The "Colvern" Centre-tapped Crystal Receiver

Radio Record, Volume II, Issue 1, 20 July 1928, Page 12

The "Colvern" Centre-tapped Crystal Receiver

A Set That Will Suit Everyone

(By

Megohm

HH ever-popylar crystal is always acceptable to a large section of the listening public, and ; ‘e even valve-set owners [Witenes find that they can get 2 pleasant change by listening-in with a erystal whilst the local station is on the sir. The owner of a not very effiecient valve set, listening to erystal reproduction, is struck by the fine quality generally, and the faithful reproduction of bass notes, however low in the seale-many notes that are suppressed in the average valve set. This sef can be made in a small and handy size not exceeding 6 by 6 by 7 inches, and may be used with a minimum of trouble if a carborundum permanent erystal is employed as the rectifier of signals. Tor distant reception some constructors will prefer ty use a Cut’s-whisker-galena detector. Tn) weh a case the detector should be plaeed outside the front panel for ease of maniilation. . AERIAL DISPENSED (WITH. oe or near the main centres, listeners with electric light in the house may use the lighting mains instead of an aerial of any kind. This is done by purchasing a "Ducon" or "Anten- ella" connection made for the purpose. This is quite a small piece of apparatus that may be cbtained at the dealers, and no objection to its use is made by the authorities. It must be clearly understood that no electric current whatever is consumed, as the construction will not let any pass through. Bat by means of suitable condensers there is a path for the high-frequency aerial eurrent picked up by the overhead lighting wires. A two-way lampholder can be purchased for about 5s. 6d., enabling the attachment to be used without interfering with the use of the lamp. One terminnl of the Ducon connects to the aerial terminal of set, and the other to the earth terminal, 2 piece of twin plex being used, though in the case of weak signals au improvement might be made by untwisting the two wires to allow them to separate. The Ducon is tried both ways round in the socket to determine which position gives the best reception. ‘An advantage of this system is that the receiver can be used in any toem with a minimum of trouble, a convenience for. those- who retire early and like listening-in whilst in bed. The receiver may of course be attached to On aerial and earth in the usual way. NEAT appearance is. given to the coil by making it upon one of the handy "Colyern". formers that can be purchased. These coils, obtainable in two sizes, are

‘more often used in valve sets, but serve well for this receiver as shown, though any other kind of former may be used. The small size of Colvern former is 2}in. in diameter, and gives a winding space of 2} inches. Upon this are wound 52 turns of 22's s.w.g: double cotton-covered wire, and when the 26th turn is reached a tap is sol: dered on, passed through a hole drilled in the former, twisted round pin No. 2, and soldered. In the viéw of wiring, a hole is drilled at K, the end of the wire passed through from outside, down through hole in lower end, and solCored to pin No. 4 after cleaning the

latter with a small piece of glass paper. | Winding is then proceeded with as above, pulling the wire tight, and pushing the turns close together as you proceed, otherwise there will not be room for the full number. The last turn is secured. by drilling a hole in former, passing the end through, twisting round pin Ne. 1, and soldering securely. Be

eareful that the soldered joints az good, and not an insulated joint in which the wire is only held by rosin, Te suit the varied purposes for which these coil formers can be used, six pins are provided, but only three ‘will be used, Nos. 1, 2, and 4. A coil base into which the B coil plugs may be used for. the. sake of neatness, or the wires from the coil may be left sufficiently long to continue to their respec- ° eonnections, 4 to earth, *phones and moving (M) plates of condenser, 1 to nerial terminal and fixed (¥) plates of condenser, and 2 to crystal. If the buse is used, the tliree connections are soldered to the numbered tags as indicated. The base is secured to the baseboard by two brass serew an inch long, and not thicker than No. > (4 and EH). PANEL AND BASEBOARD. THE panel may be of 8-16 ebonite, and if it is desired to make the "ase as small as possible, should be c.termined as to size after purchasing the condenser and sin, dial. The dial should be centrally placed on the panel in relation to the sides, and as most conflensers oceupy more space on one side than the other, this should be taken into account. Measure condenser space with the moying plates in an uprigut position to give the height of space for free working. When purchasing the condenser be careful to note that the dial fits the spindle, and that the figures read the correct way, clock-wise or antic-clock. The totul height of coil and base is 3} inches, The baseboard should be made of 3-8 in. rimu, Strengthened by a couple of lin. strips of the saiune material screwed underneath as shown. The back strip is made Jin. short at each end, to allow fur the base sliding into its cabinet on strips secured to the sides. Three-vly makes a suitable cover, size according to dimensions of panel and baseboard. Shellue dissolved in methylated spirits and rubbed on with a cotton rag gives a good finish to the woodwork with very little labour, Which is well repaid by the appearance, WIRING TP HIN wiring is clearly shown in the view of wiring. The 22's wire may be used. ‘The leads running to respective sockets on coil base are numbered accordingly. The wire from No, 1 goes round fixed plate terminal of condenser, and without cutting, continnes to aerial terminal, The wire froni No. 4 goes similarly without cutting te "phone, earth, terminal, «and

moving plate terminal of condenser. Other end of erystal connects to remaining ‘phone terminal. Carefully clean wires at all points af connection with a small piece of glasspaper. Resin-cored solder should be used for soldering coil connections, ‘The cheap‘est soldering outfit for small work is a mouth blow-pipe, which can be purchased for about is., and the lid of a metal (Colgate) shaving-soap container. A piece of stiff wire can be put round this and the two ends twisted together to form a handle. Place a tuft of cotton-wool in the lid, soak with methylated spirits, and ignite. With the blowpipe in the mouth, the tiame is directed upon the soldered

joint. IZ working ou ebonite or bakelite, it must be protected fram the flame by a piece of cardboard with a hole cut in through which the metal ; tag may be soldered to. | If desired, extra neatness may be , secured by drilling holes in the baseboard in suitable positions, and running the connecting wires underneath. In that case the wire from No. 1 would run to aerial terminal first and- con-

tinue to condenser, The ’phone end of crystal would be placed close to the ’phone terminal. The usual connection for a crystal receiver is shown in the theoretical diagram by the dotted lines, the erystal being connected to the top of the coil at B instead of the centre at A. In ease of using the large size former, fewer turns of wire would be required, say 45, or the same turns and a .0005 tuning condenser. The diameter of the large former is 2}in. AS A WAYE TRAP. THIS receiver may be used as a wayve-trup on a valve set by connecting serial t A and E to aerial terminal of valve set. By means of a condenser the unwanted station is then tuned out. Great selectivity will be added to the receiver by its use. No connection is then required to the telephone terminalis, but if the crystal is short-circuited by a piece of wire, the right-hand ‘phone terminal then provides an alternative connection for the aerial, which might suit some locations better. Any constructor. desiring to listen-in to morse transmissions on 600 metres could do so by winding a separate former of the large size with 80 turns of 26’s s.w.g. enamelled wire, tapping at the fortieth turn. The three connections to be made to the same numbered pins. All that is then required to change from broadcast to 600 metres would be to simply change the coils, as both sizes fit the same base.

eS many localities for some reason it is easier to receive wireless signals over longer distances north and south than when the direction is east and west, Pus Hungarians are coutemplating the erection of a station at Budapest with a power far greater than that of any of the British stations. [x the ordinary plug-in coil, the inner end of the winding is connected to the plug of the coil, and its outer end to the socket.

Construction Continued

Variable condenser, .005 1 8 Dial, 3in. Colvern former and base Carborundu fixed crystal 8 Ebonite.ganed c (sTze 1 4 terminals (2 'phone) 6 Baseboard, screws; etc; 3 35_

PARTS REQUIRED.