Crystal Receiver Construction

Radio Record, Volume I, Issue 24, 30 December 1927, Page 12

Crystal Receiver Construction

GENERAL PRINCIPLES AND SPECIAL CIRCUITS 7

Owing to its general simplicity and ease in both use and construction, the crystal set is bound to continue in popularity, and it makes a good appeal to these who have no time to attempt the construction of valve receivers and the accompanying mastery of technical details and theory. ‘The simplest possible circuit is shown in diagram No. 1, the circuit used in the "Record" crystal set which has given such good results. ‘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, I, is arranged to move along the coil and make contact with the wires, a small amount of insilation being scraped off each to present a clean metallic surface in the track of the slider. ‘This"is theoretically a good method, as for long distanee work it is always a good scheme to tune with the inductance as much as possible, m order to keep down the capacity of the aerial circuit. A TYPICAL CIRCUIT. In the diagram, A is the aerial, B the inductance, or tuning-coil, which is conveniently about 70 or’75 turns of No. 20’s S.w.g. wire space-wound on a 8-inch diameter former which is afterwards removed, the wires being held together by celluloid strips to which they are cemented. C is the earth connection to either a water-pipe or to a metal 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-perman-ent perikon, or a ioose cat’s-whisker_ and etystal, which latter give excellent results, especially for long-distance, but necessitate much fiddling about and loss of reception at times. If 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, PARALLEL CRYSTALS.

In diagram 2 we have the same circuit, but the crystal and ‘phones por- | tion is duplicated on the opposite side. ‘Lhis arrangement necessitating an extra erystal, gives m two pairs of ’phones volume equal tc 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 ntilised by the second crystal, so that the circuit is then a {ull-wave rectifier. ‘The connecting of the crystals is therefore important, and if the crystal side of one is connected to the aerial, then the cat’s-whisker of the other is connected to it. If the connections to one pair of headphones are altered, so that one ear-piece is connected respectively to each crystal, then double volume is obtained.

AN AMERICAN CIRCUIT. Diagram No. 8 shows an American circuit for which good long-distance 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 colldenser, and crystal. This circtiit will probably be more selective than the average. OBTAINING SELECTIVITY. 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 varving the coupling. There are innumerable variations of ervstal circuits that are worth a trial, and some experimenters will find a particular one that gives them best results. whilst others, differently located, will find success with an entirely different hook-up. With a coil or two, a variable condenser, a pait of ’phones, and a good erystal, it is possible to try out a vatietv of cirenits, provided one has the time at disposal in which to make the changes and carry out the testing. Probably many crystal users would find radio even more interesting if they went in seriously for experimenting in the same way as many valve enthusiasts do.

Next week a constrictive articte will be given dealing with the R.R. SELECTIVE CRYSTAL SET. Automatic or fixed rheos‘als are not satisfactory with type 199 tubes. There is apt to be an overload on the filament when fresh, dry cells are. installed. When a 201A tube is used in the last stage audio the best results will be obtained for loudspeaker when employing a plate voltage of 185 to 180 volts, but the proper grid hias must be nsed-from 7.5 to 9 volts. ‘The lower bias will give a better tone at 135 volts, and the higher one at 180 volts. With a 201A tube in the first stage audio, 90 volts on the plate is correct, with a 4.5 volts bias. Occasionally tighten all nuts on terminals and ‘components. Loose ter minals can cause quite a lot of noisy, and weak reception.