Our Crystal Corner

Radio Record, Volume II, Issue 24, 28 December 1928, Page 29

Our Crystal Corner

By

Galena

Types of Receivers. DURIN G the Christmas holidays, now with us, there will be many, especially among the younger folk, who will set about constructing for themselves simple crystal sets. That is where most enthusiasts start, and a better starting place could not be had. Those who missed our special crystal issue would be advised to obtain a ‘copy now. There are still a very few left. Following are three circuits showing the fundamental arrangement of the components. These circuits are the most simple yet give splendid results.

The Simplest Circuit. (THE simplest possible circuit is shown in diagram No. 1, the circuit used in all erystal outfits, but which is gonietimes slightly modified by small additions such as condensers, variometers,, etc. "There are three ways of tuning the aerial coil that may be adopted. The first, simplest and least costly is that shown in the diagram whereby a slider, F, is arranged to , move along the oil and make contact } with the wires, a small amount of insulation being scraped off each to present a clean metallic surface in the track of the slider. This is theoretically a good method. In diagram (1) A is the aérial, B the inductance or tuning-coil, which is

‘a a ee conveniently about 70 or 75 turns of No. 20’s enamelled s.w.g. wire spacewound on a 3in. diameter former which is afterwatds removed, the wires being held together by celluloid strips to which they are cemented. The wires may, if double cotton covered, be wound on to a former without the usual piece of string to separate one turn from the next. I£ the former is well shellacked such a winding could be left on it, all that is nécessaty being to cement the turns in: position with good glue. C is the earth connection to either a water-pipe or to a neutral plate or tin sunk in the ground five or six feet deep. D is the crystal detector, which may be of any, kind, permanent carborundum, Semi-permanent perikon or a loose cat’s-whisker and crystal, which latter give excellent results. especially for long distance, but necessitate much fiddling. about and loss of reception at times, BE represents the pair or two or three pairs of headphones, but unless fairly near to a broadcast station each pair of ’phones added further weakens the volume of reception. ’Phones can be taken a great distance from the set providing the wire is well insulated and fairly thick. Parallel Crystals. [X diagram 2 we have the same Circuit, but the crystal and ‘phones portion is duplicated on the opposite side. This arrangement necessitating an extra crystal, gives in two pairs of *phones volume equal to that of one pair with the single crystal, and with two pairs each side the four ‘phones would get the same volume as two with a single crystal. The reason for this is that the alternate waves not utilised by the first crystal, instead of being suppressed or wasted, are utilised by the second erystal, so that the circuit is then a full-wave rectifier. The connecting of the crystals is therefore important, and if the erystal ide of one is connected to the aerial, then the cat’swhisker of the other is connected to it. If the connections to one pair of headphones are altered so that one earpiece is connected respectively to each ervetal. then double volume is obtain-

ed. "Pentode" has developed this idea and produced his full-wave crystal set. An American Circuit. DIAGRAM No. 8 shows an American cireuit for which good long-dis-tance achievements are claimed, The circuit is not an expensive one to experiment with, as a couple of 75-turn coils, three inches in diameter, tuned each with two sliders, are all that is required in addition to the ’phones, ‘phone condenser, arid crystal, This circuit will probably be more selective than the average. Obiaining Selectivity. GELECTIVITY with New Zealand operators cannot be much more

than a term, when applied to crystal sets. The reason for this is that the powerful stations. are so far removed that it is extraordinary to get beyoud the local on a crystal. Selectivity is gained by the employment of loose-coupling between the aerial coil and a separate secondary coil, which is not directly connected to the aerial circuit, but is placed close to the aerial tuning coil, in such a way that the distance between the two coils can be varied. The energy rectified by the crystal is that transferred across the space between the two coils, and it will be found, therefore, that this distance is important, . and slight modifications will often eliminate undesired signals but will leave the broadcasting . quite clear, This coupling is most conveniently carried out by the use of plug-in coils, mounted on a holder with provision for varying the coupling. . | Another method is to wrap the primary round the secondary, but separated from it by an insulator-greased paper. | There are innumerable variations of erystal circuits that are worth a trial, and some experimenters will find a particular one that gives them best results, whilst others, differently locaton — —

ed, will find success with an entirely different hook-up. With a coil or two, a variable con‘denser, a pair of ’phones, ahd a good crystal, it is possible to try out a variety of circuits, provided ohe has the time at disposal in which to make the changes and earry out the testing. Probably many crystal users ‘would find tadio even mote intéresting if. they went in seriously for experimenting in the same way as mahy valve enthusiasts do, A Humming Noise in "Phones. SOME crystal listeners whose homes are connected to an electric supply may find that there is a silght cantinued hum in their headphones. This interference is caused by nearness to the electric wiring in the house or by the influence of street mains upon the aerial, which may be running more or less parallel to the street wires. Here is a simple method that may eradicate the trouble. Place a slightly moistened finger on the earth terminal of the set, and if this cuts out the hum then a permanent equivalent to the finger can be tried. This consists of a length of a few feet of bare copper wire attached -to the earth terminal of the set, and allowed to run loose along the floor. It may improve matters to connect the free end to a metal object, perhaps a foot square sheet of metal placed under the floor covering, but neither this nor the wire should be directly connected to earth. Altering the aerial to make it at right angles to the power lines is often effective. —