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OLD-'I'I M f. CRYSTAL DXing with a crystal set can be a lot of fun. This old-time crystal radio comes as a kit and delivers a performance that will surprise you. By JOHN HILL The story of this old-time crystal radio begins in the Victorian city of Ballarat. Like many towns these days, Ballarat is chasing its share of the tourist dollar and offers many first-class attractions for the tourist to see. The latest addition to Ballarat's sights is something that should be of interest to SILICON CHIP readers, especially those old enough to remember the early days of radio. Ballarat now boasts the Orpheus Radio Museum which is full of 56 SILICON CHIP fascinating electronic relics from the past. However, the premises are not used solely as a museum. In a separate factory area at the rear, museum owner Richard Wilson has a workforce of approximately 20 employees producing computer equipment and other hi-tech electronic gear under sub-contract, as well as their own products marketed under the "Atron" tradename. An unusual aspect of the 10 business is the production of vintage radio kits which are sold through the museum and by mail order. These kits include a 1-valve receiver, a 2-valve receiver and a "Super Crystal Set". Both of the valve radios are battery-operated "reaction" types. The crystal set The Super Crystal Set is of particular interest and is based on an early Navy circuit which is most efficient. I have built this set and its performance is so good I had difficulty in believing the station call signs I heard. Any crystal set that can separate about 15 stations and pull in interstate transmissions at listenable volume is a mighty fine design. I might add that this remarkable performance was possible even with a local 5kW station only 6km away. Without that local station, the t::o------c ,11 I ~ E .__---~· # START ) S1 CJ HIGH Z PHONES I T EARTH SUPER CRYSTAl SET Fig.1: unlike other crystal sets, this design has two separate tuning circuits with variable inductive coupling (via L2). Result - greatly improved station selectivity. ◄ The Super Crystal Set control panel is well laid out. Black bakelite , brass fittings and gold lettering give the receiver an authentic vintage radio appearance. Super Crytal Set would perform even better. Personally, I relate rather well to crystal sets, for it wa s these simple receivers that fostered my interest in radio some 40 years ago. I built them in all shapes and sizes, including some in matchbo xes. However, I was always restricted to one station listening. As I lived in Bendigo at the time, 3BO was all I ever heard on any of my crystal sets. My crystal sets were set up in my bedroom and I would often go to bed early and lay in the dark with the headphones on and listen for hours. I did a lot of listening to crystal sets simply because they were all I could afford at the time. Yes, I have very fond memories of my home made crystal receivers. Getting back to the Super Crystal Set again: it is available in kit fo rm and is very well presented. The kit is complete and comes with a bra ss stud switch, a headphone jack, and every nut, bolt and washer - in short, the lot! The front panel and baseboard are pre-drilled so tha t the set goes together with a minimum of fuss. This end of the set tunes the radio frequencies before the signal is fed to the detector. The small tuning capacitor at right is the author's modification Spiderweb coils Construction of the Super Crystal Set is quite intricate and it takes a good many hours to build. It requires the winding of thr ee The detector end of the set is fairly conventional. The small white spider web coil is the "swinging" coupler that varies the coupling between the two sections of the receiver. M ARCH 1988 5:7 Crystal sets require high impedance headphones hut you can also use low impedance phones provided you use a matching transformer. Shown is an old STC headset. This has an impedance of 2000 ohms and is guaranteed to put callouses on your ears within one hour. "spiderweb" coils, two of which are tapped. One coil is of the "swinging'' variety and is used to vary the coupling between the two stages of the receiver. These two separate stages in the receiver require further explanation. Most crystal sets have a single tapped coil and a tuning capacitor to select the stations. Such a set-up usually gives very broad tuning and if there are a number of strong local stations, as is the case in capital cities, then the set may not be selective enough to separate each signal without interference. The Super Crystal Set is not like this. It has two separate tuning circuits that are connected by a variable inductive coupling which also helps to make the receiver very selective. In other words, the set has a tuned radio frequency (RF) stage before the detector, with the two stages inductively coupled by a high-frequency transformer (the swinging spiderweb coil). Such a circuit design has a dramatic effect on selectivity, with very little loss in volume. Tuning the Super Crystal Set is a two-handed job since each circuit must be tuned separately (no fancy ganged capacitors back in the good ol' days)! The RF circuit is tuned with a brass stud switch, while the detector circuit is tuned with a variable capacitor. The brass stud switch is connected to the aerial coil which is tapped every eight turns. This eight turn tapping set-up proved to be interesting in the set that I built. Some stations were received at equal volume on two adjoining studs, indicating that the true resonance point of the coil was midway between the studs. I reasoned that a bit of fine tuning could perhaps make this excellent receiver even better. To eliminate this slight error, a small variable capacitor was added to the radio frequency circuit in order to smooth out the courseness of the 8-turn tappings. The addition of this capacitor was so successful it almost doubled the number of stations received. Note: this small capacitor is not included in the kit and w_a s strictly my own experimental modification. If you wish, you can do exactly the same. The variable capacitor is shown by the broken line on the circuit diagram (Fig .1). The general appearance of the Super Crystal Set is most pleasing to the eye. The kit has been designed in keeping with the early radio scene and all fittings are made of brass and black Bakelite. The assembled components are mounted on a stained and polished baseboard. Gold lettering on the front panel adds the finishing touch to a well-presented product. Lacquering of the baseboard is the or.ly preparation required by the constructor before assembly. No cat's whisker Anyone familiar with crystal sets will recall how tedious it was to This back view shows the complexity of this particular crystal set. Quite a few hours are required to assemble the kit. 58 SILICON CHIP find a "good spot" on the crystal and how easy it was to bump the cat's whisker off that good spot once it was found. The Super Crystal Set solves that problem by using a fixed detector which takes the form of a good-quality ger- manium diode. Mounting the diode inside a fibre tube makes it look a little more authentic. Actually, the gold-bonded diode is the secret to the set's success. If a real crystal detector were used, it would reduce the receiver's sensitivity. One novel aspect of a crystal set is the fact that it costs absolutely nothing to run, as it utilises the radio frequency energy that's picked up by aerial. Of course, the aerial must be long enough to collect sufficient energy to operate the set. A suitable aerial should be in the vicinity of 30 metres long and as high as it can be conveniently strung. An earth connection is also a must for good crystal set reception. My aerial is a single strand wire that is approximately 25 metres long and six metres high. Such an aerial is only average in crystal set terms, but the reception I obtain is the best I've ever experienced. On a good night I can pick up two Adelaide stations (5AN and 5CL), two Melbourne stations (310 and 3AR), one Sydney station (2BL), and one Queensland station (4QD). The latter is about 1,500km, as the crow flies, from my home in Maryborough, Central Victoria. If anyone had told me that a crystal set could do that prior to my building the Super Crystal Set, I wouldn't have believed them. The Super Crystal Set is well named it really does give super results! Headphones The sound reproduction from the set is also surprisingly good and the tonal quality quite acceptable through either my Brown or STC headphones. Although phones such as these are designed for maximum sensitivity rather than hifi reproduction, they aren't too bad to listen to . Crystal set headphones need to be of high impedance - around 2000 ohms. If high-impedance phones are not available, low impedance phones can be matched up to a crystal set by using an old speaker transformer. These gives the advantage of more comfortable listen- ing with better tone, but a slight drop in volume is apparent. A small speaker transformer, complete with box, plug and socket is available as an optional extra if required. Those who wish to build the Super Crystal Set can do so regardless of what type of headphones they have. In these days of computerised hitech everything, it makes a pleasant change to build a good oldfashioned crystal set. There's something about going back to basics that's hard to explain. The cost of this particular piece of nostalgia is $89.50 for the kit and $5.00 for packaging and postage. Yes, I know you could buy quite a good transistor radio for that amount, but I doubt if you would have as much fun with it. Believe me, DXing with a good crystal set is pretty exciting stuff! Footnote: the Super Crystal Set is available from Ballarat Electronic Supplies, 5 Ripen St, Ballarat, Vic 3350. Phone (053) 31 1947 . Build This Old-T1Dle Crystal Set Suppliers of * R A D · I O ~ vintage wireless kits and wireless parts. RSD B98 Ballarat, 3352. Ph. (053) 34 2513 Send for a free catalogue today! WHEN NEXT IN BALLARAT DON'T MISS THE ORPHEUS RADIO MUSEUM CNR. RING RD. & WESTERN HWY. BALLARAT OPEN 7 DAYS A WEEK 10am - 5pm MARCH 1988 59