AK 710 Omnitron REV H 2 11 03 Ak710

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User Manual: ak710

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FM WIRELESS MICROPHONE KIT

MODEL K-30/AK-710

Assembly and Instruction Manual

Elenco®Electronics, Inc.

Copyright © 2006, 1994 by Elenco®Electronics, Inc. All rights reserved. Revised 2006 REV-J 753016

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

-1-

PARTS LIST

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

If you purchased this kit from a distributor, catalog, etc., please contact Elenco®Electronics (address/phone/e-mail is at the

back of this manual) for additional assistance, if needed. DO NOT contact your place of purchase as they will not be able

to help you.

RESISTORS

Qty. Symbol Value Color Code Part #

1 R5 150Ω5% 1/4W brown-green-brown-gold 131500

2 R8, R10 1kΩ5% 1/4W brown-black-red-gold 141000

1 R7 1.5kΩ5% 1/4W brown-green-red-gold 141500

1 R3 4.7kΩ5% 1/4W yellow-violet-red-gold 144700

1 R1 8.2kΩ5% 1/4W gray-red-red-gold 148200

1 R6 10kΩ5% 1/4W brown-black-orange-gold 151000

1 R2 27kΩ5% 1/4W red-violet-orange-gold 152700

2 R4, R9 47kΩ5% 1/4W yellow-violet-orange-gold 154700

CAPACITORS

Qty. Symbol Value Description Part #

1 C4 10pF (10) Discap 211011

1 C5 12pF (12) Discap 211210

1 C6 33pF (33) Discap 213317

2 C3, C7 .001µF (102) Discap 231035

2C1, C2 .1µF (104) Discap 251010

SEMICONDUCTORS

Qty. Symbol Value Description Part #

3 Q1 - Q3 2N3904 Transistor 323904

1 LED Light Emitting Diode (LED) 350001

1 Coil FM Mic 468751

MISCELLANEOUS

Qty. Description Part #

1 PC Board 517710

1Switch (S1) 541024

1 Mic 568000

1Battery Clip (+) 590091

1 Battery Clip (–) 590093

1 Foam Cover 620002

1 Top Case 623105

1 Bottom Case 623205

1 Stand 626010

Qty. Description Part #

1Battery Cover 627002

1 Alignment Tool 629011

1 Screw 2.5mm x 4mm 641310

3 Screw 2.6 x 8mm 642109

12” Wire 22ga. Gray 814810

6” Wire 26ga. Black 816210

10.5” Wire 26ga. Red 816220

1 Solder Tube 9ST4

Resistor

PARTS IDENTIFICATION

FM Coil

Microphone

Capacitor Transistor

Battery Clips

(–)(+)

SwitchLED

Case

Stand

Battery Cover

Top

Bottom

Caution: Do not mix alkaline, standard (carbon-zinc), or rechargeable (nickel-cadmium) batteries.

-2-

FM MICROPHONE KIT

Your FM Microphone is really a miniature frequency

modulated transmitter operating in the standard FM

broadcast band. The range of frequencies for the

FM broadcast band is 90MHz (MHz = Megahertz or

90 million cycles per second). Because the FM

microphone has a variable tuned circuit, it can be

tuned to a quiet spot on your local FM broadcast

band for the best reception. When the small

microphone element is struck by sound, it converts

the audio to a change in current through resistor R1

(see schematic diagram). This electrical change is

amplified and eventually frequency modulates the

transmitter. The transmission range of the FM

microphone is approximately 100 feet, depending

on the efficiency of the antenna (properly tuned or

not) and the quality of the FM radio receiver.

IDENTIFYING RESISTOR VALUES

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

BAND 1

1st Digit

Color Digit

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Gray 8

White 9

BAND 2

2nd Digit

Color Digit

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Gray 8

White 9

Multiplier

Color Multiplier

Black 1

Brown 10

Red 100

Orange 1,000

Yellow 10,000

Green 100,000

Blue 1,000,000

Silver 0.01

Gold 0.1

Resistance

Tolerance

Color Tolerance

Silver +10%

Gold +5%

Brown +1%

Red +2%

Orange +3%

Green +0.5%

Blue +0.25%

Violet +0.1%

BANDS 12 Multiplier Tolerance

IDENTIFYING CAPACITOR VALUES

Capacitors will be identified by their capacitance value in pF (picofarads), nF (nanofarads), or µF (microfarads). Most

capacitors will have their actual value printed on them. Some capacitors may have their value printed in the following

manner. The maximum operating voltage may also be printed on the capacitor.

Second Digit

First Digit

Multiplier

Tolerance The letter M indicates a tolerance of +20%

The letter K indicates a tolerance of +10%

The letter J indicates a tolerance of +5%

For the No.01234589

Multiply By 1 10 100 1k 10k 100k 0.01 0.1

Multiplier

Note: The letter “R” may be used at times

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

103K

100V

Maximum Working Voltage

The value is

10 x 1,000 =

10,000pF or

.01µF 100V

10µF 16V

-3-

BASIC MODULATION THEORY

There are many different methods for modulating

information onto a radio wave. The two most

popular methods are Amplitude Modulation (AM)

and Frequency Modulation (FM). Figure 1 shows

the basic difference between these two methods. In

an amplitude modulated radio wave, the audio

information (voice) varies the amplitude of the RF

carrier. To recover this information, all that is

needed is a peak detector that follows the carrier

peaks. This is fairly easy to understand. In a

frequency modulated radio wave, the information

changes the frequency of the carrier as shown in

Figure 1.

The amplitude of the radio frequency carrier wave

remains constant. The loudness of the audio

determines how far the frequency is moved from the

unmodulated carrier frequency. In a normal FM

radio broadcast, the maximum deviation from center

frequency is set at +150kHz for the loudest sound.

A soft sound may move the carrier only +10kHz.

The number of times the carrier deviates from the

center frequency, each second depends on the

frequency of the audio. For example, if the carrier is

moved to +75kHz, then –75kHz 1,000 times each

second, the carrier is 50% modulated for loudness

with a 1,000 cycle audio tone.

One advantage of FM modulation over AM

modulation is the carrier amplitude is not important

since the information is carried by the frequency.

This means that any amplitude noise added to the

signal after transmission (such as lightning, spark or

ignition noise in cars, etc.) can be reduced by

allowing the amplifiers before detection to limit or

saturate. This principle is shown in Figure 2.

The standard broadcast band for FM was also

designed to have an audio range up to 25,000 Hertz

(Hertz = cycles per second). The standard AM

broadcast band has only 7,000 Hertz band width

(Figure 3). The FM band is therefore considered to

be “High Fidelity” compared to the older AM band.

Another big advantage that FM has over AM is the

“Capture” effect in FM broadcast. If two different

broadcasts are very close in frequency or on the

same frequency in AM, they will produce an audio

tweet or beat. In FM, the receiver will “Capture” the

strongest signal and ignore the weaker one. In

other words, if a local transmitter and another

distant transmitter are on the same frequency, the

FM receiver will lock in on the strong local station

and reject the weak one. In an AM radio, if the same

conditions exist, you will hear a beat (a whistle)

between the two stations, which is very annoying.

Capture works because the receiver “sees” radio

waves as the sum of each frequency present. Since

FM only looks at frequency, the weaker signal can

be eliminated by the limiter as shown in Figure 4.

The detector “sees” only the strong signal after the

limiting amplifier has stripped the weak one away.

Amplitude Modulation

Frequency Modulation

Figure 1

Figure 2

Original Transmitted

Signal

Received Signal with

Noise and Fading

Received Signal

After Limiting

Amplifier

Figure 3

Narrow Band

Wide

Bandwidth

7kHz 25kHz

AM Broadcast Band FM Broadcast Band

Audio Bandwidth for AM & FM

Figure 4

Two Frequencies

Transmitted What Limiter “sees” Capture Effect

Output from limiter

F1 only . . . F2 removed

F1 F2 F1 + F2 Limiter Levels

-4-

CIRCUIT OPERATION

Figure 5 shows a block diagram of the FM wireless

microphone circuit. The microphone element in

Block 1 acts like a resistor that changes when

exposed to sound waves. The change in resistance

causes current through the microphone element to

change when sound waves apply pressure to its

surface. This action is similar to squeezing a garden

hose and watching the water through it decrease.

When the hose is released, the water through it will

increase. When sound waves hit the microphone

element, the electrical current through the element

will increase and decrease according to the pressure

(loudness) of the sound.

Block 2 is a transistor (Q1) used as an audio

amplifier. The signal from the micro-phone element

is increased in amplitude by a factor of 3. In

electronics, this action is described as transistor Q1

having an audio gain of 3.

Block 3 is a transistor (Q2) used as an oscillator. An

oscillator is an electronic circuit similar to the

pendulum in a grandfather clock. Once the

pendulum is started in motion, it will use only a

small amount of energy from the main spring to

keep it swinging at the exact same frequency. It is

this stable frequency rate that sets the time

accurately. If the weight is moved down the stick on

the pendulum, the swing takes longer if the

frequency is lower. If the weight is moved up the

stick, the frequency increases. This is called tuning

the frequency of the pendulum. In electronics, an

oscillator circuit also has tunable elements. The

inductor in a tuned circuit is equivalent to the length

of the pendulum (see Figure 6).

By changing the position of the iron core in the

inductor, the inductance can be changed to tune the

oscillator to a desired radio frequency, just like

changing the weight of the pendulum would change

its frequency. When sound strikes the microphone

element, it is converted to an electrical signal,

amplified and used to change the capacitance

(length of the pendulum) of the electronic oscillator’s

tuned circuit. This causes the frequency of the

oscillator to make slight changes at the same rate

as the sound striking the microphone. This effect is

known as frequency modulation.

Block 4 is a transistor used as a radio frequency

amplifier. This block amplifies the modulated signal

from the oscillator and acts as a buffer stage

between the antenna and the oscilator. If the

antenna were tied directly to the oscillator without

the buffer, any capacitance added to the antenna

(touching it with your finger for example) would

produce a large change in the frequency of

oscillation. The receiver would not be able to follow

this large change in frequency and would lose the

transmission.

Block 5 is the antenna. The antenna is also a tuned

element since the length of the antenna determines

how well it will radiate the modulated signal. An

antenna acts much like a piece of string tied to a

wall and stretched tight. If you tap the string, a wave

will travel to the wall and part of the energy will go

into the wall and part will be reflected back (see

Figure 7A). If the length of the string is adjusted to

match the rate of tapping as shown in Figure 7B, the

wall receives all of the energy because it is at a

node or proper multiple of the wavelength. In

electronics, the wall is similar to the space around

the antenna. By properly tuning the antenna, all of

the available power in the antenna will be radiated

into the space around the antenna. None will reflect

back. A term used in electronics to describe the

amount of power reflected back as a ratio of the

amount of power radiated is called “The Standing

Wave Ratio”.

Figure 5

Microphone

Element

Q1 Q2 Q3

Audio

Amplifier

Radio

Frequency

Oscillator

Radio

Frequency

Amplifier

Antenna

Block 5Block 4Block 3Block 2Block 1

Figure 6

Length

Weight

C

L

C = Capacitance

L = Inductance

Pendulum Electronic Tuned

Circuit in Oscillator

Figure 7B

Figure 7A

Original Wave Reflected Wave

Wall

All energy goes

into the wall,

none is reflected.

Vibrating String Node

Introduction

The most important factor in assembling your FM Wireless Microphone 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.

DO NOT USE ACID CORE SOLDER!

CONSTRUCTION

Solder Soldering Iron

Foil

Solder

Soldering Iron

Foil

Component Lead

Soldering Iron

Circuit Board

Foil

Rosin

Soldering iron positioned

incorrectly.

Solder

Gap

Component Lead

Solder

Soldering Iron

Drag

Foil

1. Solder all components from

the copper foil side only.

Push the soldering iron tip

against both the lead and

the circuit board foil.

2. Apply a small amount of

solder to the iron tip. This

allows the heat to leave the

iron and onto the foil.

Immediately apply solder to

the opposite side of the

connection, away from the

iron. Allow the heated

component and the circuit

foil to melt the solder.

1. Insufficient heat - the

solder will not flow onto the

lead as shown.

3. Allow the solder to flow

around the connection.

Then, remove the solder

and the iron and let the

connection cool. The

solder should have flowed

smoothly and not lump

around the wire lead.

4.

Here is what a good solder

connection looks like.

2. Insufficient solder - let the

solder flow over the

connection until it is

covered. Use just enough

solder to cover the

connection.

3. Excessive solder - could

make connections that you

did not intend to between

adjacent foil areas or

terminals.

4. Solder bridges - occur

when solder runs between

circuit paths and creates a

short circuit. This is usually

caused by using too much

solder. To correct this,

simply drag your soldering

iron across the solder

bridge as shown.

What Good Soldering Looks Like

A good solder connection should be bright, shiny,

smooth, and uniformly flowed over all surfaces.

Types of Poor Soldering Connections

-5-

ASSEMBLE COMPONENTS TO THE PC BOARD

-6-

C4 - 10pF Discap (10)

Q2 - 2N3904 Transistor

(see Figure A)

L1 - Coil

C5 - 12pF Discap (12)

C6 - 33pF Discap (33)

R8 - 1kΩ5% 1/4W Res.*

(brown-black-red-gold)

C7 - .001µF Discap (102)

R10 - 1kΩ5% 1/4W Res.*

(brown-black-red-gold)

R1 - 8.2kΩ5% 1/4W Res.

(gray-red-red-gold)

R4 - 47kΩ5% 1/4W Res.

(yellow-violet-orange-gold)

R3 - 4.7kΩ5% 1/4W Res.

(yellow-violet-red-gold)

C1 - .1µF Discap (104)

R2 - 27kΩ5% 1/4W Res.

(red-violet-orange-gold)

R6 - 10kΩ5% 1/4W Res.

(brown-black-orange-gold)

R5 - 150Ω5% 1/4W Res.*

(brown-green-brown-gold)

R9 - 47kΩ5% 1/4W Res.*

(yellow-violet-orange-gold)

Q3 - 2N3904 Transistor

(see Figure A)

C3 - .001µF Discap (102)

C2 - .1µF Discap (104)

Q1 - 2N3904 Transistor

(see Figure A)

R7 - 1.5kΩ5% 1/4W Res.

(brown-green-red-gold)

Mount the

transistor

with the

flat side

as shown

on the top

legend.

Figure A

Mount

these

resistors

on end.

*

Top Legend of PC Board

Strip the insulation off of one end

of the 12” gray wire to expose 1/8”

of bare wire. Mount and solder the

wire to the foil side of the PC board

in hole J5.

Cut a 1 1/2” red wire and 1 1/2”

black wire. Strip the insulation off

of both ends to expose 1/8” of bare

wire. Mount and solder the red

wire to the foil side of the PC board

in hold J6 (+) and the black wire to

hole J7 (–).

Cut the leads of the LED so that

they are 1/4” long, then spread

them slightly apart (see Figure B).

Solder the free end of

the black wire to the

flat side lead of the

LED.Solder the free

end of the red wire to

the other lead of the

LED.Figure B

If your microphone has leads attached

to it, cut them off flush with the pads on

the microphone. Cut a 2” piece of red

wire and a 2” piece of black wire. Strip

the insulation off of both ends to expose

1/8” of bare wire. Solder the red wire to

the foil side of the PC board in hole J1

(+) and the black wire to hole J2 (–).

Solder the free end of the red wire to

the (+) pad on the mic and the black

wire to the (–) pad on the mic as shown

in Figure C.

Figure C

+

J7 J2

J1

J6

J5

Black

Red

Foil Side of PC Board

Red

Black

Flat

-7-

Mount the slide switch onto the foil side

of the PC board. The tabs on the switch

must go through the slots of the PC

board. Solder the switch to the PC

board.

Strip the insulation off of both ends on

the remaining 2 1/2” of black wire and the

7” of red wire to expose 1/8” of bare wire.

Mount and solder the black wire to the

foil side of the PC board in hole J4 (–)

and the red wire in hole J3 (+).

Solder the free end of the black wire to

the negative (–) clip.

J3

J4

Foil Side of PC Board

Insert the free end of the red wire

through the slot in the bottom case

and solder to the positive (+) clip

(see Figure D).

Pull the wire back through the slot and

insert the clip into the case and bend

the tab as shown in Figure E.

(+)

(–)

Foil Side

Figure D

Red Wire

Figure E

Pull wire down

Bend tab

-8-

Insert the mic into the slot as shown in Figure F.

Insert the LED into the hole as shown in Figure F.

Place a piece of tape over the LED to hold it in

place.

Insert the PC board into the top case, as shown

in Figure G.

Insert the negative (–) battery clip into the bottom

case as shown in Figure G.

Press the gray antenna wire and the 7” piece of

red wire through the slots in the top case as

shown in Figure G.

Place the bottom case onto the top case. Hold

the two halves together with three 2.6 x 8mm

screws and one 2.5mm x 4mm screw, as shown

in Figure H.

Insert the stand in the case as shown in Figure H.

Push the foam cover onto the case as shown in

Figure H.

Insert two “AA” batteries into the case with the

positive (+) side toward the back end of the case

(see the inscription on the inside of the case).

Caution: Do not mix alkaline, standard (carbon-

zinc), or rechargeable (nickel-cadmium)

batteries. Insert the battery cover onto the case

as shown in Figure H.

FINAL ASSEMBLY

Figure F

Mic LED

Figure H

Foam Cover

2.6 x 8mm Screws

2.6 x 8mm Screws

Battery Cover

Stand

2.5 x 4mm Screw

Figure G

Gray Antenna Wire

Red WireBlack Wire

Bottom Case

Top Case

-9-

Tug slightly on all parts to make sure that they are

indeed soldered.

A solder bridge may occur if you accidently touch

an adjacent foil by using too much solder or by

dragging the soldering iron across adjacent foils.

Break the bridge with your iron.

Make sure that all of the parts are placed in their

correct position. Check if the transistors’

orientations are correct.

Make sure that the polarity of the LED and

microphone are placed in the correct position.

FOIL SIDE OF PC BOARD

OPERATING THE FM MIC

After assembling the kit, it will be necessary to tune

in the transmitter. First, be sure that all of the parts

are in correctly and that you have good solder

connections without any solder shorts.

Get an FM radio and tune it away from any FM

stations. You should hear only noise and no

programs. Place the unit about 2 feet from the radio

with the microphone facing the radio speaker.

Remove the foam cover and tune the RF coil with

the tuning stick and listen for a howl in the radio.

This indicates that you have tuned the transmitter to

the FM radio frequency. Place the transmitter away

from the radio until the howl disappears. Talk into

the microphone and you should hear your voice on

the radio. If your voice comes through the radio

distorted, speak softer (you are over-modulating).

Push the foam cover onto the case when tuned.

Have a friend listen to the radio and move the

transmitter about 100 feet away. Your voice should

still be heard over the radio. To obtain further

distance, add a longer antenna.

TO CONSERVE BATTERIES, TURN THE POWER

SWITCH OFF WHEN NOT ACTUALLY TALKING.

TROUBLESHOOTING

SCHEMATIC DIAGRAM

-10-

GLOSSARY

Amplitude Modulation

To modify the amplitude of the carrier

wave in accordance with the desired

signal, often abbreviated as AM.

Amplify To enlarge; increase in scope or

volume.

Antenna A device by which radio waves are

released or received.

Audio Sounds that are capable of being

heard by the human ear.

Buffer Stage A circuit used to insulate signals

from other circuits.

Capacitor A device that is capable of holding

an electric charge.

Capture The capacity of an FM receiver to

pick only the strongest signal and

thus reduce co-channel

interference.

Carrier Wave The unmodulated wave radiated by

a broadcast station.

Deviation The change in frequency away from

the carrier wave due to FM

modulation.

Efficiency The ratio of energy expended to

power produced.

Frequency Modulation

To modify the frequency of the carrier

wave in accordance with the desired

signal, often abbreviated as FM.

FM Broadcast Band The range of frequency where

commercial frequency modulation

is allowed by the Federal

Communications Commission

(FCC).

FM Transmitter The sending apparatus of a radio

wave in which the message is

contained in the frequency of the

carrier wave.

Hertz A term used to indicate the number

of cycles per second.

High Fidelity A term used to indicate total

coverage of the hearing system.

Inductor A device capable of storing

electrical energy in the form of a

magnetic field.

MHz or Megahertz A million cycles per second.

Microphone A device used for producing an

electrical current corresponding in

its variations in air pressure of

sound.

Modulate To modify a characteristic of a

carrier wave in accordance with the

characteristics of a desired signal.

Noise In electronics, noise is usually the

random electrical signal produced

by the thermal agitation of atoms or

static discharges.

Oscillator A device that continually swings

back and forth between two fixed

points.

Peak Detector

A device used to recover the

modulated signal from an amplitude

modulated wave.

Pendulum A body suspended from a fixed

point so that it may swing freely.

Resistor An electric device used to restrict

the flow of electrical current.

RF Carrier The radio frequency wave used to

“carry” the desired signal.

Saturate Completely charged or at its limit of

operation.

Standing Wave Ratio A term used in electronics to

describe the amount of power

reflected back as a ratio of the

amount of power radiated.

Tuned Circuit

A collection of components used to

select a single or small group of

frequencies.

QUIZ

1. The letters FM stand for ___________ ___________.

2. In AM transmissions, the audio information varies the _____________ of the radio frequency carrier wave.

3. In FM transmissions, the audio information varies the _____________ of the radio frequency carrier wave.

4. In a standard FM radio broadcast moving the carrier +75kHz from the center frequency would represent

_____________% modulation.

5. The effect of rejecting the weaker station and accepting only the strong station is called ______________.

6. When the microphone is exposed to sound waves, it acts like a changing _____________.

7. An oscillator circuit is similar to the _______________ in a clock.

8. Sound striking the microphone is converted to an electrical signal, amplified, and used to change the

_____________ of the electronics oscillators tuned circuit.

9. Using an element to change the frequency of an oscillator at the same rate as the data to be transmitted is

called ____________ ____________.

10. The antenna is also a _____________ element.

Answers: 1. frequency modulation; 2. amplitude; 3. frequency; 4. 50%; 5. capture; 6. resistor;

7. pendulum; 8. capacitance; 9. frequency modulation; 10. tuned

Elenco®Electronics, Inc.

150 Carpenter Avenue

Wheeling, IL 60090

(847) 541-3800

Web site: www.elenco.com

e-mail: elenco@elenco.com