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

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AM/FM RADIO KIT

MODEL AM/FM-108TK

14 TRANSISTORS, 5 DIODES

Assembly and Instruction Manual

Elenco®Electronics, Inc.

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

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

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

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

THE BOX

**** SA

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

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

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

100

Tolerance

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

OSCILLATOR

AM MIXER

OSCILLATOR

FM MIXER

1ST FM IF

AMPLIFIER

AFC

1ST AM IF

AMPLIFIER

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

adio w

ave passes through the antenna, it

This change is

2ND FM IF

AMPLIFIER

2ND AM IF

AMPLIFIER

-3-

Figure 1

DETECTOR

Speaker

AUDIO

AMPLIFIER

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.

Allow the solder to flo

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.

Here is what a good solder connection looks like.

oil.

The

Component Lead

Foil

Solder

Foil

Solder

oil

Soldering Iron

Circuit Board

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

Foil

ing Iron

Gap

-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

NPN

Ω

COM

High Resistance

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

High Resistance

Ω

PNP

Ω

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

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

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

1/4” between the part and PC board.

Figure D

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

TP1 -

(see Figure A)

Q14 - MPS6560 or 8050

ansistor (see Figure C)

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

GND Pad

Nut

Foil Side

GND Pad

Nut

Pad

1.5” Wire

Backing

1.5”

Figure I

Wire

1.5” Wire er

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

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.

Amps

COM

OUTPUT BIAS TEST

Put the battery into the holder.

Figure 3

TP15

COM

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.

Figure 5

-10-

COM

TP15

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

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

olt A

Vin = __________ v

. Leave the audio

olts

-11-

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

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

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

AC BANDWIDTH

Generator

TP15

Figure 7

COM

TP15

Oscilloscope

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.

/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.

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.

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

TP15

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

ower Supply

mps

mps COM

Figure 11

TP15

Vp = Vclp/2 Vp = ______

ms = Vp x .7 Vrms = ______

Max power out = (Vrms)

/8 Max power out = ______

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

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

N in % = N x 100

er N = _______

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.

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

e sure that the

Mak

OM is set to read 9 v

olts DC

Figure 13

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

ly installed.

our batter

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.

DYNAMIC MEASUREMENTS

AM DETECTOR AND AGC TEST

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

Amps COM

V/Ω

Generator

.001µF

TP15

Figure 14

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

If your RF generator does not have amplitude modulation and

olts DC and set the

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

SYSTEM CHECK

Connect your equipment as shown in Figure 15.

.001µF

TP15

voltage at TP5 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: ____________.

Oscilloscope

Generator

TP15

Set the RF gener

ator at 455kHz, 1kHz at 80% modulation and minimum voltage output. Turn the power ON and set the v

olume control at maxim

adjust the amplitude of the RF generator output until you

um.

Figure 15

wly

Slo

TP15

hear the 1kHz tone on the speak

er. If this test fails, turn the power OFF and check R42, D4 and TP5. Turn the po

er OFF

-17-

AM DETECTOR BANDWIDTH TEST

Connect your test equipment as shown in Figure 15. Set the generator at 455kHz with 80% modulation at a modulation frequency of 1kHz. Set the oscilloscope to read .1 volts per division. Turn the power ON and set the volume at the minimum. Increase the amplitude of the generator until the signal on the oscilloscope is 4 divisions peak to peak. Check the signal to make sure that it is free of distortion. Leave the frequency of the

SECTION 3

SECOND AM IF AMPLIFIER

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 (T8) 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 16.

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

generator at 455kHz, but increase the modulation frequency until the output drops to .28Vpp. Record the modulation frequency on the generator here:

____________

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

stations. The gain at 455kHz in the second IF amplifier is fixed by the AC impedance of the primary side of transformer T8, and the DC current in Q9. The current in Q9 is set by resistors R39, R40 and R41. Both C36 and C37 bypass the 455kHz signal to ground, making Q9 a common emitter amplifier. The signal is coupled from the first IF amplifier to the second IF amplifier through transformer T7. 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.

kHz

Figure 16

kHz

-18-

ASSEMBLY INSTRUCTIONS

TP4 - Test Point Pin

(see Figure A)

T7 - AM IF Coil

(White Dot)

R40 - 10kΩ Resistor

(brown-black-orange-gold)

R41 - 470Ω Resistor

(yellow-violet-brown-gold)

STATIC MEASUREMENTS

Q9 BIAS

R39 - 39kΩ Resistor

(orange-white-orange-gold)

36 - .02µF Discap (203)

Q9 - 2N3904 transistor

(see Figure C)

C37 - .02µF (203)

or .022µF (223) Discap

Connect your VOM as shown in Figure 17. Set the VOM to read 9 volts DC and turn the power ON. The voltage at the emitter of Q9 should be approximately 1

COM

Amps

V/Ω

TP15

volt. If your reading is different by more than .5 volts, turn the power OFF and check components R39, R40, R41 and Q9.

ou do not have an RF generator and oscilloscope, skip to Section 4.

If y

Figure 17

-19-

DYNAMIC MEASUREMENTS

AC GAIN

Connect your test equipment as shown in Figure 18.

Generator

.02µF

TP15

Oscilloscope

Probe

TP15

Figure 18

Set the generator at 455kHz, no modulation and minimum voltage output. Set the oscilloscope at 1 volt per division. capacitance of 50pF or less or it will detune power ON and slowly increase the amplitude of the generator until 4 volts peak to peak are seen on the scope. With an alignment tool or screwdriver, tune T8 for a peak on the scope while re-adjusting the generator’s amplitude to maintain 4 Vpp at the oscilloscope. After T8 is aligned, move the scope probe to the base of Q9 and record the peak to peak amplitude of the signal here:

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 value is less than 50 check components R39, R40, R41, C36 and C37. Also make sure that Q9 is proper

The scope probe must have an input

T8. Turn the

Vb=________ Vpp.

ly installed.

n the po

wer OFF.

BANDWIDTH

Reconnect your test equipment as shown in Figure 18. Turn the power ON and adjust the generator for 4 volts peak to peak at TP3. Realign T8, if necessary, for maximum output while adjusting the output of the generator to maintain 4Vpp at TP3. Slowly decrease the frequency of the RF generator until the signal at TP3 drops to .707 of its original value or 2.8Vpp. Record the frequency of the RF generator here:

Fl = _______kHz.

Now increase the frequency of the generator past the peak to a point where the signal drops to .707 of its

. Record that frequency here:

peak v

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

alue

Fh = __________kHz.

Calculate the bandwidth by (FI–Fh)

Bandwidth = __________kHz.

our results should be similar to the v

Y Figure 16. Turn the power OFF.

-20-

alues sho

wn in

SECTION 4

FIRST AM IF AMPLIFIER

The operation of the first IF amplifier is the same as 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 Q8 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 Q8. This drop

ASSEMBLY INSTRUCTIONS

R34 - 1MΩ Resistor (brown-black-green-gold)

TP6 - Test Point Pin

(see Figure A)

lowers the voltage across R37 and thus, reduces the DC current through R37. Since all of the DC current from the emitter of Q8 must go through R37, the DC current in Q8 is therefore lowered. When the DC current in a transistor is lowered, its effective emitter resistance increases. The AC gain of transistor Q8 is equal to the AC collector load of Q8 divided by its effective emitter resistance. Raising the value of the effective emitter resistance, thus, lowers the AC gain of Q8.

CAUTION: Test point must not touch can of IF Coil.

STATIC MEASUREMENTS

Q8 BASE BIAS

VOM to the circuit as shown in Figure 13.

Connect y Set your VOM to read 2 volts DC and turn the power ON. The voltage at TP5 should be approximately 1.5 volts. If your circuit fails this test, check Q8 and R37. Turn the power OFF.

our

Q8 - 2N3904 Transistor

(see Figure C)

C35 - .02µF (203)

or .022µF (223) Discap

R37 - 1kΩ Resistor

(brown-black-red-gold)

Q8 CURRENT

Connect the positiv Q8 and connect the negative lead to ground point TP15. Turn the power ON. The voltage should be approximately .8 volts. Since the current in Q8 is equal to the current in R37, I(Q2) = .8/R37 or approximately .8 milliamps. Turn the power OFF.

e lead of y

our VOM to the emitter of

If you do not have an RF generator and oscilloscope, skip to Section 5.

-21-

DYNAMIC MEASUREMENTS

.001µF

TP15

Short TP3 to R38 as shown below.

Oscilloscope

TP15

Figure 19

AC GAIN

Connect your test equipment as shown in Figure 19.

The scope probe must have an input capacitance of 12pF or less, otherwise it will detune transformer T7. Using a clip lead, short TP3 to R38 as sho short prevents the AGC from lower first IF amplifier modulation, and minimum voltage output. Set the scope to read 1 volt per division and turn the power ON. Increase the amplitude of the generator until approximately 4Vpp is seen on the scope. Retune the IF transformer T7 to maximize the 455kHz at TP4. After tuning T7, adjust the generator amplitude in order

eep 4Vpp at TP4. Now move the scope probe to

to k

. Set the generator to 455kHz, no

ing the gain of the

wn. This

SECTION 5

AM MIXER, AM OSCILLATOR, AND AM ANTENNA

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 Q7 not only amplifies the RF signal, but also simulateously oscillates at a frequency 455kHz above

Positive feedback

the desired r from the collector to the emitter of Q7 is provided by coil L5 and capacitor C31.

adio station frequency

Dur

ing the heterodyning

the base of Q8 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 should be greater than 100. DO NOT TURN THE POWER OFF, GO TO THE NEXT TEST.

AGC ACTION

Move the scope probe back to TP4 and adjust the generator for 4Vpp if necessary. Remove the clip lead shorting TP3 to R38. The AGC should reduce the signal level at TP4 to approximately .8 volts. Turn the power

OFF

process the following four frequencies are present at the collector of Q7.

1. The local oscillator frequency, OF.

2. The RF carrier or radio station frequency.

The sum of these tw

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

o frequencies

, LO + RF.

-22-

The “difference frequency” is used as the intermediate frequency in AM radios. The collector of Q7 also contains an IF transformer (T6) tuned only to the difference frequency. This transformer rejects all frequencies except those near 455kHz. T6 also couples the 455kHz signal to the base of Q8 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

ASSEMBLY INSTRUCTIONS

C28 - .1µF Discap (104)

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.

R31 - 56kΩ Resistor

(green-blue-orange-gold)

C30 - 150pF Discap (151)

L5 - AM Oscillator Coil

(Red Dot)

J1 - Jumper Wire

(use lytic lead)

L4 - AM Antenna w/ holders

(see Figures J & K)

C1 - Tuning Gang Capacitor 2 Screws #3

(see Figure L) Knob (dial) Screw #3 Label AM/FM

(See Figure M)

Note: Mount the tuning gang capacitor to the f the PC

board.

oil side of

Figure J

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

errite core. Measure the

our

resistance of the coil. readings should match the appro

ximate v

alues as sho

wn.

White

Black

Red

R=9 - 11Ω

}

R=1 - 1.5Ω

1/8”

TP7 - Test Point Pin

(see Figure A)

C31 - .01µF Discap (103)

Q7 - 2N3904 Transistor

(see Figure C)

R32 - 12kΩ Resistor

(brown-red-orange-gold)

R33 - 3.3kΩ Resistor (orange-orange-red-gold)

C29 - .02µF Discap (203)

or .022µF Discap (223)

Wire3 Wire

White

R=9 - 11Ω

}

Black

Red

R=1 - 1.5Ω

}

Green

-23-

IMPORTANT: Before installing the antenna coil, determine if you have a 3 wire coil or a 4 wire coil. Assemble it to the

PC board as shown below. Mount the antenna assembly to the PC board.

Put the tab of the first holder into the right hole and twist the tab 90O. Put the tab of the second holder into the left hole and twist the tab 90O. Slide the ferrite core through the holders.

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.

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.

C (white)

B (black) A (red)

C (white)

Twisted Together

Black

B Twisted Together

C (white)

Black

Red

A (green)

Tabs

3 Wire Type Antenna: Solder the 3 colored wires to the PC board: Wire A (red) to the hole marked “RED”, Wire B (black) to the hole marked “BLK” and

Wire C (white) to the hole marked “WHT”.

Red

A (green)

4 Wire Type Antenna: Solder the 4 colored wires to the PC board: Wire A (green) to the hole marked “RED”, Wire B (red and black twisted together) to the hole marked “BLK” and

Wire C (white) to the hole marked “WHT”.

Tabs

Figure K

It is important to know which of the two types of the tuning gang capacitor you have received with your kit. Look at the gang capacitor that you have.

FM Antenna

Trimmer

FM Oscillator

Trimmer

FM SIDE

Locator Lead

AM SIDE

AM Antenna

Trimmer

AM Oscillator

Trimmer

AM Antenna

Trimmer

FM Antenna

Trimmer

FM SIDE

AM SIDE

AM Oscillator

Trimmer

FM Oscillator

Trimmer

Locator Lead

Mount the tuning gang capacitor to the foil side of the PC board with the AM and FM sides in the correct direction. Fasten the gang in place with two screws from the front of the PC board. Solder the leads in place and cut off the excess leads coming through the PC board on the front side.

Figure L

-24-

Knob Post

w Holes

Scre

ASSEMBLY INSTRUCTIONS

Figure M

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

Rotate the knob fully clockwise. Peel off the protective backing on the label. Line up the long white lines on the label with the arro on the PC board.

Screw

Knob

PC Board Stand

Insert the PC board into the stand as shown.

-25-

STATIC MEASUREMENTS

Q7 BIAS

Connect your VOM to the circuit as shown in Figure 20.

Short TP6 to the collector of Q7 as shown below.

COM V

TP15

Figure 20

Connect a clip lead from TP6 to the collector of Q7. This short prevents Q7 from oscillating. Set the VOM to read 2 volts DC and turn the power ON. The DC voltage at TP7 should be about 1.6 v

If you do not have an oscilloscope, skip to the AM Final Alignments.

olts. If the voltage

in your circuit differs by more than .5 volts, leave the power ON and check the battery voltage. If the battery voltage is greater than 8.5 volts, check components R31, R32, R33 and Q7. Tur

DYNAMIC MEASUREMENTS

AM OSCILLATOR CIRCUIT

Connect your test equipment to the circuit as shown in Figure 21.

n the power OFF.

Oscilloscope

Set the scope to read 1 volt per division and turn the power ON. The scope should display a low voltage sinewave. The frequency of the sinewave should

TP15

Figure 21

change when the tuning gang is turned. If your circuit fails this test, check components Q7, gang capacitor, C28, C29, C30, C31, L4 and L5. Turn the power OFF.

-26-

AM FINAL ALIGNMENTS

There are two different AM alignment procedures. The first alignment procedure is for those who do not have test equipment and the second is for those who do have test equipment.

Included in your kit is a special device called a “magic wand” which is used for aligning resonant circuits. It usually has a piece of brass on one end and a piece of iron on the other. When the brass end of the “magic wand” is placed near the AM antenna, the antenna coil will react as if inductance has been removed. Likewise, when the iron end of the “magic wand” is placed near the AM antenna, the antenna coil will react as if inductance has been added. Therefore, when either brass or iron is placed near the antenna coil, it will change the inductance of the antenna coil. This change in the inductance will cause the resonant frequency of the circuit to change, thus changing the frequency at which the antenna was selective. When aligning the antenna and oscillator circuits, coils L4 and L5 are adjusted at the lower end of the band, while the oscillator and antenna trimmer capacitors are adjusted at the higher end of the band. antenna and the oscillator will tr

This is done so that the

ack correctly.

AM ALIGNMENT WITHOUT TEST EQUIPMENT

It is best to use an earphone for this procedure. Make sure that the switch is in the AM position. With an alignment tool or screwdriver, turn coils L5, T6, T7 and T8 fully counter clockwise until they stop. DO NOT FORCE THE COILS ANY FURTHER. Turn each coil in about 1 1/4 to 1 1/2 turns. Set the AM antenna coil about 1/8” from the end of its ferrite rod. Refer to Figure K.

IF ALIGNMENT

Turn the power ON and adjust the volume to a comfortable level. Turn the dial until a weak station is heard. If no stations are present, slide the antenna back and forth on its ferrite core, and retune the dial if necessary. Adjust T6 until the station is at its loudest. Reduce the volume if necessary. Adjust T7 until the station is at its loudest and reduce the volume if necessary. Adjust T8 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 process peaks the IF amplifiers to their maximum gain.

Soldering Iron Tip

Shrink Tubing

Iron Slug

Brass Slug

Magic Wand Assembly

Place the piece of brass inside the end of the shrink tubing, with 1/4” outside. Heat the brass up with y

our solder around the br other end in the same manner

ing iron until the tubing shrinks

ass. Assemble the iron piece to the

OSCILLATOR ALIGNMENT

Tune the r is heard. until their broadcast frequency is announced. If no stations are present at the low side of the AM band, adjust L5 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 to that station’s frequency marking on the dial. Adjust L5 until the station is heard. Tune the radio until a known 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, adjust the AM oscillator trimmer on the gang until a station is heard (refer to Figure L). Once a station is found and its broadcast frequency is known, rotate the dial until the white pointer is aligned to that station’s frequency marking on the dial. Adjust the AM oscillator trimmer on the gang until the station is heard. Repeat these 2 steps until the oscillator alignment is optimized. This process sets the oscillator range at 955kHz to 2055kHz.

adio until a known AM station around 600kHz

It may be necessary to listen to the station

-27-

ANTENNA ALIGNMENT

Tune the radio for a station around 600kHz. With the “magic wand” place the brass end near the antenna coil as shown in Figure 23. If the signal heard at the output increases, it means that the antenna coil needs less inductance. To remove inductance, carefully slide the antenna coil along it’s ferrite core in the direction shown in Figure 23. Place the iron end of the “magic wand” near the antenna coil. If the signal heard at the output increases, this means that the antenna coil needs more inductance. To add more inductance, carefully slide the antenna coil along its ferrite core in the direction shown in Figure 23. Repeat these steps until the signal heard decreases for both ends of the “magic wand”. Tune the radio for a station around 1400kHz. With the “magic wand”, place the brass end near the antenna coil. If the signal heard at the output increases, it means that the antenna coil needs more capacitance. Adjust the antenna trimmer on the back of the gang until the signal is at its loudest. Refer to Figure 25 for the location of the antenna trimmer. Place the iron end of the “magic wand” near the antenna coil. If the signal heard at the output increases, it means that the antenna coil needs less capacitance back of the gang until the signal is at its loudest. Repeat these steps until the signal heard decreases for both ends of the “magic wand”. Since the adjustment of both the antenna trimmer and antenna coil will effect the antenna alignment, it is advisable to repeat the entire procedure until the antenna alignment is optimized. This process sets the tr radio section. Once the antenna is properly aligned,

. Adjust the antenna trimmer on the

acking of the AM

CAREFULLY APPLY CANDLE WAX or glue to the antenna coil and the ferrite rod to prevent it from moving (see Figure 23A).

This concludes the alignment of the AM radio section. If no stations are heard, verify that AM signals are present in your location by listening to another AM radio placed near the Superhet 108T. If the AM section is still not receiving, go back and check each stage for incorrect values and for poor soldering. Proceed to the FM assembly section.

ntenna Coil

Magic Wand

Antenna

Shim

If the antenna needs:

• More inductance, slide the coil

• Less inductance, slide the coil

A Ferrite Core Antenna Holder

Figure 22

Wax

Figure 23

AM ALIGNMENT WITH TEST EQUIPMENT

IF ALIGNMENT

Connect your RF generator and oscilloscope as shown in Figure 24. Make sure that the switch is in the AM

Figure 24

Generator

.001µF

TP6 to the collector Q7.

TP15

Shor

position. Place a short from the collector of Q7 to TP6. This short “kills” the AM oscillator.

Oscilloscope

TP15

-28-

Set the RF generator at 455kHz, modulation of 400Hz 80% and minimum voltage out. Set the oscilloscope to read .1 volts per division and turn the power ON. Increase the amplitude of the generator until the oscilloscope shows a 400Hz sinewave 5 divisions or .5 volts pp. With an alignment tool or screwdriver adjust T6 for a peak. Reduce the generator amplitude so that 5 divisions are maintained. Adjust T7 for a peak and reduce that amplitude again if necessary. Repeat these steps to optimize the IF alignment. This process aligns the IF amplifiers to 455kHz.

After the IF alignment is complete, lower the frequency of the generator until the voltage drops .707 of its peaked value or .35Vpp. Record the frequency of the lower 3dB corner here:

Fl = _________kHz.

ON and set the volume control to a comfortable level. Turn the tuning knob counter-clockwise until the white pointer is aligned at the 540kHz marking on the dial. With an alignment tool or screwdriver adjust L5 until a 400Hz tone is heard. Adjust L5 for a peak on the oscilloscope. Adjust the amplitude of the RF generator to maintain a level of .5 volts peak to peak or less. After peaking L5, set the generator frequency to 1600kHz. Turn the tuning knob clockwise until the white pointer is aligned to the 1600kHz marking on the dial. With an alignment tool or screwdriver, adjust the AM oscillator trimmer on the back of the tuning gang until a 400Hz tone is heard. Adjust the trimmer for a peak on the oscilloscope. Refer to Figure 25 for the location of the AM oscillator trimmer. Repeat these steps to optimize the oscillator alignment. This process sets the oscillator range at 955kHz to 2055kHz.

Increase the frequency of the generator past the peak until the voltage seen on the scope drops .707 of its peaked value or .35Vpp. Record the frequency of the high 3dB corner here:

Fh = __________kHz.

The bandwidth of the IF is equal to BW = Fh - Fl. The IF’s bandwidth should be around 6kHz. Turn the power OFF and remove the short from the collector of Q7 to TP6.

Calculate the bandwidth: ___________kHz.

OSCILLATOR ALIGNMENT

Set the RF gener modulation and a low level of output. Turn the power

Generator

ator at 540kHz, 400kHz 80% AM

ANTENNA ALIGNMENT

With the power turned OFF, connect your test equipment as shown in Figure 26.

Figure 25

AM Antenna FM Antenna

AM Oscillator

AM Antenna

Battery

AM Oscillator FM Oscillator

FM Oscillator FM Antenna

Oscilloscope

TP15

Wire loop close to antenna

Figure 26

TP15

-29-

Set the generator at 600kHz, 400Hz 80% modulation, moderate signal strength. Set the oscilloscope to read .1 volts per division. Turn the tuning knob fully counterclockwise and turn the power ON. Slowly turn the tuning knob clockwise until a 400Hz sinewave is seen on the scope. Adjust the volume control to a comfortable level. If a station exists at 600kHz, then lower the frequency of the generator and repeat the previous steps. With the “magic wand”, place the brass end near the antenna coil as shown in Figure 23. If the signal on the scope increases, it means that the antenna coil needs less inductance. To add more inductance, carefully slide the antenna coil along it’s ferrite core in the direction shown in Figure 23. Repeat these steps until the signal seen decreases for both ends of the “magic wand”. Increase the frequency of the generator to 1400kHz and turn the tuning knob clockwise until a 400Hz sinewave is seen on the scope. If a station exists at 1400kHz, increase the frequency of the generator and repeat the previous

AM RADIO HIGHLIGHTS

steps. Place the brass end of the “magic wand” near the antenna coil. If the signal increases, it means that the antenna coil needs less capacitance. Adjust the antenna trimmer for a peak. Refer to Figure 25 for the location of the AM antenna trimmer. Since the adjustment of both the antenna alignment is optimized. This process sets the AM tracking of the Superhet 108T. Once the antenna is properly aligned, carefully apply candle wax or glue the antenna coil to the ferrite rod to prevent it from moving. Proceed to the FM assembly section.

1. The number of vibrations (or cycles) per second produced by a sound is called the frequency, and is measured in hertz.

2. The distance between peaks of sound waves is called the wavelength.

3. Sound waves are produced as a certain number of vibrations per second. The more vibrations per second, the higher the frequency; the fewer

ations, the lower the frequency.

vibr Waves of v

4. waves and travel great distances through the air without the use of wires.

5. Carrier waves are radio waves used by broadcast stations to carry audio waves.

ery high frequency are called radio

6. The process of adding the audio waves to the radio

7. The amount of signal picked up by the antenna will

9. Heterodyning is the process of mixing two signals

DC VOLTAGES

The voltage readings below should be used in troubleshooting the AM section. (Switch at AM position).

TP1 3.9

Q7 B 1.5 Q11 B 4.4

E 1.1 E 3.9 C

Q8 B

E .7 E 3.9 C 8.8 C .6

Q9 B 1.7 Q13 B 8.4

E C 9.0 C 3.9

Q10 B.7Q14 B.6

E .06 E 0 C

8.8 C 8.4

1.4

1.1

3.3

Q12 B

3.3

9.0

3.9

waves is called modulation, and the process of removing the radio wave from the audio wave is called demodulation, which is performed in an AM radio by the detector.

depend on the power of the signal transmitted and the distance the signal travelled. Rectification is the process of removing half the signal, while filter that signal.

(the incoming RF signal and the RF signal from the local oscillator) to produce a third signal (the IF signal).

ing is the process of smoothing

Test Conditions

olume set to minimum.

2. Switch to the AM position.

3. Connect side of capacitor C29 (that goes to L4) to TP15 with a jumper wire.

4. Battery voltage = 9V

5. All voltages are referenced to circuit common.

oltage readings can v

10%

-30-

QUIZ - AM SECTION

INSTRUCTIONS: Complete the following examination, check your answers carefully.

1. The number of cycles produced per second by a source of sound is called the ...

A) amplitude. B) vibration. C) sound wave. D) frequency.

2. The radio frequencies used by AM broadcast stations are between ...

A) 20kHz and 400kHz. B) 5kHz and 20kHz. C) 2400kHz and 6000kHz. D) 550kHz and 1600kHz.

3. The process of removing the audio wave from the radio wave is called ...

A) demodulation. B) frequency reduction. C) modulation. D) vibrating.

4. When an electromagnetic wave (modulated radio wave) passes an antenna, it ...

A) induces a voltage and current in the

antenna. changes an audio wave into a radio

wave. C) changes the carrier frequency. D) produces sidebands

5. The power of the signal transmitted by the broadcast station and the distance travelled from the transmitter to the receiver, determine the ...

A) frequency of the modulation.

wavelength of the audio waves.

B) C) amount of signal picked up by the

antenna.

D) type of filter that is used.

, the signal

6. When the two metal plates on a variable capacitor are unmeshed the ...

A) capacitance is minimum. B) capacitance is maximum. C) capacitance is not affected. D) inductance is increased.

7. The process of mixing two signals to produce a third signal is called ...

A) filtering. B) detecting. C) rectification. D) heterodyning.

8. The magic wand is used to determine ...

A) whether more or less inductance is

required in a tuned circuit.

B) whether more or less capacitance is

required in a tuned circuit. C) the gain of an RF amplifier. D) whether the oscillator is functioning.

9. The IF frequency of your AM radio is ... A) 1600kHz. B) 455kHz.

550kHz.

C) D) 910kHz.

10.

The pur

pose of the AGC circuit is to ...

A) automatically control the frequency of

the oscillator circuit. control the band width of the IF stages.

B) C) reduce distortion in the audio circuit. D)

maintain a constant audio level at the detector, regardless of the strength of the incoming signal.

Answers: 1. D, 2. D, 3. A, 4. A, 5. C, 6. A, 7. D, 8. A, 9. B, 10. D

-31-

PARTS LIST FOR FM SECTION

RESISTORS

Qty. Symbol Value Color Code Part #

R9, 23 100Ω 5% 1/4W brown-black-brown-gold 131000

2 1 R25 220Ω 5% 1/4W red-red-brown-gold 132200 1 R3 470Ω 5% 1/4W yellow-violet-brown-gold 134700 5 R18, 22, 24, 26, 27 1kΩ 5% 1/4W brown-black-red-gold 141000 1 R11 1.8kΩ 5% 1/4W brown-gray-red-gold 141800 2 R15, 6 2.2kΩ 5% 1/4W red-red-red-gold 142200 2 R2, 7 6.8kΩ 5% 1/4W blue-gray-red-gold 146800 7 R10,12,14,16,19,20,28 10kΩ 5% 1/4W brown-black-orange-gold 151000 2 R1, 8 22kΩ 5% 1/4W red-red-orange-gold 152200 2 R4, 5 33kΩ 5% 1/4W orange-orange-orange-gold 153300 3 R13, 17, 21 47kΩ 5% 1/4W yellow-violet-orange-gold 154700 2 R29, 30 390kΩ 5% 1/4W orange-white-yellow-gold 163900

CAPACITORS

Qty. Symbol Value Description Part #

1 C9 15pF Discap (15) 211510 1 C10 30pF Discap (30) 213010 1 C6 33pF Discap (33) 213317 1 C11 220pF Discap (221) 222210 2 C4, 5 470pF Discap (471) 224717 3 C3, 7, 27 .001 3 C2, 8, 12 .005µF Discap (502) 235018 10 C13 - 22 .01µF Discap (103) 241031 1 C23 .02 1 C26 .1µF Discap (104) 251010 1 C25 10µF Electrolytic (Lytic) Radial 271045 1 C24 470µF Electrolytic (Lytic) Radial 284744

µF Discap (102) 231036

µF or .022µF Discap (203) or (223) 242010

SEMICONDUCTORS

. Symbol Value Description

Qty

1 D1 Varactor Diode 310176 2 D2, 3 1N60 Diode 311065 6 Q1 - Q6 2N3904 Transistor 323904

Part #

COILS

Qty. Symbol Value Description Part #

T5 Blue FM Detector 430110

1 1 T4 Pink FM Detector 430120 2 T2, 3 Green FM IF 430130

T1 Orange FM Mixer 430140

1 1 L1 6 Turns FM RF Amp 430160 1 L2 2 Turns FM RF Amp 430170 1 L3 5 Turns FM Oscillator 430180

MISCELLANEOUS

Qty. Symbol Description Part #

1 1 2 Antenna Screw M2 643148 1

TP8 - 14

7 1 Coil Spacer 669108

Antenna 484005

w 2-56 x 1/4” 641230

Scre

Nut 2-56 644201 Test Point Screws 665008

-32-

SECTION 6

THE FM RADIO

Section 6 begins the construction of the FM radio. The stages that we will build are shown in the block diagram below. We will begin with the FM Ratio Detector and

FM RADIO

Section 9

Section 8

Section 7 Section 6

work back to the FM Antenna. Each stage will be tested before proceeding to the next stage.

Section 1

FM RF

AMPLIFIER

OSCILLATOR

FM MIXER

1ST FM IF

AMPLIFIER

AFC

FM RATIO DETECTOR

In the AM DETECTOR section we observed that the audio was detected from changes in the amplitude of the incoming signal. In FM detection, the audio is detected from changes in frequency of the incoming signal. The RATIO DETECTOR has built-in limiting action which limits the signal so that any noise riding on the FM carrier will be minimized. The RATIO DETECTOR is redrawn below for ease of explanation. When an incoming signal is present at current flows through D2, R26, R28, R27 and D3. At no modulation, the current through the diodes D2 and D3 are equal because T5 is center tapped. Thus, no

T4 and T5, a

2ND FM IF

AMPLIFIER

Speaker

DETECTOR

AUDIO

AMPLIFIER

current is drawn through C23 resulting in zero audio output voltage. When the incoming signal is modulated, the current through one diode will be greater than the

. This causes a current to flow in C23 which will

other produce an audio voltage across C23. If the modulation is of opposite direction than before, more current will flow in the other diode, which will again cause current to flow in C23 in the opposite direction resulting in an audio v

oltage being produced across C23. The large current drawn from the audio which causes the battery voltage to vary. The ratio detector is decoupled further by the resistor R23 and capacitor C21.

µF

Figure 27

-33-

ASSEMBLY INSTRUCTIONS

R24 - 1kΩ Resistor

(brown-black-red-gold)

C21 - .01µF Discap (103)

R23 - 100Ω Resistor

brown-black-brown-gold)

(

C23 - .02µF (203)

or .022µF (223) Discap C19 - .01µF Discap (103) R21 - 47kΩ Resistor

(yellow-violet-orange-gold)

TP9 - Test Point Pin

(See Figure A)

T3 - FM IF Coil

(Green Dot)

TP8 - Test Point Pin

(see Figure A)

Q6 - 2N3904 Transistor

(see Figure C)

R20 - 10kΩ Resistor

(brown-black-orange-gold)

R22 - 1kΩ Resistor

(brown-black-red-gold)

C22 - .01µF Discap (103)

T5 - FM Detector Coil

(Blue Dot)

R25 - 220Ω Resistor

(red-red-brown-gold)

C24 - 470µF Lytic

(See Figure B)

D2 - 1N60 Diode

(see Figure E)

R26 - 1kΩ Resistor

(brown-black-red-gold)

R28 - 10kΩ Resistor

(brown-black-orange-gold)

C25 - 10µF Lytic

(see Figure B)

R27 - 1kΩ Resistor

(brown-black-red-gold)

D3 - 1N60 Diode

(see Figure E)

T4 - FM Detector Coil

(Pink Dot)

(see Figure 28)

Test Point Pin

Foil Side of PC Board

Figure A

Test Point Pin

Lytic Capacitor

Polarity Mark

( )

(+)

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

Figure B

NPN Transistor

Flat Side

EBC

Figure C

Notch

Note: Notch m

ust be pointing to the top of the PC board. Some FM detector coils have a red line or part number in place of the notch.

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

Leave

Figure 28

Diode

Band

CathodeAnode

Be sure that the band is in the correct direction.

Figure E

-34-

STATIC MEASUREMENTS

Amps COM V/Ω

TP15

Figure 29

FM VOLTAGE

Connect y AM/FM switch to the FM position. read 9 volts DC

our VOM as shown in Figure 29. Switch the

. Turn the po

wer ON. The voltage at this

Set your VOM to

Battery

point should be betw OFF. If you do not get this reading, check R25, C24 and the battery v

oltage.

een 7 and 9 volts. Turn the power

TRANSISTOR CURRENT TEST

Amps COM V/Ω

TP15

Figure 30

VOM to the circuit as shown in Figure 30.

Connect y Turn the power ON. The voltage at the emitter of Q6 should be about .7 v

Turn the power OFF. If your answer is greater than 2

olts

v Since the current through resistor R22 is equal to the

our

olts. Record the voltage here:

V(Q6) = __________.

, check R20, R21, R22, R24, Q6 and the battery.

current through tr through Q6 as follows:

Current (I) = V(Q6) / R22

Your calculated answer should be between .0005 amps (.5 milliamps) and .0011 amps (1.1 milliamps).

ansistor Q6, calculate the current

Current (I) = __________.

YOU DO NOT HAVE AN RF GENERATOR OR OSCILLOSCOPE,

SKIP TO THE RATIO DETECTOR ALIGNMENT PROCEDURE.

-35-

DYNAMIC MEASUREMENTS

AC GAIN

The AC gain of the ratio detector is set by the AC impedance of the primary side of T4 and the current through Q6. The current is set by R20, R21 and R22. Capacitors C22 and C19 bypass the AC signal to ground. Connect your RF generator and oscilloscope to the circuit as shown in Figure 31. Your scope probe must have an input capacitance of 12pF or less, otherwise the probe will detune T4 causing an incorrect measurement of the AC gain. Set the generator for 10.7MHz no modulation and minimum voltage output. Set the scope to read 50mV/division. Turn the power ON and slowly increase the amplitude of the generator until 3 divisions or 150mVpp are seen

Generator

.001µF

on the scope. With an alignment tool or screwdriver, adjust T4 for a peak. Reduce the generator input to maintain 150mVpp on the scope. Move the scope probe to the base of Q6 and record the voltage here:

Vb = __________ mVpp.

Turn the power OFF. The AC gain can be calculated as follows:

AC GAIN = 150mV / Vb

Your calculated answer should be approximately 20.

Oscilloscope

TP15

Figure 31

RATIO DETECTOR ALIGNMENT

METHOD #1 ALIGNMENT WITH NO TEST EQUIPMENT

With an alignment tool or a screwdriver, turn both coils T4 and T5 fully counter-clockwise until they stop. DO

Generator

.001µF

TP15

NOT FORCE THE COILS ANY FURTHER. Now turn both coils in about 1 1/4 to 1 1/2 turns.

Oscilloscope

TP15

Figure 32

-36-

METHOD #2 ALIGNMENT OF RATIO DETECTOR USING A RF GENERATOR AND OSCILLOSCOPE

Connect the RF generator and oscilloscope to the circuit as shown in Figure 32. Set the generator for

10.7MHz modulated at 1kHz, 22.5kHz deviation with minimum voltage out. Turn ON the radio and turn the volume control to the minimum. Slowly increase the amplitude of the generator until a 1kHz sinewave is seen on the scope. With an alignment tool or screwdriver, peak the pink coil T4 for maximum amplitude. Now peak the blue coil T5 for minimum optimized. Turn the power OFF.

METHOD #3 ALIGNMENT OF RATIO DETECTOR USING A SWEEP GENERATOR AND OSCILLOSCOPE

Connect the sweep generator and oscilloscope to the circuit as shown in Figure 32. for 10.7MHz and minimum v

Set the sweep generator

oltage out. Turn the po

wer

ON and set the volume control to a minimum. Increase the amplitude of the sweep generator until an “S” curve is seen (refer to Figure 33). Using an alignment tool or screwdriver, adjust the blue coil T5 until the “S” curve is centered, until each half of the “S” is equal. Now adjust the pink coil T4 for maximum “S” amplitude. Repeat these steps until the alignment is optimized. Turn the power OFF.

Figure 33

SECTION 7

SECOND FM IF AMPLIFIER

The purpose of the 2nd IF amplifier is to increase the amplitude of the intermediate frequency (IF) while also providing Selectivity. Selectivity is the ability to “pick out” one station while rejecting all others. T3 acts as a bandpass filter that only passes signals around

10.7MHz. The resistor R19 is used to widen the 3dB

.707

bandwidth of the 2nd FM IF amplifier. The gain at 10.7MHz is fixed by the AC impedance of the primary side of T3 and the current in Q5. The current is fixed by R16, R17 and R18. Capacitors C18 and C17 bypass the AC signal to ground. C20 is a bypass capacitor from V+ to ground.

10.625MHz

10.775MHz

10.7MHz

Figure 34

-37-

ASSEMBLY INSTRUCTIONS

TP10 - Test Point Pin

(see Figure B)

T2 - FM IF Coil

(Green Dot)

C17 - .01µF Discap (103) R16 - 10kΩ Resistor

(brown-black-orange-gold)

R18 - 1kΩ Resistor

(brown-black-red-gold)

R17 - 47kΩ Resistor

(yellow-violet-orange-gold)

R19 - 10kΩ Resistor

(brown-black-orange-gold)

5 - 2N3904 Transistor

(see Figure D)

C20 - .01µF Discap (103) C18 - .01µF Discap (103)

STATIC TESTS

Q5 BIAS

Connect your VOM to the circuit as shown in Figure 35. Turn the power ON. The voltage at the base of Q5 should be approximately 1.4 volts. Turn the power OFF.

Amps COM V/Ω

TP15

If you do not get this reading, check R17, R16, R18, Q5 and T2.

ou don’t ha

If y

ve an RF g

Figure 35

enerator and oscilloscope

-38-

skip to Section 8.

Generator

Oscilloscope

.001µF

TP15

Probe

TP15

Figure 36

AC GAIN

Connect the RF generator and oscilloscope to the circuit as shown in Figure 36. The scope probe must have an input capacitance of 12pF or less otherwise the probe will detune T3 resulting in a false reading of the AC gain. and minimum voltage output. 50mV per division and tur increase the generator until 150mVpp or 3 divisions are seen on the scope. With an alignment tool or screwdriver adjust T3 for a peak. Reduce the generator until 150mVpp or 3 divisions are seen on the scope. With an alignment tool or screwdriver adjust T3 for a peak. Reduce the generator input to maintain 3 divisions on the scope. Move the probe to the base of Q5 and record the input voltage here:

Turn the power OFF. The AC gain can be calculated as follows:

Your calculated answer should be about 20.

Set the generator at 10.7MHz no modulation

Set the scope to read

n the power ON.

Slowly

Vb = __________ mVpp.

AC Gain = 150mV / Vb

BANDWIDTH

With the power turned OFF, connect your test equipment as shown in Figure 36. Set your generator at 10.7MHz no modulation and minimum voltage output. Set the scope to read 50mV per division. Turn the power ON and slowly adjust the generator amplitude until 150mVpp is seen on the scope. Realign T3, if necessar

y, for maxim the generator to maintain 150mVpp. Slowly decrease the frequency of the generator until the voltage drops .707 of its original value, 2.1 divisions or 106mVpp. Record the frequency of the lower 3dB drop-off point here:

Fl = _________MHz.

Increase the frequency until the v its original value, 2.1 divisions or 106mVpp. Record the frequency of the high frequency 3dB drop-off point here:

Fh = ___________MHz.

The bandwidth of the 2nd IF can be calculated as follows:

um output while adjusting

oltage drops to .707 of

Record your calculation:

C Gain = __________

Bandwidth = Fh - Fl

our results should be betw

Record y

our calculation:

-39-

een 300 - 500kHz.

Bandwidth = __________

SECTION 8

FIRST FM IF AMPLIFIER

The operation of the first IF amplifier is the same as the second IF amplifier except that the gain is different.The gain is set by the AC impedance of the primary side of T2 and the current in Q4. The current in Q4 is set by the resistors R12, R13 and R15. Capacitors C14 and

ASSEMBLY INSTRUCTIONS

C15 bypass the AC signal to ground. C13 and C16 are bypass capacitors from V+ to ground to prevent feedback on the V+ line. R19 is used to widen the bandwidth of the transformer T2.

C13 - .01µF Discap (103) C14 - .01µF Discap (103)

T1 - FM Mixer Coil

(Orange Dot)

R12 - 10kΩ Resistor

(brown-black-orange-gold)

R15 - 2.2kΩ Resistor

(red-red-red-gold)

STATIC TESTS

Q4 BIAS

Connect your VOM as shown in Figure 37. Turn the power ON. The voltage at the base of Q4 should

R13 - 47kΩ Resistor

(yellow-violet-orange-gold)

R14 - 10kΩ Resistor

(brown-black-orange-gold)

TP11 - Test Point Pin

(see Figure A)

Q4 - 2N3904 Transistor

(see Figure C)

C16 - .01µF Discap (103) C15 - .01µF Discap (103)

approximately be 1.4 volts. If you do not get this reading, check R12, R13, R15, Q4 and T1.

Amps COM V/Ω

TP15

Figure 37

-40-

If you don’t have an RF generator and oscilloscope, skip to Section 9.

enerator

.001µF

Oscilloscope

TP15

Figure 38

Connect the RF generator and oscilloscope and oscilloscope to the circuit as shown in Figure 38. The scope probe must have an input capacitance of 12pF or less otherwise the probe will detune T2 causing an incorrect measurement of AC gain. Set the generator at 10.7MHz no modulation and minimum voltage output. Set the scope to read 20mV per division and turn the power ON. Slowly increase the amplitude of the generator until 3 divisions or 60mVpp are seen on the scope. With an alignment tool or screwdriver, adjust T2 for a peak. Reduce the generator input to maintain 3 divisions on the scope. Move the scope probe to the base of Q4 and record the input voltage here:

Vb = __________mVpp

Turn the power OFF. The AC gain can be calculated as follows:

Probe

BANDWIDTH

Connect your test equipment as sho Set your generator at 10.7MHz no modulation and minimum voltage output. Set the scope to read 20mV per division. Turn the power ON and slowly increase the amplitude of the generator until 60mVpp is seen on the scope. Increase the frequency of the gener the voltage drops .707 of its original v or 42mVpp. Record the frequency of the high 3dB drop-off point here:

Fh = ___________MHz.

Decrease the frequency of the generator until the voltage drops to .707 of its original value, 2.1 divisions or 42mVpp

. Record the frequency of the low 3dB drop-

off point here:

wn in Figure 38.

alue, 2.1 divisions

TP15

ator until

C Gain = 60mV /

our calculated answer should be about 10.

Record your calculation:

AC Gain = __________

Fl = ___________MHz.

The bandwidth of the first IF can be calculated as follows:

Bandwidth = Fh - Fl

Your calculated answer should be between 300 500kHz.

Record your calculation:

Bandwidth = __________kHz.

-41-

SECTION 9

FM RF AMPLIFIER, MIXER AND OSCILLATOR

In a superheterodyne receiver, the radio waves are emitted and then mixed with the local oscillator to produce the intermediate frequency (IF). The first stage is the RF amplifier which selects a radio station and amplifies it. The second stage is the local oscillator which oscillates at a frequency 10.7MHz above the desired radio station frequency. The third stage is the mixer stage where the amplified radio waves are heterodyned with the local oscillator. During the mixing process, a difference frequency of 10.7MHz is produced. This difference frequency is used as the IF in FM radios. The collector of transistor Q3 contains an IF transformer (T1) which is tuned only to the difference frequency. This transformer rejects all frequencies

MIXER ASSEMBLY INSTRUCTIONS

except those near 10.7MHz. T1 also couples the

10.7MHz signal to the first FM IF amplifier. The RF amplifier and the oscillator are the only two resonant circuits that change when the radio is tuned for different stations. Since a radio station may exist 10.7MHz above the oscillator frequency, it is important that the RF stage rejects this station and selects only the station

10.7MHz below the oscillator frequency. The frequency of the undesired station 10.7MHz above the oscillator is called the image frequency. Since this FM receiver has an RF amplifier, the image frequency is reduced significantly. The resistor R9 and capacitor C12 decouple the voltage of the tuner from the voltage of the IF stages.

R10 - 10kΩ Resistor

(brown-black-orange-gold)

C12 - .005µF Discap (502) R8 - 22kΩ Resistor

(red-red-orange-gold)

R7 - 6.8kΩ Resistor

(blue-gray-red-gold)

TP13 - Test Point Pin

(see Figure A)

R9 - 100Ω Resistor

(brown-black-brown-gold)

TP12 - Test Point Pin

(see Figure A) CAUTION: Test Point must not touch can of T1 FM Mixer Coil.

Q3 - 2N3904 Transistor

(see Figure C)

R11 - 1.8kΩ Resistor

(brown-gray-red-gold)

-42-

STATIC MEASUREMENTS

Q3 BIAS

Amps COM V/Ω

TP15

Figure 39

With the power turned OFF, connect your VOM to the circuit as shown in Figure 39. volts DC and tur base of Q3 should be approximately 1.8 volts answer varies by more than 2 volts, turn the power OFF and check components R7, R8, R11 and Q3.

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

skip to the FM Oscillator Assembly Procedure.

n the power ON. The DC voltage at the

Set your VOM to read 9

. If your

AC GAIN

The AC gain of the mixer is set by the impedance of the

imary side of T1 and by the current flowing in Q3. The

pr current in Q3 is set by the resistors R7, R8 and R11. Connect your test equipment to the circuit as shown in Figure 40. Your scope probe must have an input capacitance of 12pF or less, otherwise the probe will detune your scope to read 10mV per division. Set your RF gener output. Turn the power ON and slowly increase the amplitude of the generator until 4 divisions or 40mVpp

T1 resulting in an incorrect measurement.

ator at 10.7MHz no modulation minimum voltage

Set

are seen on the scope. With an alignment tool or a screwdr amplitude to maintain 4 divisions on the scope. Move the scope probe to the base of Q3 and record the input voltage here:

Turn the power OFF. The gain can be calculated as follows:

our calculated answer should be about 3.

Record your calculation:

Because the signal from the oscillator is injected at the emitter of Q3, the emitter resistor is not bypassed to

round. This is why the gain of the mixer is low

g compared to the other IF stages.

iver, adjust T1 for peak. Reduce the generator

Vb = __________mVpp.

AC Gain = 40mV / Vb.

AC Gain = __________

-43-

Generator

Oscilloscope

.001µF

Probe

TP15

Figure 40

BANDWIDTH TEST

Connect your test equipment to the circuit as shown in Figure 40. Set your generator at 10.7MHz no modulation and minimum voltage output. Set the scope f

or 10mV per division.

Turn the power ON and slowly increase the amplitude of the generator until 40mVpp are seen on the scope. Increase the frequency until the voltage drops .707 of its original value, 2.8 divisions or 28mVpp. Record the frequency of the generator until the voltage drops .707 of its original value

, 2.8 divisions or 28mVpp. Record the frequency of the low 3dB drop-off point here:

Fl = _________MHz.

Turn the power OFF. The bandwidth can be calculated as follows:

Your calculated answer should be between 300 500kHz.

Record your calculation:

FM OSCILLATOR ASSEMBLY INSTRUCTIONS

TP15

Bandwidth = Fh - Fl

Bandwidth = _________kHz.

C7 - .001µF Discap (102) R4 - 33kΩ Resistor

ange-or

(or

Q2 - 2N3904 Transistor

R5 - 33kΩ Resistor

(orange-orange-orange-gold)

R6 - 2.2kΩ Resistor

C8 - .005µF Discap (502)

ange-or

(see Figure C)

(red-red-red-gold)

ange-gold)

L3 - FM Oscillator Coil

(5 Turns)

C9 - 15pF Discap (15)

C10 - 30pF Discap (30)

C11 - 220pF Discap (221)

-44-

STATIC MEASUREMENTS

Q2 BIAS

Connect your VOM to the circuit as shown in Figure 41. Set your VOM to read 9 volts and turn the power ON. The voltage at the base of Q2 should be about 4

Amps COM V/Ω

TP15

volts. Turn the power OFF. If you do not get this measurement, check R4, R5 and Q2.

Figure 41

AFC

When a r desirable for the radio to “lock” in on the station. Due to changes in temperature, voltage and other effects, the local oscillator ma oscillation. If this occurs, the center frequency of

10.7MHz will not be maintained. Automatic Frequency Control (AFC) is used to maintain the

10.7MHz center frequency. When the local oscillator drifts, the ratio detector will produce a DC “correction” voltage. The audio signal rides on this DC correction voltage. This signal is fed to a filter network which removes the audio so that a pure DC voltage is produced. This voltage is fed to a special

adio is tuned to a station, it w

y change its frequency of

ould be

AFC ASSEMBLY INSTRUCTIONS

actor. A v

diode called a v internal capacitance when a voltage is applied. The ratio detector diodes are positioned in such a way that when the 10.7MHz center frequency increases the DC correction voltage will decrease. Likewise, when the 10.7MHz center frequency decreases, the DC correction voltage will increase. This voltage change causes the capacitance of the varactor to change. The varactor is connected at the emitter of Q2, so any capacitance change in the varactor is seen at the emitter of the oscillator. A change in capacitance at the emitter of Q2 will change the frequency of oscillation of the local oscillator.

aractor will change its

R30 - 390kΩ Resistor

w-gold)

ange-white-y

(or

Varactor Diode

D1 -

(see Figure F)

C27 - .001µF Discap (102)

ello

R29 - 390kΩ Resistor

(orange-white-yellow-gold)

C26 - .1µF Discap (104)

-45-

If you don’t have an RF generator, skip to the RF Amplifier Assembly Procedure.

.001µF

Generator

Amps COM V/Ω

TP15

Figure 42

Connect the RF generator and VOM to the circuit as shown in Figure 42. Set your VOM to read 9 volts DC. Set your generator at 10.7MHz no modulation and moderate signal strength output. Turn the power ON. Record the voltage of D1 here:

V(D1) = ________.

While watching your VOM, slowly decrease the frequency of your generator. As the frequency decreases, the voltage at D1 should increase. Increase

RF AMPLIFIER ASSEMBLY INSTRUCTIONS

R1 - 22kΩ Resistor

(red-red-orange-gold)

C3 - .001µF Discap (102) C2 - .005µF Discap (502)

Q1 - 2N3904 Transistor

(see Figure C)

TP15

the frequency of the generator until the voltage is equal to V(D1). While watching your VOM, increase the frequency of your generator. As the frequency increases, the voltage at D1 should decrease. This correction voltage is what keeps the oscillator from drifting. If the voltage at D1 still does not change at D1, check D1, R29, R30, C26 and C27. If these parts are inserted correctly and the voltage at D1 still doesn’t change, then increase the amplitude of your generator and repeat the same steps again. Turn the power OFF.

L1 - FM RF Amp Coil

(6 Turns) see Figure N

L2 - FM RF Amp Coil

Turns) see Figure N

C5 - 470pF Discap (471)

C6 - 33pF Discap (33)

R2 - 6.8kΩ Resistor

(blue-gray-red-gold)

C4 - 470pF Discap (471)

R3 - 470Ω Resistor

(yellow-violet-brown-gold)

TP14 - Test Point Pin

(see Figure A)

Figure N

-46-

STATIC MEASUREMENTS

Q1 BIAS

Connect your VOM to the circuit as shown in Figure 43. Set your VOM to read 9 volts and turn the power ON. The voltage at the base of Q1 should be about 1.6 volts. If you do not get this reading, check R1, R2, R3 and Q1. Turn the power OFF.

Figure 43

COM

TP15

ANTENNA FM ASSEMBLY

Antenna FM 2 Antenna Screws M2

3.5” Wire #22 Insulated

(extra wire in AM Section)

(see Figure O)

Mount the antenna to the PC board with two screws as shown. NOTE: Some antennas have only one

threaded hole.

Cut a 2 1/4” wire and strip 1/4” of insulation off of both ends of the remaining jumper wire. There are no holes

or the wire in this location, so tack solder the wire to the

f pads as shown.

FM FINAL ALIGNMENTS

There are two procedures for the final alignment steps. The first alignment procedure is for those who do not

ve test equipment and the second is for those who do

ha have test equipment.

our “magic wand” will be used to align the FM

Y oscillator circuit and the FM RF amplifier. When the brass end of your “magic wand” is placed near the FM oscillator coil L3, the coil reacts as if inductance has been removed. Likewise, when the iron end of the “magic wand” is placed near the coil L3, it reacts as if inductance has been added. The same is true for the RF coils L1 and L2. When the inductance of a resonant

Threaded Holes

Figure O

Solder Jumper Wire

circuit is changed, the resonant frequency is changed also. When aligning the oscillator

, changing the resonant frequency changes the frequency of oscillation. Lik

, when aligning the RF amp

wise

, changing the

resonant frequency at which it was selective.

When aligning the oscillator and RF circuits, coils L1 and L3 will be adjusted at the low

er end of the band, while the oscillator and RF trimmer capacitors are adjusted at the higher end of the band. This is done so that the RF amp tracks the oscillator properly.

-47-

ALIGNMENT WITH NO TEST EQUIPMENT

With an alignment tool or screwdriver turn coils T1, T2 and T3 fully counter-clockwise. DO NOT FORCE

IF ALIGNMENT

COM

Amps

V/Ω

THE COILS ANY FURTHER. Turn each coil in about 1 1/4 to 1 1/2 turns.

Battery

TP15

Figure 44

With an alignment tool or screwdriver turn coils T1, T2 and T3 fully counter-clockwise. DO NOT FORCE THE COILS ANY FURTHER. Turn each coil in about 1 1/4 to 1 1/2 turns. Use the earphone provided for best results. Switch to the FM position. Connect your VOM to the circuit as shown in Figure 44. Turn the radio ON and tune the radio to a weak station. It is best to keep the volume at a low level. Adjust T1 for the minimum voltage on your VOM. Reduce the volume if necessary. Adjust T2 for minimum voltage on your VOM and reduce the volume control if necessary. Adjust T3 for minimum voltage on your VOM and reduce the volume control if necessary.

ou adjust the coils you should hear less distortion

As y and noise. Repeat this procedure until the FM IF gain is optimiz

ed. This process peaked the FM IF amplifier

to their maximum gain.

DETECTOR ALIGNMENT

Adjust for minimum distortion. Repeat these 2 steps until the ratio detector alignment is optimized.

T4 for minimum voltage on your VOM. Adjust T5

heard, this means that L3 needs more inductance. Carefully press together L3 until the station is heard. Pulling apart or pressing together L3 just a small amount will have a great effect on the coils resonant frequency. Repeat this step until the pointer is aligned to the station’s frequency. Tune the radio to a station around 106MHz. 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 on the dial. Place the brass end of the “magic wand” near L3. If the station is heard, it means that L3 needs more capacitance. Carefully (as shown in Figure L, until the station is heard.

adjust the FM oscillator trimmer

page 23), on the back of the gang

Place the iron end of the “magic wand” near L3. If the station is heard, it means that L3 needs less capacitance.

Carefully adjust the FM oscillator trimmer located on the back of the gang until the station is heard.

“Magic Wand”

OSCILLATOR ALIGNMENT

Tune the radio to a known station around 90MHz. Once a station is found and its broadcast frequency is known, rotate the dial until the white pointer is aligned with that stations frequency on the dial. Using the “magic wand”, place the brass end near coil L3. Refer to Figure 45. If the station is heard, this means that L3 needs less inductance. Carefully pull apart L3 until the station is heard.

Place the iron end near L3. If the station is

-48-

Spread apart the coil for less inductance Press the coil together f

or more inductance

Figure 45

RF Coil

Repeat this step until the pointer is aligned to the station’s frequency. Adjusting both the oscillator coil L3 and the oscillator trimmer capacitor will effect the oscillator’s frequency, so it is advisable to repeat this procedure until the FM oscillator alignment is optimized. This process sets the FM oscillator range at

98.7MHz to 118.7MHz.

RF ALIGNMENT

Press together L1 and L2. Spread apart coil L1 so that it resembles Figure 46. The gaps or spaces should be between 1/32” and 1/16” wide. This procedure sets the tracking of the RF section. Use the special coil spacer provided to gap the coil as shown. Carefully slide the coil spacer between the coils to get the spacing shown in Figure 46. This concludes the alignment of the FM radio section. If no stations are heard, verify that FM signals are present in your location by listening to another FM radio placed near the superhet 108. If the FM section is still not receiving go back and check each stage for incorrect values and for poor soldering.

Approx. 1/16” gap

L1 L2

Figure 46

Spacer

Top View

ALIGNMENT WITH RF GENERATOR AND OSCILLOSCOPE

IF ALIGNMENT

Switch to the FM section. Connect your RF generator and oscilloscope to the circuit as shown in Figure 47. Set your RF generator at 10.7MHz modulated at 1kHz deviation with minimum voltage output. Set the scope

Generator

TP15

to read 50mV per division. With a clip lead, short the base emitter junction of Q2. This short “kills” the local oscillator.

Short the base of Q2 to the emitter (as shown below).

Battery

Oscilloscope

TP15

Turn the power ON. Slowly increase the amplitude of the generator until a 1kHz signal is seen on the scope.

eep the generator at a low level of output to prevent

K the IF sections from limiting. With an alignment tool or

, adjust

wdr

scre

T1 for a peak on the scope. Reduce

Figure 47

the amplitude of the input signal if necessary. Adjust T2 for a peak and reduce the amplitude of the input signal if necessar is optimized. This procedure aligns the FM IF amplifiers to 10.7MHz.

-49-

y. Repeat these steps until the IF alignment

OSCILLATOR ALIGNMENT

Remove the clip lead and set your generator at 88MHz modulator at 1kHz, 22.5kHz deviation and minimum voltage output. Tune the radio until a 1kHz signal is seen on the scope. It may be necessary to increase the amplitude of the generator. Rotate the dial until the white pointer is aligned to 88MHz. Using the “magic wand” place the brass end near L3 as shown in Figure

45. If the signal seen on the scope increases, this means L3 needs less inductance. To remove inductance, carefully spread apart coil L3. Pulling apart or pressing together coil L3, a small amount will have a great effect on the coil’s resonant frequency. Place the iron end of the “magic wand” near L3. If the signal seen on the scope increases, it means L3 needs more inductance. To add inductance carefully press together coil L3. Repeat these steps until the signal decreases for both ends of the “magic wand”. Increase the frequency of your generator to 108MHz. Tune the radio until a 1kHz signal is seen on the scope. Rotate the dial until the white pointer is aligned to 108MHz. Place the brass end of your “magic wand” near L3. If the signal on the scope increases, it means that L3 needs more capacitance. Adjust the FM oscillator trimmer on the gang (as sho signal is at a peak. Place the iron end of the “magic wand” near L3. If the signal increases, it means that coil L3 needs less capacitance. Adjust the FM oscillator trimmer on the gang until the 1kHz signal is at a peak. Repeat these 2 steps until the signal decreases for both ends of the “magic w oscillator coil L3 and the oscillator trimmer will effect the frequency of oscillation, it is advisable to repeat this procedure until the oscillator alignment is optimized. This process sets the FM oscillator range at 98.7MHz to 118.7MHz.

wn in Figure L on page 23) until the 1kHz

and”. Since adjusting both the

RF ALIGNMENT

Set your generator at a frequency around 90MHz modulated at 1kHz, 22.5kHz deviation and minimum voltage out. Tune your radio until a 1kHz tone is heard. Place the brass end of your “magic wand” near RF coil L1. If the signal on the scope increases, it means that coil L1 needs less inductance. Carefully spread apart the coil L1 to reduce its inductance. Place the iron end of the wand near L1. If the signal increases, it means that coil L1 needs more inductance. Carefully press together the coil L1 to increase its inductance. Repeat these steps until the signal on the scope decreases for both ends of the “magic wand”. Increase your generator to a frequency near 106MHz. Tune your radio until a 1kHz tone is heard. Place the brass end of your “magic wand” near L1. If the signal increases, it means that the coil L1 needs more capacitance. With an alignment tool or screwdriver, adjust the FM antenna trimmer (see Figure L on page 23). If the signal increases, this means coil L1 needs less capacitance. Carefully adjust the FM antenna trimmer until a peak is seen on the scope. Repeat these steps until the signal on the scope decreases for both ends of the “magic wand”. Since adjusting both the RF coil L1 and the antenna tr advisable to repeat this procedure until the RF amplifier alignment is optimized. This process sets the RF stage to “track” the FM oscillator stage.

This concludes the alignment of the FM radio section. If no stations are heard, ver present in your location by listening to another FM r near the Superhet 108. If the FM section is still not receiving, go back and check each stage for incorrect values and for poor soldering.

immer will effect the gain of th RF stage, it is

ify that FM signals are

adio

FM RADIO HIGHLIGHTS

1. The FM broadcast band covers the frequency range from 88MHz to 108MHz.

2. FM signals are usually limited to line a sight.

Audio signals up to 15kHz are transmitted on the

3. FM carrier.

The amount that the RF carrier changes frequency is

mined by the amplitude of the modulating signal.

deter

5. The number of times the carrier frequency changes in a period of time is exactly equal to the audio frequency.

The change in frequency is called the de

6. is limited to 75kHz for monaural FM.

7. The bandwidth assigned for FM is 200kHz.

-50-

viation and

DC VOLTAGES

The voltage readings below should be used in troubleshooting the FM section. (Switch at FM position.)

Q1 B 1.6 Q4 B 1.3

E .9 E .7 C 7.0 C 7.5

Q2 B 3.3 Q5 B 1.3

E 3.0 E .6 C 7.1 C 7.5

Q3 B 1.6 Q6 B 1.2

E 1.3 E .6 C 7.0 C 6.6

1. Volume set to minimum.

2. Connect TP14 to TP15 with a jumper wire.

3. Battery voltage = 9V

4. All voltages are referenced to circuit common.

5. Voltage readings can vary +

Test Conditions

10%

SPECIFICATIONS

Audio:

Frequency response 3dB drop into 8 ohm resistive load.

Low end 800Hz - high end 120kHz

Maximum power out at 10% total harmonic distor

500 MilliWatts Typical audio gain at 1000Hz: 150 times Typical % distortion at 100 milliwatts output <2%.

tion.

AM Radio Specifications:

Tuning range - 520kHz to 1620kHz IF frequency 455kHz Tracking = + 10dB signal to noise at 200 microvolts typical

3dB from 700kHz to 1400kHz

FM Radio Specifications:

Tuning range = 88MHz to 108MHz IF frequency 10.7MHz Tracking +5dB from 90MHz to 106MHz 10dB signal to noise at 12 micro Uses ratio detector and full time auto frequency control.

volts typical

-51-

QUIZ - FM SECTION

INSTRUCTIONS: Complete the following examination, check your answers carefully.

1. The FM broadcast band is . . . A) 550 - 1,600kHz. B) 10.7MHz. C) 88 - 108MHz. D) 98.7 - 118.7MHz.

2. The maximum audio frequency used for FM is ... A) 7.5kHz. B) 15kHz. C) 20kHz. D) 10.7MHz.

3. The frequency of the modulating signal

determines the . . .

A) number of times the frequency of the

carrier changes per second. B) maximum deviation of the FM carrier. C) maximum frequency swing of the FM

ier

carr D) amount of amplitude change of the FM

carrier.

4. The AFC circuit is used to ... A) automatically hold the local oscillator on

frequency.

B) maintain constant gain in the receiver to

prevent such things as fading.

C) prevent amplitude variations of the FM

carrier. automatically control the audio

frequencies in the receiver.

5. The r

atio detector transf

A) 10.7MHz. B) 88MHz.

455kHz.

C) D) 10.9MHz.

ormer is tuned to ...

6. The ratio detector is used because ... A) it is sensitive to noise. B) it is insensitive to noise. C) it provides amplification. D) it doesn’t need a filter.

7. The device most often used for changing the

local oscillator frequency with the AFC voltage is a ...

A) feedthrough capacitor. B) variable inductor. C) varactor. D) trimmer capacitor.

8. The capacitance of a varactor is determined by ... A) the voltage level. B) the amount of current in the circuit. C) the signal strength of the RF carrier.

the amount of resistance in the circuit.

Limiting in FM receivers is the process of ...

9. A) removing interfering FM stations. B) providing greater station selectivity. C) separ

D) removing noise from the FM carrier.

10. A detector circuit that does not require a limiter is

a ...

A) slope detector B) ratio detector. C) Travis detector. D)

ating the FM stations from the AM

stations.

Foster-Seeley detector

Answers: 1. C, 2. B, 3. A, 4. A, 5. A, 6. B, 7. C, 8. A, 9. D, 10. B

-52-

AM/FM-108T RADIO BAFFLE

OTICE: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.

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

Suspension.

Screw 2-56 x 1/4”

Nut 2-56

AM/FM-108T Kit Carton

Cut Off

Baffle

2. Cut off two pieces of the flap as shown and bend the four flaps upward as

1. Start at one edge and carefully remove the baffle from the bottom of the kit box.

3. Bend the top side upward as shown. 4. Bend the two sides upward. Attach the three sides using scotch tape or glue (Elmer’s, Duco Cement, or other).

shown.

brown

side

5. Bend the bottom side upward and attach it to the other sides using scotch tape or glue down as shown in the figure.

. Bend one mounting flap

2-56 x 1/4” Screw

2-56 Nut

Back View

Insert the baffle as shown in Step 6 . Secure into place with the 2-56 x 1/4” screw and a 2-56 nut as shown in Step 6. Secure the side of the baffle with a piece of tape as shown in Step 7.

o make an air tight seal, place a bead of seal between the PC board and the baffle.

Optional:

Back View

Tape

-53-

SCHEMATIC DIAGRAM - AM/FM-108T RADIO

-54-

Elenco®Electronics, Inc.

150 Carpenter Avenue

Wheeling, IL 60090

(847) 541-3800

Website: www.elenco.com

e-mail: elenco@elenco.com

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

Specifications and Main Features

Frequently Asked Questions

User Manual