MAKING A TRANSISTOR RADIO

A LADYBIRD BOOK

making a
Transistor Radio

Series 724
How to make it

With superb colour diagrams and an equally
easy-to-follow text, this fascinating book shows
how to make a transistor radio capable of
receiving a number of stations.

First to be described is a simple crystal set
that really works. Stage-by-stage the set is added
to (each stage a complete working radio) until
a powerful transistor radio is completed.

Easily, inexpensively, and learning as you go,
as each section is added you will experience the
thrill, shared by all radio constructors, of
listening to sounds coming from a radio receiver
that is your own work.

Contents

The world of radio
Radio reception
Reading circuit diagrams
A crystal set—the tuned circuit
A crystal set—the crystal diode
Tools for radio construction
Fixing the components
Making the circuit board
Winding the coil
Building the crystal set ..

The crystal set—its aerial and earth
An experimental P.O.W. radio
The transistor
Adding a transistor

Resistors .. .

Biasing the transistor
Adding a second transistor
The second transistor stage
Adding a loudspeaker
Regeneration
Adding regeneration
Wiring the regeneration stage ..
Using the radio
The completed radio

A Ladybird 'How to make it' book

i

making a
Transistor Radio

by G. C. DOBBS

with illustrations by B. H. ROBINSON

© Wills & Hepworth Ltd 1972
Printed in England

f \

The world of radio

Today, radio is part of our everyday life, yet only
at the beginning of this century it was a miracle. It
is still something to marvel at—and a fascinating
subject to study.

With this book you will learn, stage by stage, how
to build more than one simple receiver. Everything
you should know is explained simply and illustrated
clearly, and you will have the pleasure of constructing
something marvellous that really works.

All the components used are easily available, usually
from your local radio dealer. Most large towns have
shops specialising in stocking components, , but it is
also very easy to obtain any you need by post. All the
popular electronic construction magazines contain
advertisements for mail order supplies. Many of these
suppliers provide catalogues which list all the com¬
ponents you will need.

0 7214 0324 7

4

Radio reception

Radio waves cannot be seen or heard. They are
electro-magnetic waves, and they flow backwards and
forwards ( oscillate ). Each backward and forward
movement is known as a cycle. The rate of this back¬
ward and forward movement (< oscillation ) is known as
the frequency of the wave.

A radio wave is also a carrier wave. It can ‘carry’ an
electro-magnetic wave of a frequency exactly similar
to the sound waves of the speech or music which is being
transmitted. This frequency, which is imposed on the
carrier wave, is known as an audio frequency. It is this
electro-magnetic audio frequency that a radio receiver
translates into sound via the loudspeaker. For a more
detailed explanation of this, you might like to read
pages 20-26 of the Ladybird book: ‘How it Works—
Television’.

When radio waves carrying an audio frequency
arrive at a receiver, three things happen:

1. TUNING. The radio frequency carrier wave is picked

up by the aerial. Along this same aerial pass the
radio frequency carrier waves from many other
transmitting stations, but the signal we want from a
particular transmitter must be selected (tuned) from
them.

2. DETECTION. The next stage is the detection stage,

in which the electro-magnetic audio frequency wave
is detected from the carrier wave.

3. AMPLIFICATION. The electro-magnetic audio

signal wave is amplified, or made bigger, and
translated from an electrical signal into SOUND
by the loud-speaker, from which we hear a repro¬
duction of the transmitted sound.

6

AERIAL

The aerial picks up electro-magnetic,
radio frequency, carrier waves

TUNING

A carrier wave of a particular
frequency is selected

The electro-magnetic, audio
frequency wave is detected
from the carrier wave

AUDIO

AMPLIFICATION

hHMHI

wmmmmmmmmtm

The electro-magnetic, audio
frequency wave is amplified

LOUD ■
SPEAKER

The electro-magnetic
audio frequency wave is
converted into a sound wave

Reading circuit diagrams

Before building a piece of electronic equipment, a
constructor needs to know how to wire up the various
components. This can be done with the help of a
diagram showing how the components are joined.
Electronic engineers have developed a ‘shorthand’
method of showing the various components and their
joining wires. This is the circuit diagram.

A circuit diagram is like a map. All the electronic
components have symbols, and the connecting wires
are shown as straight lines. The positions of the com¬
ponents on the circuit diagram do not normally corres¬
pond to their actual positions in the completed equip¬
ment. Sometimes a layout diagram is also used to show
where to position the components on a base board.

Many symbols are used in circuit diagrams and we
will learn them as we meet them in the course of our
construction. To make construction easier we will use
both circuit and layout diagrams.

8

Battery

LAYOUT DIAGRAM

r

A crystal set - the tuned circuit

A crystal set, the simplest form of radio receiver, is
the first stage of our radio. It can operate entirely on
its own as a radio receiver, even if no further stages
are added.

As we have read on page 6, an aerial picks up all the
radio signals (the radio frequency carrier waves) in its
path. But we need only signals of a certain frequency—
those sent out by a particular transmitter. We therefore
need some sort of filter—to allow only one radio station
to be received at once. A tuned circuit is this filter,
because it can be tuned to any frequency we want, and
can select one of the many radio frequency carrier
waves reaching the aerial.

A tuned circuit consists of a coil of wire and a tuning
capacitor. On the opposite page you can see a circuit
diagram for a tuned circuit, and beneath are shown the
actual components—a coil and tuning capacitor.

The particular radio frequency waves which a tuned
circuit selects depends on the number of turns of wire
on the coil and the ‘value’ of the capacitor. Since it is
inconvenient to keep changing the number of turns of
wire on the coil, the capacitor is made with movable
plates so that the tuning can be varied and the signals
we want selected.

10

A crystal set - the crystal diode

After the tuned circuit has selected the particular
carrier wave frequency required, the audio frequency
signal which it carries must be extracted from it. The
crystal (sometimes referred to as a diode) is a simple
device that does this by allowing current to pass only
one way. It is used in the crystal set as a ‘detector’.
The action is a complex one, but we can simply say that
it detects from the radio frequency carrier wave the
electrical waves of the same frequency as sound {audio)
waves and which a loudspeaker or headphone can
convert into sound.

On the opposite page you will see a circuit diagram
showing the tuned circuit mentioned on the previous
page but with the symbols for the diode and the ear¬
piece included. With the addition of the aerial and earth
symbols, we now have a circuit diagram for a complete
working radio set.

Also illustrated are a typical earpiece and a crystal
diode.

12

^ fAerial

Ferrite

Coil

Earth

Diode

000054JF
Tuning Capacitor

THE CRYSTAL SET CIRCUIT DIAGRAM

Symbol

positive

Diode

Earpiece

Tools for radio construction

Simple radio construction, of the sort described in
this book, requires few tools. Most of these can be found
in a normal household toolbox or are inexpensive to
buy.

During the building of the radio, wires have to be
bent into place and sometimes cut to the correct length.
Some components are joined by P.V.C. covered copper
wire which has to be cut to size and its covering removed
from the ends to make the connection. The most useful
tool for this is a pair of pointed-nose pliers. These will
also strip the plastic from the wire before the connec¬
tions are made. A pair of wire cutters may also be used
to cut and strip wire.

It is possible to buy an inexpensive little tool called a
‘wire-cutter and stripper’ to do both jobs. A pocket
knife is useful for scraping the ends of the wire to
clean them before the connections are made. Sandpaper
or the rough edge of a matchbox can also be used to
clean wires before they are connected. A medium-sized
screwdriver will be needed for making the connections.

This radio is to be built on a wooden base, so a few
simple woodworking tools—handsaw, gimlet, brace
and bit—are also required.

14

Pointed-Nose
Pliers

Gimlet or Bradawl
(for starting screws)

Screw to adjust Stripper
different wire sizes

and Stripper

Stripping

Cutting

Knife

CLEANING WIRES

Fixing the components

In electronic construction it is usual to solder the
components into their places in the circuit. This radio
is designed for the beginner, so a non-soldering method
of connection is used. For those who wish to solder the
connections, the technique is explained on the inside
back covers of this book.

A simple method of connecting the components
uses woodscrews and washers. The screws are No. 6
brass countersunk, half inch screws and the washers
are No. 6 brass screwcups. Two dozen of each are
required. All are obtainable from your local iron¬
monger.

The method is explained in the diagram opposite, but
remember the following tips: (1) Make sure the wire
to be connected is scraped clean until it shines. (2) Where
several wires are trapped under one washer, do not
finally tighten the screw until all the wires are in place.

It is possible to test a connection with the simple
battery and bulb arrangement in the diagram. Touch
the two leads onto some of the bare wire either side of
the connection. If the connection is good, the bulb will
light up.

16

■unsafe

TESTING A CONNECTION

Making the circuit board

In the early days of radio, receivers were made on a
baseboard sometimes called a ‘bread board’. We are
using a form of bread board mounting.

Softwood is used for the base. Mount the screws and
screwcups, used for connections, at one inch intervals
in two rows of eight. The rows should be an inch-and-
a-half apart. This is shown in the drawing facing this
page. The two large holes are made with a brace and
bit and are for mounting the control components.

The accuracy of the measurements is not vitally
important, but it would be helpful to follow those given
as closely as possible. Not all the screws are used in
building the first part of the radio; the additional
screws are needed later as the radio is completed.
Although this board is very simple to make, it is
important to try to get the screws in straight so that the
screwcups press firmly onto the wires.

18

Counter-bored hole Screw centres Counter-bored hole

With a t" bit, drill from the marked side With a }" bit, drill from the other
until the point of the bit just appears on side, half way through the board
the other side. (Do not drill right through)

MAKING THE HOLES

Marked
side of
board

j Pencil
lines

Section of the counter-
bored holes

Use a gimlet or bradawl
to make holes for the screws. (Do not
fully tighten the screws at this stage)

Winding the coil

Before the crystal set can be built, we must wind a
tuning coil for the tuned circuit. This is done on a piece
of | inch diameter ferrite rod. Ferrite is iron dust
stuck together. Usually ferrite rods are 6 inches long,
so cut a notch all round the centre and smartly snap the
rod in half (Figs. 1 and 2).

Wrap about 1 \ inches of sticky tape around the centre
of the rod on which to wind the coil (Fig. 3).

Obtain some 36 s.w.g. (standard wire gauge) enamel¬
led copper wire from your radio or electrical dealer.
Leaving about 3 inches of wire, secure one end of the
wire with sticky tape (Fig. 4) and wind 5 turns, side
by side, onto the rod (Fig. 5). Pull out a 3 inch loop
and twist the wire until it is tight up to the rod (Fig. 6).
Then wind a further 45 turns, side by side, securing the
final end with tape. The whole coil can be covered with
tape to keep the windings secure.

Remember to leave about 3 inches of spare wire at
both ends and at the twisted wire (the ‘tapping’ point).
Later the enamel must be scraped off so that connections
can be made.

The illustrations opposite will guide you through
each stage.

20

%y \y '-v

(dh
en

thumbsonly (otherwise it may splinter)

1) Cut a deep notch all 0 . 0 _ ,

the way around the rod V ^ nap ^ rod by holding close to

the notch with forefingers and

3) Wind the tape in separate parallel
bands , slightly overlapping. (If spirally
bound the tape will wrinkle)

Twist the wire

Form the coif
either by winding the
wire or rotating the rod,
whichever you find easier

6) Twist the tapping point with a finger
in the loop, holding the rod still with
the wire trapped under finger
and thumb

Aerial

Coil

SYMBOLS

Building the crystal set

Having made the coil, the ferrite rod can be mounted
on the baseboard by twisting a loop of stiff wire around
each end and screwing the wire under two screwcups,
as shown opposite.

Next the tuned circuit can be completed by the addi¬
tion of the tuning capacitor. You can buy one from
your local radio dealer and you should ask for a
0-0005 pF solid dielectric type. The illustration opposite
shows how it is mounted on the baseboard, the capacitor
spindle passing through the f inch hole. The wiring
connections are made to the two terminal nuts on the
back of the capacitor. The connecting wire used should
be PVC covered copper wire, the ends being bared
before they are trapped under the screwcups.

The illustration also shows how the diode is included
in the circuit by means of the screwcups. The earpiece
is also shown inserted. When purchasing this, you
should ask for a crystal earpiece. A pair of high
resistance headphones could be used in place of an ear¬
piece, and may give even better results. Resistance is
discussed on page 32, in the section headed ‘Resistors’.

On the following two pages there is some useful
information about the aerial and earth.

22

HOW THE CAPACITOR IS
MOUNTED THROUGH THE BOARD

Lead

THE COMPLETED CRYSTAL SET

The crystal set - its aerial and earth

The aerial is the part of the receiver which ‘picks up’
the radio signals. Unlike our crystal set, many modern
transistor radios are sufficiently sensitive to pick up
enough signal from the ferrite core of the tuning coil.
Although the final radio which we will make will not
need an aerial and earth, the crystal set relies on the
electrical current set up between a good aerial and earth
to work well.

A suitable aerial can be made from inexpensive PVC
covered copper wire. The idea is to suspend as much
of this wire as possible, as high as possible. The far end
can be attached to a tree, the top of a pole or even the
eaves of a friendly neighbour’s house, but this can be
dangerous and is best done by an adult or under the
supervision of an adult. The wire should be fastened
onto a large screw eye or hook. The near end of the wire
may be taken in the top of an upstairs window and
connected to the aerial connection on our radio.

For the aerial to be effective, the other side of the
tuned circuit has to be connected to earth. The simplest
method is to attach the earth lead to the nearest house¬
hold water pipe. These pipes lead to the ground. If
your water pipes are plastic, make your own earth by
driving about a yard of copper pipe into soft earth and
connecting the earth wire to the top.

24

Aerial Wire

Tree
or Pole

(A permanent aerial should have an
insulator at each end)

To

Riidio

An experimental P.O.W. radio

During the last war, some prisoners in prison camps
used home-made radios to keep in touch with events
in their native lands. Often, all the components had to
be made from items picked up within the camp. Diodes
were almost impossible to obtain, so several ingenious
home-made alternatives were tried.

The diagram shows one simple method of improvising
a diode. Take a small piece of wood, about 1 inch thick,
and a piece of washed coke about the size of a pea.
Mount the coke on the wood as shown, using a screw
and screwcup, and taking a lead from beneath the
screwcup. Wind about a foot of steel wire—a length of
piano or guitar wire is ideal—around a number 4 or 6
knitting needle, leaving about 1 inch on each end. The
wire should now be slightly springy. Fasten one end to
the board with a screw and screwcup, taking another
lead from this screwcup. Stretch tfie wire until the free
end lightly touches the coke.

Remove the diode from the crystal set and screw the
wires in its place. It should be possible to hear some
local stations by adjusting the tuning capacitor. If
nothing is heard, keep moving the position of the wire
on the coke until a signal is heard.

26

Shallow 1"
hole bored
with brace
and bit

The screw is at a slight angle
so as to grip both coke and
lead securely

Pencil

Lines

The transistor

To most people the word TRANSISTOR means a
small radio set, but a transistor is really a single electronic
component. The transistor is one of the most important
inventions of the last 25 years. It has helped to make
electronic equipment smaller and less dependent upon
high voltages.

A transistor is like two diodes working together inside
one case. Most transistors have three leads, emitter,
collector and base. A transistor can be damaged unless
the correct leads are used when connecting it into a
circuit. The base lead is usually the middle wire and the
collector lead is set slightly apart from the other two
wires. The transistors we are to use also have a red
spot painted on the casing to indicate the collector lead.

We shall be using transistors as amplifiers; that is,
to make electrical signals stronger. Transistors are
designed so that a small, changing current between the
base and emitter produces a large current change
between the collector and the emitter. So small signals
applied to the base will appear amplified (or made
bigger) at the collector. Our first transistor will be used
to make louder the sound coming from the crystal set.

28

/

Base'

/

L

V

\

/

I /

v

am

i wsr
I TV

JLJ

V

Collector

\

/

/

/

Emitter

THE SYMBOL FOR A TRANSISTOR

Collector

Base

Emitter

0C71

*

TRANSISTORS

OC71 TRANSISTOR (Actual size)

Adding a transistor

The circuit diagram shows how a simple transistor
amplifier is inserted where previously we had the ear¬
piece. Compare this diagram with the circuit diagram
of the crystal set on page 23.

The transistor used is an OC71, and other extra
components for this stage are a 2-7K resistor and a 9-
volt battery. The battery is the small transistor radio
type PP3, for which can be obtained an inexpensive
snap connector. Resistors are discussed on the next page.
This resistor forms an output ‘load’ for the collector,
and the earpiece is connected across the resistor.

Take care to connect up the circuit as shown in the
drawing. The connections of the transistor are im¬
portant. Join the emitter to screw 5, the base to E and
the collector to F. The earpiece is removed and re¬
connected to F and G. Connect the red battery con¬
nector lead to screw 5, and the black lead to screw G.
The radio is switched on by pushing the connector into
place on top of the battery.

This one transistor should make a big improvement
to the crystal set; the same stations will be heard but
they will be much louder.

30

■iHTnnnjw

2'7K

Resistor

Earpiece

MjD

9 volt
Battery

0C71

+

CIRCUIT DIAGRAM

To

Aerial

0C71

9 volt Battery (PP3)
and Connector

Resistors

Resistors are very common components in electronic
construction. Look at most pieces of equipment and
you will see several small, wire-ended tubes with brightly
coloured bands. These are called resistors because they
‘resist’ the flow of electrical current in the circuit.
This is done to adjust the voltages and flow of current
at various places in a circuit.

The little tubes are filled with a carbon composition,
and the coloured bands give the value of the resistor.
This value is called the resistance and is measured
in ohms. When one knows the colour code opposite,
it is easy to work out the resistance from the coloured
bands. The tolerance of the resistor, that is, how close
one can expect it to be to the value, is sometimes in¬
dicated with another coloured band. Our radio can use

/

resistors of any tolerance, so this band may be ignored.

The symbol for resistance is the Greek letter omega Q.
Quite often, very high resistances are used—sometimes
thousands or millions of ohms. To reduce the number
of noughts, we call thousands K £2 (Kilo ohms) and
millions M £2 (or Mega ohms). The ohm sign is usually
omitted from circuit diagrams when the Kilo (K) and
Mega (M) signs are used.

32

THE SYMBOL FOR A RESISTOR

Tolerance Band

RESISTOR COLOUR SCOPE

BROWN

RED

ORANGE

YELLOW

0

(or 0 noughts)

(or 1 nought)

(or 2 noughts)

(or 3 noughts)

VL

(or 4 noughts)

GREEN

BLUE

VIOLET

GREY

WHITE

TOLERANCE COLOURS

1 (or 5 noughts)

I (or 6 noughts)

(or 7 noughts)

8

(or 8 noughts)

(or 9 noughts)

GOLD 5% SILVER 10% SALMON 20%

BROWN, BLACK, BROWN

100 «47000 or 47k

I

Biasing the transistor

The transistor amplifier we have built is the simplest
arrangement possible. If a transistor is to be used effi¬
ciently, one has to supply the correct voltages to its
various parts; this is called biasing. In our simple
arrangement the signal from the diode provided a
voltage to bias the base of the transistor. Resistors can
be added to make the biasing correct.

The circuit shows how the transistor is biased with
two resistors. (33K Cl —orange, orange, orange, and
150K Cl —brown, green, yellow.) They give the correct
voltages between the collector, base and emitter. Take
care to connect the resistors in the places shown or the
effect will be ruined.

This addition to the radio will not increase the volume
in any noticeable way, but the biasing is essential be¬
fore the next stage is added.

The wire ends of resistors, like any other components,
can become dirty and greasy, so scrape the wires clean
before screwing them into place.

34

To

Earpiece

Lines

9 volt Battery

Adding a second transistor

Before we can add another transistor to our radio,
two new components must be introduced—a poten¬
tiometer and a capacitor.

The potentiometer is a variable resistance. Look at
the drawing on the opposite page and you will see that
it has three contacts. The outer two are each end of a
circular carbon track. The centre tag is joined to a
movable contact which slides over the track when the
shaft is rotated. By moving the shaft, which is usually
fitted with a plastic knob, any part of the resistance can
be used and tapped off through the centre tag. Poten¬
tiometers are often used, as in this case, as volume
controls.

A capacitor is a component which allows the signal
to pass but not the fixed voltages used to power a
circuit. It consists of two plates which are separated by
insulating material; the circuit symbol shows this.
Electrolytic capacitors must be connected into a circuit
the right way round. One end is marked either with a
red band or a + sign, and this side goes to the positive
side of the circuit. The negative (—) wire is connected
to the metal can which forms the body of the capacitor,
and which may be marked with a black band. Sometimes
this is not obvious, but follow the circuit and layout
diagrams with care to avoid mistakes. Other fixed
capacitors can be connected either way round: we will
meet these later. The value of capacity is written in
microfarads, (pF or sometimes MFD). We require
lOpF at a working voltage of 16-volts.

36

The second transistor stage

The circuit for the second stage is shown on the
previous page. The signal is taken from the collector of
our first transistor, through the capacitor to the sliding
contact of the volume control potentiometer. The nearer
the centre contact is adjusted to the base side of the
control, the more signal is allowed to go to the base
of the second transistor. The greater the signal reaching
this transistor, the louder will be its output.

The potentiometer is mounted in the spare f inch hole.
Its tags are too close together to use the large screwcups,
so small screws are used. These are screwed down
through the holes in the potentiometer tags, trapping
the leads under the tags.

Remove the 2-7K resistor from F and G and connect
in the new components carefully, following the layout
diagram facing this page. As you work, compare the
layout diagram, connection by connection, with the
circuit diagram. This additional transistor will greatly
increase the volume, enough to drive a loudspeaker,
which is the next addition.

38

Bend tags to lie
flat on board

Cut with a hacksaw, to the
same length as tuner spindle

Make a hole in the board with a screwdriver
to accommodate this projection

10K

potentiometer

IOajF

Pencil

Lines

WOK

1

Adding a loudspeaker

Using our radio with an earpiece means, of course,
that only one person at a time can hear the programme.

A loudspeaker changes electrical waves into sound
waves in the same way that an earpiece does, but it ,
has a large cardboard cone so that several people can
hear the sound. Loudspeakers can have very large
cones, but a smaller cone speaker with a diameter of 3
inches is ideal for our purpose.

Loudspeakers are made with inputs of varying
resistances, and our radio requires a 3 or 5 O type.
Unfortunately the loudspeaker cannot directly replace
the earpiece because its resistance is much lower than
that of the earpiece, so it has to be matched into the
circuit with a transformer. A transformer can change
a current of one voltage to a smaller current of a higher
voltage or to a larger current of a smaller voltage. It
consists of a ‘primary’ and ‘secondary’ winding of
fine wire round a metal ‘former’. The transformer we
require is the Eagle LT700. This transformer has rather
long leads, and these may be shortened by winding them
tightly round a knitting needle, "fhis makes spiral leads
which can then be pulled out to reach the screws.

Screw the transformer onto the base, as shown;
remove the 2-7K resistor across G and H and connect
the correct leads (colour coded) to the screwcups. Two
more screwcups are added at the end of the board to
connect the output leads to the loudspeaker, using two
small crocodile clips.

40

TRANSFORMER

Cut off centra! white lead

Bend tags to lie
flat on board

EAGLE LT700
TRANSFORMER

Pencil

Lines

3n 3 inch
LOUDSPEAKER'

Regeneration

We shall now replace our simple crystal set stage with
a transistor stage, using regeneration. Regeneration will
enable our first transistor in the radio to be used three
times.

The circuit shows the regeneration stage. The signal
;■ from the aerial is tuned by the coil and variable
capacitor, the orange marked signal then passes through
capacitor Cl to be amplified by the transistor and
appear at a radio frequency choke (R.F.C.). Part of this
signal, marked in blue, passes through C2 to a pair of
diodes which change the radio signal into an audio
signal, which is amplified again by the transistor. The
signal, now marked red, passes through C3 back to the
coil. The part of the coil tapped between the transistor
collector (through Cl) and earth, allows the signal to
reach the collector again.

This means that some of the signal at R.F.C. has been
amplified three times, so this is a very economical use
of one transistor. Passing signals back again like this is
called feedback. If the feedback is too great, the circuit
will produce a howling sound, so C3 is an adjustable
capacitor to control the amount of feedback. The final
signal is taken from R.F.C. through capacitor C4.

42

HOW REGENERATION WORKS

In circuit diagrams , wires which cross one another, (but do NOT touch and
are NOT connected), are sometimes shown tike this -j- (as in the diagram)
or like this Shown like this -A- always means they are connected.

Adding regeneration

The circuit shows the new stage with the components
added. The adjustable capacitor is a 10 pF (picafarad)
trimmer. This consists of two copper plates mounted
on a pot base; the distance between the plates can be
adjusted with the small screw.

Fixed capacitors may be flat or cylindrical with two
leads; the type we require, ceramic capacitors, may look
like a disc.

The radio frequency choke is really a coil with a lot
of turns of wire on a small former. The choke acts like
a resistance to radio-frequency waves.

Two diodes are needed to provide the feedback path
from the 500pF capacitor. If the top diode were omitted,
the signal would not be changed into audio frequencies
before it reached the transistor base, and if the bottom
diode were omitted, the signal would go straight to
earth and be lost.

The 1K resistor lowers the battery voltage needed to
power this stage. It is possible for some of the audio
signal to feed back to this stage through this IK resistor
and distort the signal. This is prevented by the lOOpF
capacitor which takes any stray signals to earth without
affecting the battery voltage.

44

lOpF TRIMMER

postage stamp n ,cr
type'

CERAMIC CAPACITOR

FREQUENCY

CHOKE

R.F.C.

Wiring the regeneration stage

The drawing shows how this stage is wired onto the
board. Before wiring the stage, all the components
from the crystal stage, up to screws E and 5, are
removed except for the coil and tuning capacitor. The
diode is removed to become one of the two diodes in
the new stage. Follow the drawing carefully, adding one
component at a time and checking its position on the
circuit diagram. There are many bare wires which come
close to each other, and great care must be taken to
ensure they do not touch each other. The IK resistor
must not touch screws E and F.

The trimmer is trapped by its tags under screws A
and B; if it is too long or too short, the screws may be
moved. All three connections to the coil are now used,
and if the wires are not placed in the correct positions,
regeneration will not oqcur. The drawing shows how to
place these three wires. The lOpF capacitor takes the
signal from the new stage to the base of the next OC71
transistor. The advantage of this new stage is that our
radio will have a greater output and no longer need a
large aerial and an earth. Regeneration is tricky to get
used to, but once mastered, the results are surprising.

46

100Ah

Pencil
Lines '

R.F.C.

Pencil

Lines

Using the radio

Operating a regenerative radio requires care at first
because the trimmer has to be adjusted to suit the
strength of each signal. Begin by connecting about 3
yards of wire to the radio as an aerial; no earth is
required. Set the tuning capacitor about midway and
screw in the trimmer until the radio begins to howl.
Unscrew the trimmer until the noise just stops, and try
to tune in a station. The trimmer should then be adjusted
until it is just short of the point when the radio howls.
The general method for receiving any station is to tune
it in with the tuning capacitor and to adjust the feed¬
back trimmer until the howling just disappears.

The feedback not only makes the radio more sensitive
but also the tuning becomes sharper, so tjae tuning knob

s

must be rotated very slowly. This sensitivity makes the
radio ideal for the medium wave band, which is rather
overcrowded. The tuning should be sharp enough to
tune in individual stations without the background
sounds of other stations. In areas of good signal strength,
it will be possible to use the radio without an aerial at
all, merely relying upon the signal picked up by the
ferrite rod coil.

48

390K 4-7K

IQpF

V/ 1

IK

150K

v, 2 p» j

4-7K

150 K

> 17700 §ji?

IQsuF

R.F.C.

3n

OIajF

0A81

Sanaa

0A81f\

n/A

IOOajF

jj Bf

§ i /

Ci

0C45

500pF

33K

T

9 volt battery

The completed radio

Now that the radio is completed, you may wish to
make the circuit board into a simple case. The circuit
board is used as a front panel, and end and bottom
panels are added.

These can be made from similar wood to the circuit
board (4 inch by \ inch planed softwood). The bottom
panel is the same size as the circuit board and the
measurements for the end panels are shown opposite.
The end panel next to the output transformer may be
used to hold the loudspeaker. A series of f inch holes
are drilled with a brace and bit to allow the sound
to leave the case, and the speaker is secured with screws
and cups. The panels are joined with panel pins and
glue.

The battery may be firmly mounted above screws D
and E on the circuit board, using 2| inches of £ inch
elastic and two screws and cups. Tw6 smart plastic
pointer knobs and a tuning scale complete the front
panel. The tuning scale is a paper gummed label which
can be marked with the positions of the main stations.

If you have followed these pages carefully, you will
now have a useful working radio. You will also have
shared the thrill of thousands of other radio construc¬
tors, listening to sounds coming from a radio receiver
which is your own work.

50

How to mark out the position The Loudspeaker is mounted
of the Loudspeaker and i" holes over the circle with 4 screws

SOLDERING TECHNIQUES

MATERIALS REQUIRED

Solder Wire
with a core of flux

Electric Soldering Iron.
25 watt with pencil bit
is idea!

or the edge of a knife. The metal surfaces should
be dean and bright

2) Join the wire to the tag to make a firm joint
that will not move

3) Put the hot bit of the iron firmly upon the joint
and allow a few seconds for the joint to heat up.
Take care. The soldering iron bit and metal shaft

can burn the skin!

and allow the solder to flow freely over the whole joint.
DO NOT apply the solder to the bit, the heat of the
joint should do the melting.

5) Allow the solder to coo! until it goes solid.

GOOD SOLDER JOINTS should be bright and shiny, if the
joint looks very dull and grey it may be a bad joint, called
a DRY JOINT. If this is so, remake the joint.

Components are usually soldered to copper tags. It is possible
to replace the screw cups of the radio with small copper nails
and solder the components onto these. Wrap the wires
around the nail before soldering so that the joint
is made firm.

»

Series 724

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