THE OSCILLATING CRYSTAL

Radio Record, Volume I, Issue 11, 30 September 1927, Page 14

THE OSCILLATING CRYSTAL

AS A HIGH-FREQUENCY AMPLIFIER

much experimented with by amateurs, "rection of increasing the sensitivity, Oscillating crystals have not been! owing, no doubt, to the case with which a valve cirenit can be put to--gether to replace the crystal, is, however, much scope for interest: ing and useful experiment in the dia and thus the range, of a crystal detector. Experiments already conducted have shown surprising results in: comparison with those obtained with the ordinary crystal circuit. As an KLE. amplifier as much as 15 to 1) amplification has been obtained, but as an I,. amplifier an increase of volume about 3 to 1 is all that may) be expected, The oscillating circuit can be added to an ordinary crystal set. The extra components required will be a variable condenser, capacity according to the one already In use, 400 olum potentiometer, resistance, 4-volt and 12-volt battery, and a good specimen of zincite crystal. The potentiometer must be a — smoothworking one, as any break will throw the crystal off oscillation. ‘Ihe resistance may bea fixed one of about 2000 ohms, and for this purpose the secondary winding of an J... transformer may be used. A variable resistance can be used with some advantage. Zincite is the only crystal used to

give oscillations, and of this a goo specimen must be obtained. A suitable sample has a deep, dull-red, glassy appearance, and if not this colour all over, there must be a very substautial patch of it, or trying to obtain oscillation will be a mere waste of time. A crystal having the appearance of a piece of coke will be useless. Zincite is a somewhat brittle crystal, and on that account should uot be too tightly clamped in the cup. The usual cat’s-whisker must be replaced by a spiral of No, 380 steel wire, or a copper spiral with an ordinary pin at the end. The point of cat’s-whisker must not be too sharp, and may be blunted slightly on glass-paper. ‘The oscillator should rest upon a piece of felt or spongy rubber, in order to deaden all outside mechanical vibration, as this is an important factor towards success. The resistance already mentioned must not on any account be_ less than 1000 ohms, as it must»exceed the resistance of the cat’s-whisker contact, which is usually 800 to 850 chms, Another trial resistance that may be

used is one ear-piece winding of a 4000 ohm pair of ’phones, It should be noted that the zincite is connected to the positive end of the battery. For the detector a perikon combination is particularly suited, but any stable crystal of high resistance can be used, but one of low resistance, such as hertzite, would be liable to cause howling. Condenser A .0005, condenser B .00035. Operation. The method of operation is somewhat as follows: ‘The cat’s-whisker is taken off the zincite and a station tuned in on the detector crystal, condenser B, at zero. Condenser Bis then turned to full capacity, whilst the surface of the zincite is explored for a sensitive spot, the potentiometer being set about half-way over, and cat’s-whisker contact light, When oscillation is obtained a rushing sound will be heard in the ’phones, and a moist finger placed on the aerial terminal will give clicks. The oscillation must be allowed to steady down before tuning is proceeded with. Care must be taken not to jar the set in any way. ‘The condenser LB is then turned until-the whistle of a carrier wave is heard, and tuning is carried out with the two condensers and potentiometer, and, provided that the tuning coil is of suitable value, reception should be satisfactory, A lit-

1 tle practice will be required to regulate the battery potential, so that oscillations are just ceasing when the carrier wave is tuned in, Improying the Crystal. A good specimen of zincite will give oscillations, but it cam be improved by the process of fusing in au electric arc in coutact with manganese dioxide. An amateur may carry out the equivalent of this process on an ordinary fire by placing the crystal in a tin lid with a little manganese dioxide. Place this in a hot fire until the crystal shows a white heat, when it may be taken from the fire and allowed to cool gradually. The above notes have- been compiled from several reliable sources, and are given at the request of a number of readers who wish to experiment in this direction, but the circuit has not been personally tested. ‘"Megohnv" will be glad to hear from anybody who gets good results, which will probably, not be very diflicult to obtain. aaa