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THE CL MATCH ASSIC BOX CRYSTAL SET By STEVE PAYOR Have you ever built a crystal set in your life? If you've never a built a crystal set before, this is the one to have a go at. You '11 be surprised at just how well it performs. This interesting little project is easy on the budget but big on satisfaction. And it won't cost you a cent to run after it's finished, even if you let it play continuously until the year 2088. Make no mistake about it, this crystal set is a real, working AM radio receiver, not just a nostalgic novelty. Until the invention of electronic amplification, the humble crystal set was the only form of wireless reception in use. Circuit development reached a peak around the 1920s and the "Super Crystal Set" described in the March 1988 issue of SILICON CHIP is a typical example of the type of receiver used for shipto-shore communications over thousands of miles. Simpler crystal sets, using only a single tuned circuit, were used for listening to domestic broadcasts and the crystal set described in this article is in the same category. It's cheap, easy to use, and capable of receiving all local broadcast stations at worthwhile volume. There is a price to pay for free entertainment however. First, you have to string up an antenna and provide a fairly good earth. Second, you will have to find a pair of sensitive, high-impedance headphones. More about the headphones later. Indoor antenna Our new crystal set has been designed to work with 15 metres of hookup wire as an indoor antenna. In most cases just draping it around A1 MATCHBOX CRYSTAL SET Fig.1: the complete circuit for our matchbox crystal set. Unlike most other crystal sets, it uses variable inductance tuning to give constant bandwidth and volume over the entire tuning range. The headphones should be high-impedance types (see text). a skirting board will do the trick. But if you live in a poor signal area it may be necessary to run the wire out the window and up and away from the house. In any case, stick to a length of 15 metres to start with, because the antenna capacitance and resistance form an integral part of the tuned circuit. As you can see from the circuit diagram and photographs, the tuned circuit consists of an inductor, wound on the outside of a matchbox, in parallel with a fixed capacitor and the antenna capacitance. The inductance is varied over a wide range by sliding two pieces of ferrite rod in and out of the matchbox. The ratio of minimum to maximum inductance that can be achieved in this way is about 4:1, which is not quite enough to cover the entire broadcast band in one go. This band extends from 531kHz to 1602kHz; ie, a 3:1 frequency ratio which requires a 9:1 change in inductance. We solved this problem by providing an antenna tapping, A2, at the centre of the coil. This tapping is used when tuning the top half of the band, and the antenna capacitance is transformed to an equivalent capacitance of only 32pF or thereabouts. When the antenna is connected to Al, the full antenna capacitance (approximately 135pF) is in parallel with the tuned circuit. This effectively doubles the tuning capacitance and shifts the tuning range down to cover the bottom half of the band. The two antenna tappings also perform another vital function they enable us to optimize the antenna coupling for each tuning range. The subject of antenna imOcT0BER 1988 53 1~NJ~~:Els El--0 400 EQUIVALENT TO EART!!J:""° ... · V 135PF 300uH FIXED SIGNAL INDUCTOR Fig.2: equivalent circuit for a 15-metre long antenna. It can be represented by a 135pF capacitor in series with a 400 resistor and an AC voltage source. pedance, and its loading effect on the tuned circuit, is perhaps the most important consideration in the design and operation of a crystal set. So let's start by considering the antenna as an electrical circuit. Equivalent circuit Fig.2 shows the equivalent circuit of our 15 metres of hookup wire at broadcast band frequencies. The wire is too short for its inductance to be significant at these frequencies. Thus, the antenna can be effectively represented by a capacitance of 135pF in series with a small voltage source [the received signal) and a certain amount of resistance. This resistance can be broken down into two parts. The major component is just the highfrequency resistance of the antenna wire itself. The other component is the "radiation resistance", which represents the coupling of the antenna to the surroun~ing space. The total series resistance of our 15-metre indoor antenna [including the earth connection) is about 400 at 1MHz, of which only a few ohms is the actual radiation resistance. Thus the antenna's efficiency is quite low. One way to increase the antenna efficiency is to make it longer and higher. The wire resistanc:::e increases in proportion to the length, whereas the radiation resistance goes up with the square of the effective height above ground. This should answer the old question "why is a longer antenna better than a short one?" . 54 SILICON CHIP Fig.3: the traditional "textbook" crystal set. The need for tap-changing as the set is tuned makes operation awkward. If we could run a very thick antenna wire straight up to a height of a quarter of a wavelength (75 metres at 1MHz), we would find that the series inductance would exactly cancel out the capacitive reactance. Thus, the only impedance left would be the radiation resistance, about 370. To achieve this ideal performance, this antenna would also require a perfectly conductive ground plane - a copper disc 150 metres in diameter would be near enough! Believe it or not, at a distance of 20km from a typical broadcast station, such an ideal antenna would receive more than 20mW of power. This is enough to provide quite respectable sound level from a loudspeaker! Obviously such an antenna system is not practicable, but one should bear in mind that a crystal set needs the best possible antenna PARTS LIST 1 wooden matchbox 9 metres 0.355mm (27 B&S) enamelled copper wire 5 solder lugs 1 polystyrene or silvered mica (low loss) capacitor; approximately 1 OOpF 1 germanium diode, OA47, OA91 , OA90 or equivalent 4 miniature alligator clips Miscellaneous 1 5 metres of insulated hookup wire for antenna, additional wire for earth lead as required, highimpedance headphones (see panel) . and earth system to function effectively. A microwatt of received power will provide good volume with a sensitive pair of headphones, a nanowatt is perfectly listenable, and a signal of around 50 picowatts is just readable. A good earth For the record, this crystal set will receive stations at listenable volume with less than a metre of antenna. However, it won't work at all without an earth connection, for the simple reason that the antenna current has nowhere to flow. One can get away with a large metal object buried in damp soil, but since dirt is not as conductive as metal the best ground connection would be a large metal grid, laid on or under the ground, and stretching as far as possible in all directions. Domestic water pipes fit this description admirably. Selectivity and coil tappings Sharpness of tuning is always a problem with simple crystal sets the resistive component of the antenna impedance and loading by the diode circuit both tend · to reduce the selectivity of the tuned circuit. One solution to the problem is to provide several taps on the inductor, as in the typical "textbook" crystal set of Fig.3. If we tap the antenna and diode into the coil a few turns from the "earthy" end, the tuning will be very sharp because the loading will be negligible. But the signal coupling will be correspondingly small. Selectivity and "Q" Factor The "Q" or Quality Factor of a tuned circuit is the ratio of its centre frequency to its -3dB band., width (see Fig.4). For example, a tuned circuit with a centre frequency of 1 MHz, and a -3d8 bandwidth of 1 0kHz, has a Q of 100. Normally, a Q of this magnitude is only just achievable with a welldesigned low-loss inductor. The unloaded Q of the Matchbox Crystal Set at 1 MHz was measured at around 95 to 98. Conveniently, Q is also equal to the ratio of the impedance of either the inductor or capacitor (they are equal at resonance) to the resistance causing the energy loss in the tuned circuit. For example, consider again a circuit with a Q of 100 at 1MHz. If the tuning capacitance is 1 00pF, it has a reactance of -j16000 at 1 MHz and the inductor has a reactance of +j1600O (253µH). For a Q of 1 00, the equivalent series resistance of the tuned circuit is 1600/100 = 160. The equivalent parallel resistive load would be 1600 x 100 = 160k0. (Fig.5.) Thus a high-Q tuned circuit must have very little series resistance (in the coil for example), and a very high parallel load resistance. The unloaded Q of the Matchbox Crystal Set is almost entirely determined by the coil resistance which, as we have just calculated, is about 160 at 1 MHz. Compare this with the DC resistance which is only about 1.30! When the crystal set is in use, the Q is considerably lower, because of the combined loading The optimum tapping points can be found by experiment but (and here's the catch) they vary with each frequency setting. For example, at the low frequency end of the band, one might obtain best results with the antenna tapping at 100% (ie, at the top of the coil) and the diode at about 60% of the turns. At the high frequency end of the band, the optimum antenna tapping may be only 10%, and the diode tapping about 25%. BW AMPLITUD OdBl+-----t-~ Q= folBW lo FREQUENCY RESONANT FREQUENCY Fig.4: the "Q" of a tuned circuit is the ratio of its centre frequency to its - 3dB bandwidth. 25311Ht:D OR~ lo = 1MHz Q = 100 Fig.5: equivalent high•Q tuned circuits (Q = 100 at 1MHz). of the aerial resistance and the diode circuit. With a 5k0 headphone load and a 1 5-metre antenna connected to "A2", the Q of the Matchbox Crystal Set was measured at around 50 at 1MHz, giving a -3dB bandwidth of 20kHz. Since the typical minimum separation between local broadcast stations is about 50kHz, this would put the interference from an adjacent carrier at -14dB. Unless the wanted signal is very weak, the major component of the interference will be ultrasonic, and therefore inaudible, so you will find that the Matchbox Crystal Set has adequate selectivity for most applications. There are three reasons why the tappings change so much. First, the RF resistance of the antenna wire increases with the square root of the frequency (due to the "skin effect"). But worse than this, the radiation resistance goes up with the square of the frequency. Thus, the damping effect of the antenna resistance is considerably greater at the top end of the band than it is at the bottom end. Second, to obtain the same selec- tivity at the top end, the tuned circuit needs to have a "Q" three times higher than at the bottom end. Finally, to make matters worse, the impedance of the L and C elements is three times higher at the top end of the band. The result is that we need to reduce the effective antenna and diode loading by more than a factor of nine as we tune up the band. For this reason the fixed inductor/variable capacitor approach is just about the worst arrangement possible. The only reason why this circuit is so popular is that variable capacitors are ·(or were) readily available. Variable inductors, on the other hand, were something you had to make yourself. Variable inductance is best If we want a tuned circuit that maintains constant selectivity with the antenna and diode connected to a fixed tapping point, the first thing we must do is use a variable inductor instead of a variable capacitor. Oddly enough, all of the earliest crystal set designs used variable inductors. These were invariably quite cumbersome affairs, using either a sliding contact arrangement or a pair of coils which slid or rotated inside one another. Nowadays, making a variable inductor is almost too easy, thanks to the availability of low-loss RF ferrites. With a fixed tuning capacitor and a variable inductor, the impedance of the L and C elements is three times lower at the top end of the band. This means that, for a given loading, the tuned circuit Q will automatically be three times higher. In practice, the effect is not quite this good, since we have ignored the resistive losses in the coil itself. Another problem is that the loading is not constant. Remember that the antenna resistance increases with frequency, so the optimum tapping point still needs to be moved, although over a much smaller range. This design solves the problem by electrically "moving" the A2 tapping as the tuning is adjusted. Although this tapping is nominally half-way up the coil, the effective OCTOBER 1988 55 Headphones for the Matchbox Crystal Set Owning a good pair of highimpedance headphones is a must for any crystal set enthusiast. You may get lucky poking around the disposal stores, but most of the headphones you are likely to find will have a fairly low impedance Don't let this worry you too much . If you find a sensitive pair in good condition, then a small audio transformer with the appropriate turns ratio is all you will need to turn them into a first class pair of crystal set headphones. The most sensitive type to look for are those with a "balanced armature" mechanism and a light aluminium diaphragm . Old telephone earpieces of this type are excellent and, even allowing for the losses in the impedance matching transformer, they are just about the most sensitive transducers ever made. Next, in order of decreasing sensitivity, come the even older style of headphones with sheet iron diaphragms. These are still being manufactured in 20000 impedance, but the sensitivity is not as good as it was in the old days. Next come the older style of "modern" headphones, the ones coupling varies depending on how far the ferrite rods are inserted into the top of the coil. When the rods are only partially inserted, the inductance of the top half of the coil is greater than the inductance of the bottom half. Also, not all of the magnetic flux from the bottom half of the coil passes through both halves - some of it leaks out the sides of the matchbox. Thus, the effective tapping is less than half-way up. When the ferrite rods are fully inserted, all of the flux flows through both halves of the coil and the effective tapping point is exactly half-way. So our "fixed" antenna tapping actually "moves" whilst we are tuning from station to station. There is an obvious advantage of this tuning system - it's dead easy to use. As a bonus, the Matchbox 56 SILICON CHIP with small 80 loudspeakers in them, and very large earpads. A good scheme is to connect both 80 drivers in series (get the phasing right) and use a 1 k0:80 transistor radio type audio transformer (eg , the M-0216 from Dick Smith Electronics). You may lose a little bass response, but you will have a pair of 20000 headphones. Note that the DC resistance looking into the matching transformer will be a lot less than 20000, so the transformer looks like a short circuit to the DC voltage from the rec'titied carrier. Actually, it is not quite this bad because the diode still has a few thousand ohms of forward resistance. Nevertheless, it is worth taking some steps to avoid excessive damping of the tuned circuit. Fig.6 shows how a series resistor can be added to bring the DC resistance up to 2k0. A bypass capacitor prevents any attenuation of the audio signal. This technique can also be used to make a pair of medium impedance phones from a couple of old telephone earpieces. These Crystal Set automatically maintains a constant bandwidth and volume over its entire tuning range. There is a second and not-soobvious bonus: we can accurately mark a tuning scale on the sliding part of the matchbox. This is normally not possible with an "ordinary" crystal set, since a slightly different tuning scale is required for each antenna tapping - hardly worth the bother! Actually, the Matchbox Crystal Set has two tuning scales, one for the top end of the band (with the antenna connected to AZ), and one for the bottom end (with the antenna connected to Al). As you can see from the photographs, both scales are almost linear, and all of the Sydney stations are spread out quite nicely across each range. The same will apply for other locations. " 20000" 1k·BO AUDIO HEADPHONES TRANSFORMER HEADPHONE DRIVERS 80 Fig.6: this diagram shows how to convert a pair of low impedance headphones into 20000 highimpedance headphones. All you need is a small audio tran~former, a resistor and a capacitor. 0.22·0.47 BALANCED ARMATURE TELEPHONE EARPIECES 200 (0C) Fig.7: this circuit will give good results with 200 telephone earpieces. have an AC impedance of a few hundred ohms each, but a DC resistance of only 200. Connected as shown in Fig. 7, they perform surprisingly well, although with 'a proper matching transformer Once your crystal set has been calibrated, the tuning scales will always be spot on, so long as you maintain a fixed antenna length. Diode circuit For the sake of simplicity, the AZ tapping is used for the diode as well as the antenna. Loads of between Zk0 and 10k0 are about right for this tapping. If you intend to use a very high impedance load, such as a pair of piezoelectric "crystal" earpieces, then connect the diode to the top of the tuned circuit (ie, to Al). The load resistance with crystal earpieces will be somewhere between 100k0 and lM0. Headphone impedance At this point we should explain why the headphone impedance required for crystal sets is always DIODE IN~&T PIEZOELECTRIC TRANSDUCERS 4.7k-10k Fig.8: a 4.7kll-10kll resistor should be added in parallel with piezoelectric transducers to prevent distortion due to insufficient shunt resistance. the results are phenomenal. If you don't want to go to all this trouble, try a pair of piezoelectric transducers. Murata make some very sensitive piezo inserts which are easy to fit into any old-style headset. These are available (along with lots of other goodies) from Orpheus Radio, RSD 898, Ballarat 3352. Telephone (053) 34 2513. Piezoelectric transducers have the opposite problem to electromagnetic transducers - they have an infinite DC resistance, and a fairly low AC impedance. Electrically, a typical pair of piezoelectric transducers in parallel looks like a 0. 1µF capacitor. If you connect them to the diode without a load resistor, all you will hear is a faint, distorted crackling. very high, compared with the impedance of modern high-fidelity headphones (usually 80 to 320). Why can't we just move the diode tapping way down towards the bottom of the coil and use low impedance headphones? The answer is we could, if we could find a diode with a resistance of about 100 at a forward voltage drop of say lOmV. The best all-round diode for crystal set use is the gold-bonded germanium type OA47. At lOmV applied voltage, a typical device exhibited a forward static resistance of 26k0 and a reverse resistance of 3 lk0. So, for signal voltages of ± l0mV, no effective rectification takes place. When the voltage was increased to lO0mV, the forward resistance dropped to 7k0 and the reverse resistance increased to 73k0. Rectification now takes place but, for Fig.8 shows the demodulated signal envelope as delivered by the diode into a load which has too much capacitance and not enough shunt resistance. Severe audio distortion results because the capacitor can't discharge rapidly enough to follow the audio frequency variations. Adding a 4. 7k0 to 1 0kO resistor in parallel with the transducers will fix this problem. Last, but not least, you might like to try a pair of "crystal" earpieces. These use tiny crystals of a naturally-occurring piezoelectric salt and their impedance is very high. A pair connected in parallel will only need a shunt resistance of between 1 0OkO and 1 MO for best audio quality. Often, the reverse leakage of the germanium diode alone will be sufficient. reasonable efficiency, a load in excess of 10kO is necessary to prevent undue loading. There are two possibilities for improved rectification with low impedance loads. One is the so-called "backward diode", which is really a zener diode with a reverse breakdown voltage of 0V. These are normally used only in microwave detectors and are hard to come by. Another approach is to use a battery and potentiometer to bias a silicon diode into forward conduction. By adjusting the bias, you can select the best compromise between forward and reverse resistance for a given load. This approach was often used in the past, especially when using a "carborundum detector". This was a diode made from a crystal of silicon carbide with a sharpened steel point sticking into it. It needed This old telephone earpiece was converted to high impedance operation using the circuit shown in Fig.6. The parts all fit in the plastic housing. a forward bias of about 1V for best results. However, high impedance headphones and an ordinary germanium diode are still the most convenient arrangement. If you want to, you can make a suitable pair of phones using currently available parts (see panel). What, no .00tuF capacitor? Conspicuous by its absence from the circuit diagram is the usual .00lµF capacitor across the headphones. This is included in all the "textbook" circuits to provide RF filtering of the audio output, but its presence is usually undesirable. The impedance looking into a rectifier with a capacitive filter is close to half that of the load resistance, so with our headphones already lower than the optimum load resistance, this capacitor only makes things worse. By contrast, an inductive filter can increase the effective load resistance to slightly more than the headphone resistance. A lmH or 2.5mH RF choke in series with the diode may well improve the performance with low impedance headphones (eg, lkO or even 6000) - at least in theory. In practice, the choke can be left out as there will be sufficient inductance in the headphone windings or in the matching transformer. Construction Let's begin construction by locating a suitable matchbox. This OCT0BER1988 57 glue is incredibly tough, and it is the only one which can take soldering temperatures without embarrassment. Start with the three lugs on the "antenna" side of the matchbox. Spread an ultra-thin smear of Urethane Bond on the matchbox, and a little more on the solder lugs. Position one lug at each end, and one exactly in the middle. Leave the box standing on its side for several hours until the glue hardens. Next, turn the box over and glue the two headphone lugs exactly opposite the "A2" and "E" lugs, checking the location with a ruler. Stand the box between two heavy objects to keep it level until the glue sets. The tuning scale was carefully hand-drawn on the inner part of the matchbox. Note the diode running across the centre of the matchbox and the polystryrene capacitor between the "Al" and "E" terminals. L1,L2 = 37 TURNS, 0.355mm DIA. ENAMELLED COPPER WIRE, CLOSE WOUND. 100pF TUNING CAPACITOR FERRITE RODS GLUED TO INSIDE OF MATCHBOX TRAY f \ V ' SOLDER LUGS GLUED TO MATCHBOX COVER WITH URETHANE ADHESIVE. Fig.9: here's how to build your matchbox crystal.set. Each of the coils consists of 37 turns of 0.355mm guage wire, but other wire gauges can also be used (see text). is not so easy as it sounds because wooden matchboxes are scarce these days, and the cardboard ones are a little too flimsy to wind a coil on. In any case, you will need to make up a wooden "dolly" to prevent the box from being crushed during the winding. Carefully plane a block of wood so that it just fits tightly inside, then glue a short piece of broom handle to it and put it aside. Next, give the insides and outsides of both halves of the matchbox a coat of Satin Estapol or similar clear polyurethane finish. This strengthens the assembly considerably. Allow it to dry overnight, then lightly sand all over with 600 58 SILICON CHIP grade paper to smooth any furry bits. The top coat of Estapol will be added later, when the coil windings, etc are in place. Meanwhile, prepare the five solder lugs by firstly tinning them - this will minimise the amount of soldering heat needed later on. Once tinned, thoroughly scrub away all traces of the flux resin with a tissue and methylated spirits, then bend the lugs through 90° as shown in the photographs. Glueing the lugs The best glue for this job is Dow Corning "Urethane Bond" (formerly sold under the Selleys label). This Winding the coil The recommended wire gauge is 0.355mm (27 B&S) but don't worry if you don't have the exact size good performance can be obtained with any gauge from 0.315mm to 0.5mm (28 B&S to 25 B&S). Of course, the number of turns that will fit in the available space will vary, but the appropriate choice of tuning capacitor will compensate for any variation in inductance. For example, on one prototype, wound with 0.355mm wire, two coils of 37 turns each just fitted between the lugs. This crystal set required a tuning capacitor of l00pF. A second prototype was wound with 0.5mm wire. The two coils ended up having only 23 turns each, and a 270pF tuning capacitor was required. Performance wise, there was little difference between the two windings, although the second set had a slightly restricted tuning range, and it preferred a lower headphone impedance. When it comes to winding the coil, you can use one of two methods. Either wind each half of the coil separately, terminating the wire on the solder lugs as you go, or wind the whole lot in one go, with a few extra turns across the central gap which can be snipped out later. Keep a heavy tension on the wire all the time, and don't let go or you will find yourself having to start all over again. In fact, it's a good idea to have on hand several short pieces of masking tape, for holding flanges, and soldered the end of the antenna wire to it. A band of flat elastic keeps the wire in place when it is wound up. For the earth lead, we suggest that you use a few of metres of wire with a miniature alligator clip at one end, and a large jumper lead clamp at the other end, which can fit around the kitchen tap, or a water pipe. Initial testing We made up an antenna reel using a tin can with a couple of large "Milo" lids as flanges. The headphones were put together from a pair of discarded telephone headsets and a small audio transformer. the wire in place should you need to stop for any reason. It isn't necessary to count the turns, just fill up the available space between the lugs with a neat, close-wound layer of wire. When the time comes to solder the wire to the lugs~ you can temporarily anchor the turns in place with more masking tape, or secure the windings with "Super Glue" or a tiny smear of "Araldite". Thoroughly scrape and tin the ends of the wire, and solder them to the lugs with a minimum of heat. Mounting the components The diode runs from the middle "A2" lug, across the front of the matchbox, to the headphone lug on the other side. Bend the leads neatly to follow the shape of the box. The tuning capacitor runs down the side of the matchbox, between the "Al" and "E" lugs, but its exact value needs to be determined experimentally, so do not solder it in place at this stage. The only other components are the two pieces of ferrite rod, but first they need to be cut to length. Ferrite is quite hard and brittle. About the only way to cut it is to file a groove right around the rod and then break it by hand. Trim the rod if necessary by rubbing it on a sheet of silicon carbide paper (wet or dry), using kerosene as a lubricant. Check that both pieces fit snugly lengthwise down each side of the "drawer" part of the matchbox. If everything is correct, give them a wipe over with methylated spirits, then glue them in place with 5-minute AL'aldite. By the way, any type of round or flat ferrite rod will do, providing it is "antenna" grade ferrite. Accessories The crystal set is now ready for testing but first we need some accessories to complete the setup. Number one on the list is a suitable pair of headphones, and we have prepared a panel describing a number of practical alternatives. Fit a pair of miniature insulated alligator clips to the headphone leads. Next; measure out 15 metres of medium or heavy duty hookup wire for the antenna, and fit a miniature alligator clip to one end. You may wish to secure the other end to some sort of spool, so that you can wind the antenna up without getting it thoroughly tangled. We made a spool from a tin can with two large "Milo" lids as Now for the exciting part. Connect up the antenna, earth and headphones, and temporarily attach a 100pF capacitor to the "Al" and "E" terminals with a pair of short clip leads. Check the coverage of the top end of the band with the antenna connected to the "A2" tapping, then the bottom end with the antenna connected to "A 1 ". If you are missing some stations off the top end of the band, you will need to reduce the tuning capacitor, and vice-versa. A small assortment of capacitors will provide quite a variety of values if you Handy Hints Hint #1: nail polish is a handy item in your workshop. It will secure a pointer to a dial cord, prevent a knot in a dial cord from coming undone, or can be used to lock screws in place. Nail polish can also be used to anchor the terminations on small coils and to permanently set trimmer capacitors and potentiometers. Hint #2: to hold small components while you solder them together, use a large spring clip (normally used for holding papers together). The spring clip can either be screwed to your work bench or to a solid block of wood to stop it. from moving around. Hint #3: having trouble getting a replacement tip for your Tandy, Dick Smith or Scope temperature-controlled iron? Many of the tips for these brands are interchangeable. Take your old tip with you to compare it with the new one to make sure it matches. OCT0BER1988 59 This view shows how the two ferrite rods are glued to the inside of the matchbox tray. Any type of round or flat ferrite rod can be used, providing it is "antenna" grade ferrite . connect them in series and parallel combinations. Once the required capacitance has been determined, it can be made permanent, but make sure you use a low-loss capacitor. A loss factor of less than .001 % at 1MHz is desirable. The best capacitors are polystyrene, silvered mica, or NPO ceramic types specifically designed for RF work. Finishing touches The ruggedness of your Matchbox Crystal Set can be improved by giving the lower parts of the solder lugs, and the area around them, a thin coating of epoxy. Use slow-setting Araldite, and spread it thinly and evenly with a pointy instrument. It will flow out smoothly and set crystal clear if you warm it up a little while it is setting. A warm window sill in the afternoon sun is ideal. Put a little of the epoxy oh the diode and capacitor as well but don't bother to coat the coil windings - these are best held in place with a final coat of Satin Estapol. Before you get out the Estapol, stick some small pieces of sticky label material on the matchbox and label the terminals "A 1" , "A2", "E", " Phones" and "Phones" with your neatest writing. Don't use a spirit-based felt tip pen - use drawing ink or similar water-based 60 SILICON CHIP The matchbox crystal set is tuned by sliding the tray up or down to change the inductance of the coils. A good earth connection is vital for correct operation. medium. Also, now would be a good time to draw a tuning scale on the sliding part of the matchbox, so that it will be protected under the final coat of Estapol. Calibration As you can see from the photographs, we managed quite a comprehensive tuning scale. This was hand-drawn over the first coat of Satin Estapol with Rotring drawing ink and a fine pen. This surface takes ink beautifully if it is pretreated to render it slightly hydrophilic (ie, to make it attract water). Just moisten a tissue with saliva (yes, that's right), rub it all over the Estapol, and then wipe it dry. If the cardboard is darkly coloured, you can stick some white label material over it, or use one of the decorative white or silver inks available at most artists' supply shops. Next, consult a list of AM broadcast stations (in any recent DSE catalog for example) and make a short list of all the stations in your area and their frequencies. Log as many of the stations as you can, drawing a light pencil line against the edge of the matchbox for each one. Draw a base line on the scale and measure the positions of all the station lines relative to it. Next, plot a graph of these measured distances against the listed carrier frequency of each station. Carefully join the points with a smooth curve. You can now read off the position of any intermediate frequencies (eg. every 50kHz or lO0kHz), as well as the exact position of any stations which happen to be too weak to be received from your location. Transfer all this information with ruler and pencil onto the sliding part of the matchbox, and ink it in. If you make a mistake, it can be cleaned off with a moistened cotton bud. Be careful of smudges - the ink takes a long time to dry on this surface. Finally, give both halves of your crystal set a final coat of Estapol all over, keeping clear of the exposed parts of the solder lugs of course. Operational hints Although the circuit should be tuned by moving the ferrite rods in and out of the top of the matchbox, you can also tune in the same stations by moving them in and out at the bottom. The difference here is that the lower half of the coil will have a greater inductance, effectively moving up the "A2" tapping. This will give you more volume, but less selectivity - the stronger stations will be noticeably louder but some of the weaker ones will be lost altogether. Use whichever setting suits you best. ~