The "R. R." Selective Crystal Receiver

Radio Record, Volume I, Issue 25, 6 January 1928, Page 16

The "R. R." Selective Crystal Receiver

No Difficulties in Construction and Makes

a Good Wave Trap

By

MEGOHM

_ Here ic a first-class, easily constructed, selective crystal set. The "Record" crystal receiver, which was not, of course, designed for particular selectivity, can be easily converted by making a few slight alterations. Those who live within crystal range of two stations, and thie to cut out one of them will be able to do so with this set. Uys

This set makes a good wave trap for. valve sets. Next week ‘Megohm" will give an article showing how this set can be used as an efficient wave trap. Owners of valve sets know how essential a wave trap is to . satisfactory reception, and the details, which will be given next week, will make its construction a comparatively simple matter.

This crystal set has been designed to add high selectivity to the efficiency of the "Record" receiver, which was not produced with any idea of particular selectivity. Users of that circuit will be familiar with the way in which the condenser may be rotated through perhaps twenty or more degrees either way without "losing" the station. Those who convert their "Record" to this hook-up by the necessary small additions and alterations will be surprised at the different action of the circuit. At its best selectivity, two miles from 2YA, the writer gets, if anything, more foudspeaker volume than from the "Record," but here is the differenceturn the dial one degree either way, and ZYA is only just audible; another degree and it has disappeared, even from *phones. This. result is attained by jntroducing auto or semi-aperiodic coupling into the circuit. Many readers who live within crystal range of two stations and, have hitherto been unable to cut out the mearest one, will be able to do so with this set, whilst interference from morse on certain wavelengths should be materially reduced. The plug-in method of ‘varying the aerial coupling and detector connection will enable listeners to find which combination best suits their conditions and locality. Some — will find probably that one station will come in best with a certain combination and another with a different one, Within a few miles of a main station, good construction and a good aerial will give weak loudspeaker strength, sufficient for listening-in quite comfortably near to the speaker, ADAPTING THE "RECORD" SET. Those who have already made the "Record" set according to specifications only required to purchase six-panel or valve sockets and three pins to fit them, and also procure about half a yard of single flex’ble insulated wire. Then a [ttle work has to be done in the way of altering connections and putting four taps on the tuning-coil. The lower end of coil, instead of connecting to FE terminal, connects to socket 6, left-hand ’phone clip, and moving plates of condenser. The aerial and earth terminals are now only connected to their respective flexes. The tcp of the coil is now connected only to socket 1 and fixed plates of con. denser. The back end of the crystal is only connected to a flex. Check all connections so that they are just as shown in the diagrams. THE PANEL. A piece of 8-16 ebonite six inches by seven inches is required for the panel, which is to be drilled according to the diagram. As the exact type of variable condenser to be used is not known, care must be taken before drill- | ing the panel to see that room will be allowed for the condenser plates to totate withont catching or passing too near to the detector on one side, or the tap wires on the other. The detector

may be placed as much in the corner as necessary. THE VARIABLE CONDENSER, The ‘capacity of the variable condenser is .0005 mfd., for which a neat knob and 3in. dial may be procured. A vernier dial is a good refinement, but not altogether necessary. THE BASEBOARD AND COVER. Three-eighths of an inch will be a suitable thickness for the baseboard,

which will be the exact width of the panel. ‘The depth from back to front ‘may be seven or eight inches dependmg upon how much space the condenser requires. The coil may be placed slightly towards the side where the sockets are, but should not be much nearer to the cover than one inch A batten is screwed under the baseboard at both back and front. ‘The front one

projects forward the thickness of the panel, as shown on inside view of the set. The back button is sufficiently short at each end to allow room for the guide strips inside the cover The cover consists of three sides and atop. On the inside at the bottom of each side two strips jin. square are screwed, so that the baseboard may slide in between them. The back might be made of din. rimu, and the top and sides of three-ply, and finished with shellac and methylated spirits applied and polished with a rag, but individual tastes may please themselves in the exact construction of the cover. THE TUNING COIL. Only a low-loss coil is worth while. No. 20’s s.w.g. enamelled wire is to be used, and 54 turns should be put on Where: 4YA (Dunedin), 463 metres, is to be received, 58 or 60 turns will prob. | ably be necessary. A cardboard cylindrical former 8ins in diameter and’5 or 6 inches long isrequired on which to construct the coil, the former being afterwards removed, leaving the turns of wire supported by only the four double strips of celluloid. The celluloid used for the strips is that used for motor-hood lights, the thicker the better. If scratched with a sharp point it breaks clean where the scratch is made. Now cover the former with a sheet of good paner, fastening bv sticking the overlapping edge, but do not let it stick to the former anywhere. This is to prevent the coil sticking fast to the former. Cut four strips of celluloid, gin. wide, and length of the former, Lay the strips along the former at equal intervals, amd fasten in place with twine bv tving round the end of the former and the ends of the strips, this being done at both ends A ball of twine the same thickness as the wire or a shade less is required. ‘The twine atid wire are fastened to a hole in one end of the former, and winding is proceeded with, the wire and twine heing wound side-by-side tightly until the required number of turns has been put on. ‘The end of the wire, leaving a foot over for connecting, is then fas- | tened down temporarily. The is now carefully nnwound from between the turns of wire. The wire is then fastened to the celluloid strips by a liberal application of cement made bv dissolving chips of celluloid in liquid sectone, obtainable at the chemists. Another strip of celluloid is then laid over the cement and the coil put awav until next day to set. The former is removed hv taking a sharp knife and cutting through the inside along one of the celluloid strips, when the two edges can be curved inwards, releasing the cardboard. The coil may be fastened to the base by drilling the end of two opposite strips and screwing to the two ends of a strip of wood, which is screwed to the baseboard. ‘The direction of the winding of the wire on the coil does not affect the working of the set. If the condenser has to be full in to give best tuning, then a few more turns must be added to the coil. With the auto-coupled coil, the length of aerial has tiot much effect upon the tuning.

The coil is to be placed not to touch or be too near to either ‘the case or the condenser. MAKING THE TAPS. Tour taps not less than 5 inches long have to be attached to the coil, reckoning from the bottom, at the LUth, 18th, 28th, and 40th turn. ‘here are two ways of making these taps, one by taking some very thin copper sheet and cutting strips 1-8th-inch wide, with a small wider portion at one end to take a hole sufficiently large to go over the back of sockets. The wire on the coil is scraped clean all round the position of the tap. working from inside and outside with a sharp knife, as all the enamel must be removed, a narrow strip of glass-paper also Leing of assistance. A small hook is then bent on the end of each strip, hooked round the wire, and soldered on. Another method is to twist up three pieces of 26’s enamel wire, and, after scraping clean, hook the ends in place and solder on. The 20’s wire is rather thick to pass between the turns, hut it could be used by hammering the end flat. . THE CRYSTAL. A semi permanent crystal could be

used, but a carborundum cartridge is recommended, as it is always set for use, and eluninates all the worry of adjusting, and sometimes missing important items. GENERAL HINTS, The wiring is clearly shown in thie diagrams, and should present no difficulty. ‘he same wire as used for the coil may be employed Solder all connections possible. The flex may be single lighting flex, or twin "bell flex," which is very pliable. the twin wires to be connected and used as one. The end to be attached to the pins may be soldered to a solder tag and then secured under the nut, screwing up very tight. The ’phone clips are made from 26s hard brass }-inch wide‘by 1 5-8 for the ccntre ones, and 1 inch for the two etid ones, curled round a 1-8-inch drill and adjusted to fit the thick part of *phone tags. Thev are drilled and holted to the panel as shown, with 1-8-

ineli brass bolts, When using one pair of *phones, a tag goes in each of the two onter clips; two pairs, ‘one tag in left-hand clip, cne i next elip, lag of second pair in second clip from jeft, and other tag in right-hand end clip; three pairs, each pair of tags in two adjacent clips, but tags of the same pair of ’phones not to he put into the same piece of brass. ‘Terminals can be used in place of the clips if desired. A loudspeaker would be connected in the same way as one pair’, of ‘phones. The above arrangement is for ‘phones "in series," or all following one: ‘another Sometimes for two pairs more volume is obtained by using them ‘in parallet.’* This can be tested by joining together the two left-hand clips with a piece of thin bare wire twisted tound the curled portions, the right-hand pair being joined the same way, ‘Then one, two. or three "phones are connected by plugging .one tip of each pair to the’ left and one to the right set of clips. The secret of obtaining smooth action of the condenser dial is to cut a lin, washer of felt and slip this over the spindle to work between the panel and dial. Yo secure the dial put the condenser plates full in, and fix the dial with 100 at top, making a white mark on the panel When purchasing the condenser and dial, notice that the numbering on the dial runs in the direction, clock ot anti-clock, to suit the condenser. A small or tmedium-sizeu speaker would. give bettter results than a large one, but the constructor is advised to borrow one in order to test the capabilities of the set. ' OPERATION. In use, the most selective combination is with crystal plugged into socket 4, aerial 5, earth 6. With the crystal plugged in this position the set is much more selective than with it connected te the top of coil, the usual position. It should always piug in above the aerial pin With earth in 6, its normal position, the lower the aerial is brought the more selectivity. Another combination is crystal 1, aerial 2, earth 8, less selective. Other combinations may be tried, and the best determined. For local use a ‘Ducon" gives good ’phone strength, or weak speaker, and tuning is very sharp, but results depend to some extent on tlie particular lighting circuit, and the one on which the test was made is not a particularly favourable one. MATERIALS REQUIRED. The following is a list of parts required. with approximate prices :-

; : 8. Iibonite panel, 3-16 by 7 by Gin... 3 Variable condenser, .0005 mfd. ... 18 Dial and knob, 3in. ...... 6 vcsccssocveee 1 6 finsh sockets, 8 pins ....-..0 2 Carhorundum detector cartridge... 7 Q termitialsS ....ccccccccescrseeeeeeeceee eeee Alb. 20’s enamelled wire .......... 1 Copper foil, sheet brass, screws bolts, baseboard, ete. 2 oe 82 » bo