AN EFFICIENT CRYSTAL SET

Radio Record, Volume I, Issue 4, 12 August 1927, Page 14

AN EFFICIENT CRYSTAL SET

GAN BE MADE BY ANYBODY .

There is a simple crystal set for the beginner in construction. It is not intended to be a permanent set, but can be quickly constructed and put into use while the owner takes his leisure to construct a more pretentious set. All that is required in addition to the items given are a pair of headphones and an aerial. Articles Required.-Cardboard former about 5in. long and 3in. diameter,. 3 brass ‘terminals, t1b. 20’s enamelled wire, permanent crystal detector, piece of voard about 9 x 5 inches, 1 dozen #in. brass screws, sheet brass, ete: As no condenser is to be used in tuning this set, it is necessary to provide a means of tapping the coil at the exact turn that tunes in the required station. Any means that will accomplish this will answer well, but the simplest and most convenient is a metal arm moving in an are across the wires, the insulating material having been scraped away to give good metallic contact between the metsl arm and the wire. The emul of this arm that makes contact

with the wire on the coll must have’ uron it a small flat which will rest rpon one wire without touching the wire on either side of it. As a crystal set is generally used only on the local station, once the arm is set it will only require moving in the event of the Station getting several metres off its usual wave-length. It is convenient, therefore, to fix the pivot of the arm itt a suitable position below the few turus upon which contact is to be made. . If two stations can be heard, then the pivot may be fixed half-way beween the two positions. ‘The new Wellington station, 2YA, comes it best at about 80 turns on a three-inch former, with an 80 feet aerial, whilst about 100 turns should be provided to include 4YA if that happens to be the local station, and for 2YA 90 turns should be put on to leave a margin, and this same winding will suit for 1YA and 8YA, as they will tune in on fewer turns than 2YA as their wave-length is lower The Coil. Twenty’s enamelled wire is to be used fer thé coil, and wound close together goes about 23 turns to the inch, 90 tutns occupying about four inches, so that a former five inches long will give a good margin at the ends ‘The diameter is to be three inches. Two holes ate made near one end with a fine bradawl, the wire is passed in through cne hole and out through the other, ‘eaving about five inches of end. Wind-

ing is now proceeded with as described elsewhere on this page, and this method will be found good whether the former is bought or made at home. The ends of the coil can be supported on small blocks of wood, and the coil clamped down by a lath running through the bottom, a hole being drilled in each | end of the lath through which screws may be passed to fasten the whole to! the baseboard. ‘The finish of the coil is fastened through two holes in the former, but cut off close as it does not connect to anything. The Tuning Arm. The brass for the tuning arm should be hard brass of 18’s gauge, 2}in. long, at least, but the length can best be determined after the coil is fixed in position. When straight up it should contact the wires well above the centre of the coil.. The wide part of the atm is about Zin. across, drilled to fit over the terminal on the portion that usually holds a wire. Half an inch at the other end is turned over at right angles, the width at the bend being about jin. ‘The end is tapered nearly to a poiut, a small flat being filed at the exttemity. The block ‘should be thin enough to allow of the terminal projecting at the back through the hole drilled for it, so that a mut can be screwed on to keep it secure. The enamel coating is scraped off the end of a piece of wire, the end curled into a loop and secured between the base of the terminal and the block, or, better still, between the nut and the block.

This wire is -craped clean wliere it is looped round the aerial terminal, and without cutting, continues to one end of the crystal, whatever type may be used as detector. The enamel must be carefully and thoroughly scraped off the wire for a distance of half an inch or more, wherever it is to connect to auother part of the circuit. Even a small portion of enamel remaining will sometimes stop signals from coming through. The Crystal. The crystal shown in the drawing is a good type ¢f semi-permanent crystal that will go for days without resetting, and will give good volume’ Some may prefer the carborundum type, whicli is put up in similar cartridge form, but has the additional advantage of never requiring setting, and always gives good and consistent volume and tone. It pays to buy a good crystal to eusure continued good reception, and it can always be used in a more pretentiouslooking set tliat may be coustructed later on. Of course, a cat’s whisker aud loose crystal can be used, but is not recommended for loval reception. ‘Phone Connectors. The ’phone connectors shown are made from 30’s hard brass sheet, curled roand a nail or drill shank jin. in diameter. ‘The brass for the two outside pieces mieasures lfin. hy jin., and for the centre piece 1ljin. by fin. Holes tin. diameter are drilled or punched as

shown, and the connectors fixed to the board with fin, No, 4 brass screws, The loose end of wire from the left-hand of the coil is now cleaned at the proper position, clamped under the earth terminal, and continues to clamp tinder the brass ’phone connection (1). These ‘phone connectors will allow of two pairs of ’phones being used. If one pair is in use, plug the tags into 1 and 4; if two paies, plug the first pair into 1 and 2, and the second into 8 and 4.

Final Remarks. The baseboard may be finished with shellac, dissolved in methylated spirits, rubbed on with a rag, but this must. be done before attaching any of the parts. The crystal is shown supported by the connecting wires, but a brass angle piece drilled #in. on one face and drilled for two screws on the other, can be screwed to the board, the wire from aerial clamped underneath. The broad end of crystal mount will then slip through the din. hole and is secured by the nut provided, and stands in the same position as shown, but is miore securely held. Provided that a reasonably good aerial is erected, this set will give all that can be desired in the way of crystal reception. Where the aerial used is short, extra turns should be put on the coil to compensate, The track of the tuning arm should be bare copper tin. wide, the enaniel being scraped off the wires with a sharp knife.