Other Crafts and Hobbies books available Stamp Collecting Cooking Swimming and Diving Sailing and Boating Biding Indoor Gardening Coarse Fishing Tricks and Magic Knitting Sewing Heraldry Crochet Coin Collecting Handwriting Woodwork Metalwork Camping Knots Pats Ladybird titles cover a wide rango of subjects and reading ages. Write for a free illustrated list from the publishers : LADYBIRD BOOKS LTD Loughborough Leicestershire England 0 7214 D32e 7 Prlnisd I n Efiglhitii 24 p Hat Lea rna bout... making a Transistor Radio m > f dean both surfaces to be joined with sandpaper or the edge of a knife. The metal surfaces should be dean and bright . SOLDERING TECHNIQUES MATERIALS REQUIRED Solder Wire with a core of flux Electric Soldering iron. 25 watt with pencil bit is ideal i Jam the wtre 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 sclderrng iron bit and metaJ shaft can burn the skin! 4J Lightly push the end of the solder onto the joint and allow the solder to how 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 cool until it goes solid. GOOD SOLDER JOINTS should be bright and shiny, if [he joint looks very dull and grey it may be a bad poini, caFFed a DRV JOINT. If tftrs is so, remake the joint. Components are usually soldoiod to copper tags. It is possible to replace ihe screw cups of the radio with small copper nails Arid Solder the components onto these. Wrap rhe wires around die nail before soldering so rhet The joint \s made firm A Ladybird Book Series 634 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. (c: Ladybird Books Ltd (formariy Wills & Hep worth Ltd) T 372 Athlghts rwswvBd No part Shis; pubficatf&n maybe repraducdii.. Ltoisd m s fdzrtAv&l .sysla, 7). or f rftmmttfirf m or?y form or by any maenj. pfscimme, mac fjaniwF. photo- copying, fcrcpxding nr othorwigd, wittiPuT ihn prior consent pf fhp OP.O yrigb if cmw making a Transistor Radio by G. C. DOBBS with illustrations by B. H. ROBINSON Ladybird Books Ltd Loughborough The world of radio Today, radio is part of our everyday life, yet only at the hegi tining of this century it was a miracle. Jt 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 dearly, and you will have the pleasure of constructing something marvellous that really works. All the components used arc easily available, usually from your local radio dealer. Most large towns have shops specialising in stocking components, hut if 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. G 7 2 14 Q'32* 7 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, ft can 'carrv' 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, ft 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 i lie 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 Y The asn$t picks up electro- magnetic, radio frequency, carrier waves TUNING _A_ DETECTION 4 earner wave of a particiff&r frequency *$ seeded The efectm* magnetic. audio frequency wave is defatted from the earner wavs The deetto -magnetic, audio frequency wave is amplified The dectm- magnetic audio frequency wave is converted into a sound wove 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- ponerus 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. 3 Bait CiRCUlT DIAGRAM o Swttch Battery LAV OUT 03AGRAM 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 wc 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, Jt is used in the crystal set as a 'detector*. I he 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 sec 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 Asriai Eorth THE CRYSTAL SET CIRCUIT DIAGRAM Diode. 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 or the wire to clean them before the connections are made. Sandpaper or the rough edge of a matchbox can also be used to dean wires before they are connected. A medium-sized screwdriver will be needed for making the connections. i This radio is to be built on a wooden base, so a Tew gfc simple woodworking tools — handsaw, gimlet, brace ' and bit — are also required. 14 Gimtet Btjfowf ftor Scfewdmzr Culling Stripping i' A fo. 6 Bra$s 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 wood screws 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: (J) Make sure the wire to be connected is scraped dean until il shines. (2) Where several wires are trapped under one washer, do not finally lighten the screw until all the wires are in place. Il 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 1 t .(.tipprf Wite to. 6 Brass Screwcup TESTING A CONNECTION Making the circuit board In the early days of radio* receivers were made on a baseboard sometimes called a ‘bread board', Wc are using a form of bread board mounting. Softwood is used for the base. Mount the screws and screwoups, 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. IS "f j ■ * r ui M » ; W*th a i h it, drift from trie marked side With a i bk f drift from trie airier on tit trie point of trie hit just appears on side half wap through trie hoard trie other side. (Dp not driii right through) MAKING THE HOLES Marked w tie of hoard \ Pencil tines Fine t ton of trie cannier bored hates Use e gimfet or Grade wf to make hates fa: rite strews (Do not fully tighten the screws at trite 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 i 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, 3 and 2). Wrap about 1-} inches of sticky tape around the centre of the rod on which to wind the cod (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 oT the wire with sticky tape (Fig, 4) and wind 3 turns, side by side, onto the rod (Fig. 5). Full out a 3 inch loop and twist the wire until it is tight up to the rod (Fig. 6). Then wind a further 45 sums, side by side, securing the final end with tape. The whole coil can be covered with tape to keep the windings secure. Re member 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 Shat connections can be made. The illustrations opposite will guide you through each stage. 20 1} Cut a deep natch afl the way around the rod 2) Snap the rod by holding doze to the notch with forefingers and dnfmhs..Qnly .{[otherwise It may splinter) 1 1 Wind if re tape m separate parallel Umds* slightly overlapping- l If spirally hound the tape wifi wrinkle) S) Form the coil, v ' either by winding the wire or rotating the rod. whichever you find easier M Twist the tapping point with a finger in the hop. holding the rod stiff with the wire trapped under finger and thumb ^ Aerial The finished coil 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 Lhe baseboard, the capacitor spindle passing through the jj- 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 lhe screwcups. The illustration also shows how the diode is included in the circuit by means of the sc re we ups.. The earpiece is also shown inserted. When purchasing this, you should ask for a crysial 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 ihc following two pages there is some useful information about the aerial a rut earth. 22 UiJ-Ml HOW THE CAPACITOR IS MOUNTED THROUGH THE BQARO THE COMPLETED CRYSTAL SET The crystal set - its aerial and earth The aerial is the part of the receiver which ‘picks up' [he 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 | he final radio which we will make will not need an aerial and earth, the crystal set relies on the electrical current set tip 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, bui ihis 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 he connected to earth. The simplest method is to attach the earl h 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. 14 Clip Support Wtfu (A permanent amiai shotiid have ah mtxterof at each end# ■ Earth Wife Scrape pipe dean at point of con tad EARTH CONNECTION TO A METAL WATER PJPE mvRi An experimental P.O.W, radio During the last war, some prisoners in prison tamps used home-made radios to keep in touch with events in their native Sands, Often , ail 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 l 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 screw-cup, 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 f 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 the wire until the free end lightly touches the coke. Remove the diode from the crystal set and screw' the w-ires 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 wi rc on the coke until a signal is heard. 26 The screw is at s 'slight' s/tgh so as to grif) hath coke and had securely ■■ 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 or the most important inventions ol~ 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 wirp& The transistors we are to use also have a red spot painted on the casing 10 indicate the collector lead, Wc 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. 2S CoMector THE SYMBOL FOR A TRANSISTOR OC7t 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 GC7I, 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 27K Resistor ; I QfffhvB & volt Beltary CIRCUIT DIAGRAM To far piace To firth 9 volt Battery fPP3) 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 Tesist* the flow of electrical current in the circuit. This is done to adjust the voltages and flow oT 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 or any tolerance, so this band may be ignored. The symbol for resistance is t lie Greek letter omega iL Quite often, very high resistances are used — sometimes thousands or millions of ohms, To reduce the number of noughts, we call thousands K O (Kilo ohms) and millions M Q (or Mega ohms). The ohm sign is usually omitted from circuit diagrams when the Kilo (K) and Mega (M) signs are used. 32 A/VVv — THF. SYMBOL FOR A RESISTOR Number of Noughts I Tolerance Band RESISTOR COLOUR SCOPE 0 (or 0 noughts) GREEN 5 (or 5 noughts} BROWN 1 (or 1 nought) BLUE 6 for 6 noughts) 2 for 2 noughts) VIOLET 7 (or 7 noughts) ORANGE 3 for 3 noughts) GREY 8 for 8 noughts) YELLOW 4 for 4 noughts/ WHITE 9 for 9 noughts) TOLERANCE COLOURS GOLD 5" . SB VLB IB” , SALMON 20';-. YELL 47000 nr 47k mm 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 ihc 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 il — orange, orange, orange, and I50K, fl— 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. 1 his addition to the radio will not increase the volume in any noticeabie 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 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. I he 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 (his case, as volume controls, A capacitor is a component which allows the signal to pass but not (he fixed voltages used to power a circuit. It consists of two plates which arc 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 H sign, and this side goes to the positive side of the circuit. The negative ( — ) wire is connected to the metal can which forms ihe 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 lOgF al a working voltage of [6-volts. 36 »rn Origins} Crystal Set □ SAG RAM 9 votes ELECTROLYTIC CAPACITOR POTENTIOMETER The second transistor stage The circuit Tor 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. Tire 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 $ inch hole. Its tags arc too close together to use the large screwcups, so small screws are used. These arc screwed down through the holes in the potentiometer tags, trapping the leads under the tags. Remove the 2-7 K 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 fie fiat on board Cot. with a WW fength as tuner sptndfe Afl&ke a ho/v in the hoard with a sere wdt ivt*r to accommodate this prOfectfon To Earpiece t \ 1 0K / potentiometer Adding a loudspeaker Using our radio with an earpiece means, of course, l ha 1 only one person al 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 0 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 or a higher voltage or to a larger current of a smaller voltage, Tt 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 lightly round a knitting needle. This makes spiral leads which can then be pulled out to reach the screws. Screw the transformer onto the base, as shown; remove I he 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 oft 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 dreuil 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 ihe 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 loo 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 ipja another, (but do NOT touch and urc NOT connected), are sometimes shown tike this + fas in the diagram) or tike this Shown tike this - 4 - aiwa V s .means they are connected. Adding regeneration The circuit shews the new stage with the components added. The adjustable capacitor is a 30 pF (picafarad) rrimmer* This consists of two copper plates mounted on a pot base; the distance between the plates can he adjusted with the small screw. Fixed capacitors may be fiat or cylindrical with two leads; die type we require, ceramic capacities, may look like a disc. i he radia 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 50GpF capacitor. If the top diode were omitted, the signal would not be changed into audio frequencies before il reached the transistor base, and if i he bottom diode were omitted, the signal would go straight to earth and be lost* The I K 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 Lhe battery voltage. 44 *wvW IK lOOvF 500pF I > 7* R.F.C TOpF TRIMMER po^age Stamp D J| C ceramic CAPACITOR type NCY CHOKE 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 Lo become one of Lhe two diodes in the new stage. Follow the drawing carefully, adding one component at a time and checking its position on the ritxuii 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 screw’s 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 occur. 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 OC7I transistor. The advantage of this new stage is that our radio will have a greater outpuL 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 Using the radio Operating a regenerative radio requires care at first because the trimmer has to be adjusted to suit the strength or 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 unlit 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 shorl of I lie 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. I he feedback not only makes the radio more sensitive hut also the tuning becomes sharper, so the tuning knob 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 (he signal picked up by the ferrite rod coil. 48 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 t inch planed softwood). The bottom panel is the same size as the circuit board and the measurements for the end panels arc shown opposite. The end panel next (0 the output transformer may be used to hold the loudspeaker. A series of J inch holes arc 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 arc joined with pane) 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. Two 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 f 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 ow n work. 50 How to mark out the position of the- Loudspeaker and i" ho fas The Battery is mourned above strews D and E Thu l.muJufj-Miikftr in ItlOUtititf over this orclw with *1 no rows ■i Hltl THE CQMI’I.MH) HAIKU Contents The world of radio , . . , * ft ft: 4 ■Radio reception , a 6 Reading circuit diagrams 8 A crystal set the tuned circuit . . .. 10 A crystal set— the crystal diode 12 Tools for radio construction 14 Fixing the components . , 16 Making the circuit board k * .. 18 Winding the coi] .. 20 Building the crystal set , . . . .. 22 The crystal set its aerial and earth . . - ■ 24 An experimental P-O.W. radio *.ft .. 26 The transistor + ft .. 28 Adding a transistor # ► ft J ,. 30 Resistors ■ T , . 32 Biasing the transistor * T T ‘a ik 34 Adding a second transistor + a .. 36 The second transistor stage ft a .. 38 Adding a loudspeaker ■ 1 .. 40 Regeneration 42 Adding regeneration 44 Wiring the regeneration stage . . - t- T I' .. 46 Using the radio' H . a a a . , . 48 The completed radio + ft .. 50 Components List At each stage you have a complete working radio. Stage 1 . Crystal set Fethte rod 1“ (1 Omni) diameter. 2 y da (2 metres) 30 s.w.g. enameHed copper wire. ■QQGSuF (50QpF) variable capacitor (solid dielectric type).. Crystal diode OA81 - Alternatives; 0A70, OA71* \ QA79 or any general purpose diode. Crystal earpiece. Stage 2. Adding a transistor GC7t transistor - Alternatives: NKT214, AC 122, A Cl 25 or any general purpose FNP audio transistor. 2'7K resistor. 9 volt battery (PP3) and connector. Stage 3, Biasing the transistor 33K resistor, 1 50 K resistor. Stage 4, Adding the second transistor QC 71 - Alternatives NKT214. AC122, AC1 25, AGY35. 4 7K and 160K resistors, 10K potentiometer. IGuF (tS volts, working) efecirolytsc capacitor. Stage 5* Adding a loudspeaker Eagle LT700 transformer. 3 ohm loudspeaker (about 3 s (76 mm) diameter). Stage 6. Adding regeneration QC45 transistor ■ Alternatives: OC44, 2SA12. AF116, A FI 26. A FI 27 or any general purpose PNP RF transistor Crystal diode (as stage 1) r Radio frequency choke (transistor type). 10pF trimmer (postage stamp type). 1 QuF electrolytic capacitor (16 volts working), IOQuF electrolytic capacitor (16 volts working). 5oopF ceramic capacitor. 01 uF disc ceramic capacitor. 390K r 4-7 K and' 1 K resistors. COMPONENTS fOR THE BASE BOARD 4 fc ■* y (1 Q2 mm * 13 mm) planed softwood 12’ (305 mm) long. 2 dozen No. 6 brass screwcups. 2 dozen No. 6 brass countersunk Y (13 mm) screws. 1 yd £1 metre) single pv.c. covered copper boll wire.