R43. Identify the resistor by its color and install as shown in Figure A. Be careful
to properly mount and solder all components. Diodes, transistors and electrolytic capacitors are polarized, be sure
to follow the instructions carefully so that they are not mounted backwards. Check the box when you have completed
each installation.

Test Point Pin

Foil Side
of PC Board

Figure A

Diode

Be sure that the
band is in the
correct direction.

R43 - 100Ω Resistor

(brown-black-brown-gold)

Lytic Capacitor

Polarity Mark

( )

(+)

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

Figure B

Band

CathodeAnode

Figure E

NPN Transistor

Flat
Side

EBC

B

E

C

Figure C

Mount so E lead is
in the arrow hole
and flat side is in
the same direction
as shown on the
top legend. Leave
1/4” between the
part and PC board.

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

TP15 - Test Point Pin

(see Figure A)

C39 - 470µF Lytic

(see Figure B)

TP2 - Test Point Pin

(see Figure A)

R44 -

3.3kΩ Resistor

(orange-orange-red-gold)

D5 - 1N4148 Diode

(see Figure E)

R45 - 470kΩ Resistor

(yellow-violet-yellow-gold)

R47 - 330Ω Resistor

(orange-orange-brown-gold)

Q10 - 2N3904 Transistor

(see Figure C)

R46 - 47Ω Resistor

(yellow-violet-black-gold)

C42 - 47µF L

ytic

(see Figure B)

R48 - 100Ω Resistor

wn-gold)

k-bro

lac

wn-b

(bro

R49 - 100Ω Resistor

(brown-black-brown-gold)

C40 - 10µF Lytic

(see Figure B)

C41 - 10µF Lytic

(see Figure B)

C44 - .001µF Discap (102)

Q11 - 2N3904 Transistor

(see Figure C)

C43 - 470µF Lytic

(see Figure B)

Q13 - MPS6562 or 8550

Transistor (see Figure D)

est Point Pin

T

TP1 -

(see Figure A)

Q14 - MPS6560 or 8050

ansistor (see Figure C)

r

T

Q12 - 2N3906 Transistor

(see Figure D)

-7-

ASSEMBLY INSTRUCTIONS

Note: Mount the Pot/SW, earphone jack,

and speaker to the foil side of the PC
board.

Knob
Nut

Washer

Cut off
locating pin

Plastic Washer

Solder all 5 tabs to PC board

Figure F

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

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

(see Figure F)

Earphone Jack
with Nut

(see Figure H)

Speaker
Speaker Pad
Wire #22 Insulated

(see Figures G & I)

Figure G

If the speaker pad has center and outside pieces, then
remove them. Peel the backing off of the speaker pad and
stick the pad onto the speaker. Then stick the speaker
onto the solder side of the PC board as shown.

Remove

Backing

Speaker

Figure H

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

Foil Side

2

3

GND Pad

Nut

1

Foil Side

2

3

1

GND Pad

Nut

Pad

1.5” Wire

Backing

1.5”

Figure I

Wire

1.5” Wire
er

T

From

minal 3

1 - GND

Tip

2 ­3 - N.C. Tip

Mount the jack with the nut from the foil side of the PC board (terminal #1
on the GND pad of the PC board). Be sure to line up the tab with the pad
on the copper side of the PC board.
PC board.

Part # 622130

1 - GND

Tip

2 ­3 - N.C. Tip

Solder terminal #1 to the pad of the

Part # 622131

1” Wire

ar

P

t # 622130

1” Wire

Part # 622131

1.5” Wire

Cut three wires 1”, 1.5” and 1.5” and strip 1/4” of insulation
off of both ends. Solder the 3 wires as shown. Save the
extra wire for the FM Section.

-8-

STATIC MEASUREMENTS

POWER TEST

For all measurements, connect your equipment GND to
circuit GND TP15. Set your VOM (Volt-Ohm-Millimeter)
to read 2 amps DC. Connect the meter to the circuit as
shown in Figure 3. Make sure that the volume control
is in the OFF position (turned fully counter-clockwise).
While watching you VOM, turn the volume to the ON
position (rotate clockwise until a “click” is heard). The
VOM should indicate a very low current. Adjust your

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

+

--

+

--

A

Amps

COM

OUTPUT BIAS TEST

Put the battery into the holder.

Figure 3

TP15

COM

V

V

Figure 4

-9-

Adjust your VOM to read 9 volts and connect it as
shown in Figure 4. Make sure that the battery, or a 9
volt power supply (if available), is properly connected
and turn the power ON. The voltage at TP1 should be
between 3 to 6 volts. If you get this reading, go on to
the next test. If your circuit fails this test, turn the power

TRANSISTOR BIAS TEST

Move the positive lead of your VOM to the base of Q11.
Make sure that the power is ON. The voltage should be
between .5 and .8V higher than the voltage at TP1. All
silicon transistors biased for conduction will have
approximately .7V from the base to the emitter. Now
move the positive lead of your VOM to the base of Q12.
The voltage at this point should be between .5 and .8V

If you do not have a variable power supply, skip to the next test.

DYNAMIC MEASUREMENTS

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 E next to it. Check that all resistor values
are the correct value and not interchanged.

lower than the voltage at TP1. This is because Q12 is
a PNP type transistor. Turn the power OFF. If your
circuit fails this test, check the Q11 and Q12 are
properly inserted in the circuit board. All static tests
must pass before proceeding to the Dynamic Tests or
the next section.

DC GAIN

The DC gain of the audio amplifier is set by the current
in transistor Q10. Looking at the circuit and assuming
the output bias is 1/2 of V+ or 4.5 volts, the base of Q11
will be .7V higher or 5.2 volts. This is because there is
a negligible voltage drop across R48. This means there
is a 3.8 v
R44 can no
which equals 1.15 milliamps. Since D5 and R42 are
used for biasing transistors Q11 and Q12, the current
through Q10 can be assumed to be 1.15 milliamps.
The DC gain of Q10 can be calculated as the collector

oltage drop across R44. The current through

w be calculated as 3.8/R44 or 3.8/3.3k

resistor, R44, divided by the emitter resistor plus the
Effective Emitter Resistance. The effective emitter
resistance is actually the dynamic resistance of silicon
and can be calculated by the approximate equation:
Rj = 26 / I(in milliamps)

ore, Rj = 26 / 1.15 = 22.6 ohms. Now the DC gain

theref
can be calculated as:

R44 / (R46 + Rj) or 3300 / (47 + 22.6) which equals

47.4.

V

Figure 5

-10-

COM

TP15

V

It is advisable to use a digital meter because of the
small voltage changes in the following test. Connect
your VOM to the circuit as shown in Figure 5. Set your
VOM to read 1 volt DC and turn the power ON. Record
the base of Q10 here:

Vb1 = _____ volts.

Now set your VOM to read 9 volts and connect the
positive lead to test point TP1. Record the output bias
voltage here:

Vo = ____ volts.

Turn the power OFF. With a 1M ohm resistor (brown­black-green-gold), R34, connect the power supply to the
circuit as shown in Figure 6.

+

–

Power Supply

TP15

Turn the radio ON and turn the power supply ON.
Increase the supply voltage until the voltage at TP1 is
equal to Vo. Now increase the voltage of the supply
until the voltage at TP1 decreases by 1 volt. Move the
positive lead of your VOM to the base of Q10 and
record the voltage here:

Vb2 = ______.

It may be necessary to change scales of your VOM for
a more accurate reading. Turn the power OFF and
disconnect the power supply. Since the DC gain equals
the DC change at the output divided by the DC change
at the input, the DC gain of the audio can be calculated
as: 1 / (Vb2 - Vb1). Your answer should be near the
calculated DC gain of 47.4.

1MΩ

R34

Figure 6

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

AC GAIN

The AC gain can be calculated in the same manner as
the DC gain e
capacitor C42 bypasses the emitter resistor R46
leaving only the effective emitter resistance, and there
is a resistance seen at the output of Q13 and Q14.
AC gain of Q10 can be calculated as R44 / Rj or 3300
/ 22.6 which equals 146.
positive, there will be a current flowing in Q11, which we
will call I(Q11).
the Beta (β) of transistor Q13 or β x I(Q11). The total
current at the output is equal to I(Q11) x (1 + β). The
resistance of R48 is also seen at the output. The
resistance is eff
Assuming β of the output transistors are equal to 100
than the resistance seen at the output is equal to 1
ohm, 100 / 100. This means that there is a voltage
divider between the output and the 8 ohm speaker. The
signal is now divided down so that the output is equal
to the A
which equals 130. This is also true when the input

C (gain of Q10) x (8 / (1+8)), or 146 x (8 / 9)

xcept for two differences. For AC,

The

When the input signal is

This current will then be multiplied by

ely divided b

ectiv

y

β, R48 / β.

COM

TP15

signal is negative. The only difference is that Q12 and
Q14 are no
generator to the circuit as shown in Figure 7.

mally the AC gain is measured at a frequency of

Nor
1kHz. Your VOM, however may not be able to
accur
Therefore, it is recommended that this test be

ormed at 400Hz. Set the audio generator at 400Hz

perf
and minimum voltage output. With the power ON, set
your VOM to read an AC voltage of 1 volt at test point
TP1.
Slowly increase the amplitude of the audio generator
until y
generator at this setting and move the positive lead of
your VOM to the base of Q10. Record the AC input
voltage to the amplifier here:

w conducting. Connect the VOM and audio

oltages at this frequency

ately read A

Increase the v

OM reads 1 v

our V

C v

olume control about half w

C

olt A

Vin = __________ v

. Leave the audio

.

olts

-11-

V

V

.

ay.

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

enerator

G

output voltage divided by the AC input voltage, or 1/Vin.
The gain should approximately equal the calculated
gain.

Hz

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

AC BANDWIDTH

Generator

Hz

TP15

Figure 7

COM

TP15

Oscilloscope

V

V

TP15

Connect the oscilloscope and audio generator to your
circuit as sho

wn in Figure 8.
Set the audio generator for a frequency of 1kHz and
minim

oltage output.

um v

Set the oscilloscope to read
.5 volts per division. Turn on the power and slowly
increase the v

olume control to a comfortable level.
Increase the amplitude of the audio generator until the
oscilloscope displays 2 volts peak to peak, (Vpp), at

It may be necessary to adjust the volume control.

TP1.
Move the oscilloscope probe to the base of Q10 and
record the input v

oltage here:

Vin = _______ Vpp

Figure 8

-12-

TP15

(at this point, you may want to verify the AC gain).

Move the oscilloscope probe back to TP1 and slowly
increase the frequency from the audio gener

ator until

the waveform on the oscilloscope drops to .7 of its

iginal reading (1.4Vpp or 2.8 divisions). The

or
frequency of the generator, when the output drops to
.7 of its original value, is called the high frequency 3
decibel (dB) cor

ner. Record this frequency here:

(f high 3dB) = __________ kHz.

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

2.8 divisions. This frequency is called the low
frequency 3dB corner - the low frequency 3dB corner or
(f high 3dB) - (f low 3dB). Your calculated answer
should be greater than 30kHz.

DISTORTION

Connect the generator and oscilloscope as shown in
Figure 8. Set the generator at a frequency of 1kHz, turn
the power ON and turn the volume to maximum. Adjust
the generator output until the peaks of the sinewave at
TP1 are clipped as shown in Figure 9A. One side of the
sinewave may clip before the other depending on the
DC centering at TP1. If oscillations are seen, connect
a clip lead from the GND of your generator to the GND
of the circuit.

Clipped

Crossover
Distortion

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 350
milliwatts.

2

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

EFFICIENCY

By measuring the DC power taken from the battery at
the maximum power output level, the efficiency to the
audio amplifier can be calculated. Power from the
battery is equal to the current taken from the battery
times the voltage of the battery during maximum power
output. Efficiency can then be calculated as follows: Eff
= Max audio power/Battery power. It is best to use a
power supply (if available) to prevent supply voltage
from changing during these measurements. Connect
the generator, oscilloscope and current meter as shown
in Figure 11. Set your current meter to read 1 amp DC.
Turn the power ON and rotate the volume control to
maximum. Slowly increase the amplitude of the audio

ator until the output is clipped as shown in Figure

gener

Record Vclp here:

9A.

A

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

Using a wire short out diode D5 and resistor R47 as
shown in Figure 10. The waveform should resemble
Figure 9B. The “flat spots” near the center of each
sinewave demonstrate what is called crossover
distortion. Most of this distortion should disappear when
you remove the shorting lead. Turn the power OFF.

Figure 9

Vclp = _______ Vpp.

B

Wire Lead

or Clip Lead

Vclp = _________ Vpp.

This should be equal to Vclp in step 4. Record the DC
current dr

Measure the supply voltage and record the V supply
here:

Turn the power OFF. Calculate the maximum power
output as done in the

awn from the 9 volt supply here:

Current (I) max = ________ Amps.

V supply = ________ volts.

Maximum Power Output Step.

Record your answers on the next page.

Figure 10

-13-

Generator

Hz

TP15

TP15

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

ower Supply

P

mps

A

mps COM

A

Figure 11

TP15

Vp = Vclp/2 Vp = ______

ms = Vp x .7 Vrms = ______

Vr

Max power out = (Vrms)

2

/8 Max power out = ______

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

:

er

w

po

Battery power = Imax x V supply Battery power = ______

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

er out/Batter

N = Max po

w

N in % = N x 100

.

er N = _______

w

y po

N = _______%

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

-14-

SECTION 2

AM DETECTOR AND AGC STAGE

The purpose of the detector is to change the amplitude
modulated IF signal back to an audio signal. This is
accomplished by a process called detection or
demodulation. First, the amplitude modulated IF signal
is applied to a diode in such a way as to leave only the
negative portion of that signal (see Figure 12). The
diode acts like an electronic check valve that only lets
current pass in the same direction as the arrow (in the
diode symbol) points. When the diode is in conduction
(On Condition), it will force the capacitors C33 and C38

to charge to approximately the same voltage as the
negative peak of the IF signal. After conduction stops
in the diode (Off Condition), the capacitors will
discharge through resistors R36 and R42. The
discharge time constant must be small enough to follow
the audio signal or high frequency audio distortion will
occur. The discharge time constant must be large
enough, however, to remove the intermediate
frequency (455kHz) and leave only the audio as shown
in Figure 12.

Figure 12

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

ASSEMBLY INSTRUCTIONS

Switch

J2 - Jumper Wire

(use lytic lead)

1/8”

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

-15-

ASSEMBLY INSTRUCTIONS

C34 - 100µF Lytic

(see Figure B)

T6 - AM IF Coil

(Yellow Dot)

27kΩ Resistor

R35 -

(red-violet-orange-gold)

TP5 - Test Point Pin

(see Figure A)

C32 - 10µF Lytic

(see Figure B)

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

C33 - .02µF (203)

or .022µF (223) Discap

R38 - 100Ω Resistor

(brown-black-brown-gold)

TP3 - Test Point Pin

(see Figure A)

T8 - AM IF Coil

(Black Dot)

D4 - 1N4148 Diode

(see Figure E)

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

(red-red-red-gold)

STATIC MEASUREMENTS

AGC ZERO SIGNAL BIAS

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

k that the VOM is adjusted to

Chec
read 9 volts DC and turn the power
ON. The voltmeter should read
approximately 1.5 volts DC. If your
reading varies by more than .5 volts
from this value, turn the power OFF
and check the polarity of D4. Also
check R36 and R35 and check that

ansformer T6 is properly installed.

tr

Amps COM V/Ω

TP15

T8 TEST

er turned OFF, connect the positive lead of

With the po
the VOM to TP3 and the negative lead to ground pin
TP15.
and turn the power ON. The voltage on the VOM should

w

e sure that the

Mak

OM is set to read 9 v

V

olts DC

V

Figure 13

be the same as y
voltage. If not, turn the power OFF and check that T8
is proper

ly installed.

our batter

T

y voltage or power supply

urn the po

wer OFF.

ou do not ha

If y

ve an RF g

-16-

enerator

, skip to Section 3.