Our Crystal Corner

Radio Record, Volume II, Issue 31, 15 February 1929, Page 29

Our Crystal Corner

By

Galena

The "Signal" Crystal Set. _ HIS crystal set was designed after a series of experiménts, with, the object of simplicity, clarity, and volume from. the local station. It is suitable for local reception, either as a simple erystal set or with valve amplification. The tuning is accomplished by means of a copper disc moved over the face of a spiderweb* coil, which-, once ad--justed, needs no further attention. The coil is wound upon, a former, -and to suit 2YA a 60-turn coil is required ; to find a coil suitable for other stations, divide the. wave-length by. 7. This: gives the number of turns in a coil; suitable: for a standard 100ft. aerial and lead-in.. The former'is made: of a disc of wood or metal 1}in. dia--meter with 24in. or 3in. nails set radi.ally around its circumference, like #spokes. ii a cart wheel, any. odd numbér, 9 to 15, may bé used. The wire, used, 24g. d.c.¢, is wound in and out the nails until the réquired number of turns has been put on, and then the wire must be held in place by tying with strong thread where the Wwirés cross, or fixed with celluloid cement before removing the former. The Tuning. Dise. "THE tuning disc is cut’ from sheet copper, 26 or 28 guage, the same diameter as. the coil, leaving a lug on oné side to fit on the aerial screw.

The tuning is affected by the distance that the copper disc stands away from the face of the coil, which should ‘be from fin. to lin. If the disc will not swing over the face of the coil, bend it away or reniove a few turns from coil; if the loudest position is when the dise is full over, bend it Pioser until maximum’ volume is atined; move the disc with a piece of wood, not the hand, when tuning. The diagram shows the side view, with the theoretical diagram inset.

The base board is 4in. by 4in., and the back piece 4in, by 2in.; they can be made out of 3-8in. rimu, rubbed. over with linseed oil and: shellaced, or, better still, is 3-8in. "Sindanyo" or other insulating material. / The writer runs a medium size loudspeaker with this set and one valve, 9 miles from. 2YA. A Novel Set. A CORRESPONDENT, Mr. R. Garlick, has sent us a description of a noyel crystal set. For this, the usyal components ate necessary, to whit, 4 terminals, a crystal, quarterpound of D.C.C. wire, quarter pound of 82 enamelled ‘wire, a 2-inch former, and a 83-inch former. The set is loose coupled as the two sizes of wire and formers would suggest. The first task is to wind the coils on the 3-inch former. Wind on 40 turns of the 22 wire. There is no need to space wind the wire when it is cotton covered, so that this should present no difficulties. When wound on neatly and tightly, remove the former, and on the 2-inch fotmer wind 75 to 90 turns of 24 D.C.C. wire. This, as before, will be close wound. A box is then taken (the writer used an Epsom salts box), and on each end of the box four holes are drilled, so that a jJine between each pair crosses at right angles. The holes on the vertical plane are 3 inchés apart, while those on the horizontal are 2 inches apart, that is, the respective diameters of the two coils. These are drilled similarly on the other end. Through these, string is threaded so that a cage of four wires is made in the box. Upon the inner pieces of string, which are 2 inches apart, slip the coil of fine wire and solder a piece of heavier wire about 10 inches of, say, 22, on to,each end. Now over the top of this eéil and on the remaining two pieces ‘of string, which are 3 inches apart, slip the remaining coil; pass the ends through the lid of the box to terminals marked "aetial" and "earth." This completes the primary winding. Take the two ends of the finer wire, that is, the sécondary, and pass them through the top of the box. One end passes to one side of the ¢rystal, the other side of the crystal to a phone terminal. The other phone terminal connects with the free end of the secondary wire. This completes the set. The set is tuned by moving one coil in relation to the other. y Of this set the writer adds: "I can get really good phone strength at 24 miles from 3YA, but I do not know the range of thé set. I have heard it said that some listeners have heard 2YA here in Christchurch. There is plenty of room for improvement, and I would like to hear results which listeners #ét from this set." In passing this set on to readers, it may be added that the loose coupled style, of which this is a variation, is usually more sélective than the tight coupled, though seldom delivérs the’ same volume. However, for anyone in-

terested in sets, this should make a very interésting and novel experiment, Full-Wave Rectification. oO" (Christehrch) writes com: plaining of poor results. His trouble seems to be similar to that, of "Puzzled" (Wellington), who was answered in Vol. 11, No. 27 Certain specific questions are asked: 1. What should be the respective positions of one detector be to another? Answer: That is immaterial, but keep the leads short. 2. Can you adécount for greater volume on one crystal than on another. Answer: It appeats as though one crystal is more efficient. 8. Should I get much more volume from the speakér at, say, 13 miles from a station, with a forty-foot aerial than I would at two miles with a

RS UTI ALL UUU eCLLe ILA twenty-foot aerial. Answer: Providing both aerials point to the same direction and are the same Jength-~Yes. 4, Would a horn speaker be suitable? Answer: Yes, if it is not too large. 5. Would an aerial 60-feet high one end and 40-feet high the other end, give much more volume than one 20feet high each end and _ the same length? Answer: It should~Yes. 6. As I want to make my earth wire as short as possible, would the follow-. ing be an advantage. (my present earth is connected to the water pipe, which is about 9ft. from set): I intended putting an old copper, well down in thé earth, filling it with coke and leaving a pipe above the: ground to pour water down, This-copper is to be conhected to bare copper wire, which would run along under the ground and connéct to the water pipe,

still under the ground. Then thé leadit would be connected to the copper and bé brought in through the window. The distance betwéen copper and the water pipe would be 10 or 15ft. Would this give both the benefit ef the copper earthing and the water pipe earthing in one, or would one be robbing the othér? Answer: The earth should be more efficient than the present one, but see our article on "Harths." The correspondent remarks that the dials work 6n 60 and 100, so that it appears that turns have to come off the coil controliéd by thé condenser whose dial reads 100. Choice of Components. N this respect the amateur is very liable to err. So maty components are.on the market -at most at- | tractive prices that the young enthusiast who wishés to put his crystal ott~ put on to a loud-speaker, is inclined to under-estimate. the value of good .components. Cheap compottents injure toné more than volume, although the latter certainly suffers. Transformers afford one of thé most outstanding examples of the difficulties that beset the purchasér of cheap parts, A little. while back, it was quite safe to say that a small transormér Was a poor one, but to-day this can no longer be said with any dégree. ef truth. Already, some very fine transformers, which are quite dimjnutive in size, have appeared on the market, avid are giving the greatest of satisfaction. Particular among these. avé the Philips and Mullard type with silver wound ptimaries. Thése transformers are by no means cheap, bt are well worth the extra expenditure. However, at the price paid for these, one has a wide choice of gaod transformers, and the constructer should always consult a-reliable dealer whose advice he may quite profitably take ‘before purchasing this component. ' The coming season, too, will mark the advent of very many fine but lo\vpriced transformers. If the young constructor will watch advertisements, he will see that many of these are now on the market, The effect of a small transformer is to spoil the tone of the set. Besides this they havé a véry shert life, so that it really pays both from the point .of view of tone and economy to purchase the better type transformer. The ratio very often worries the young amateur. For valve sets the usual ratio is about 34 to 1, but in amplifiers for crystal sets where only a limited volume is to be handled, the ratjo is not so important. So long as the curve of the transformer is flat or relatively so, a ratio of 6 to J, 6r more, can be quite profitably used. In addition, this higher ratio helps to step up the output, The curve of the transformer is"ustially published by all reliable makers, so that the déaler should be able to show the young én‘thusiast just what his transformér is. capable of doing.

READERS in difficulty, or who feel they are not getting the best from their crystal should send in their difficulties. They will be discussed in the Corner, that is providing they ave not too particular to one case, when they will be replied to by post. If an owner has found a circuit he considers superior to the usual, send tt in and if worth publishing ét will be passed on to others. Remember, address all inquiries "Technical Editor,’ and mark the letters "Crystal Corner."