Our Crystal Corner

Radio Record, Volume II, Issue 25, 4 January 1929, Page 29

Our Crystal Corner

By

Galena

, Difficulty to Log 2YA. "(GOULD you please inform me as to whether I am likely to get 2YA in Paekakariki on a crystal set? I have already tried without any results whatever. Is this due to my set? enclose diagram, which I took from a back number of the ‘Radio Record.’ I did not follow the cireuit exactly, as I had not the necessary space in my box t6 use a 8in. former. Does this make any difference? My aerial is ‘Blectron’ wire, 7ft. high, 75ft. long, without insulators, as I thought the rubber insulation would do» I hope you can help me.-Keith Dixon (Paekakariki). The erystal set comprises 77 turns f 24 d.c. wound on.a 2in. former and pped at the thirty-ninth turn. The ‘tuning is carried out by means of a condenser. Paekakariki is about 25 Iniles air line from 2YA, so that it is, or should be, just within crystal range. In testing the actual timing capacity K.D. must make certain that his tapping is at the right turn. Try varying it. The aerial is not particularly good. It should be carefully insulated and if possible increased in length and height. Stranded copper wire enamelled would probably be better. Seven feet is far too low to receive 2YA in Paekakariki. Try about 30 feet. The type of earth has not been specified. From reports, certain places in Paekakariki have difficulty in getting a good ground. ‘Try earthing a kerogene tin and keep it filled with water. A 2in. former can be used, but a Zin. is preferable. In general a smaller former means more turns and finer wire less turns. To Prevent the Aerial Snapping. ‘A CORRESPONDENT, HB. R. Hill, has sent in quite a good sugges: tion concerning the aerial. Where a tree is to be used as one aerial mast and an ordinary mast for the other he suggests that three sashweights be secured to the mast end of the halyard supporting the aerial. When the tree sways the weight will move up and down, accordingly keeping an even strain and prevent the tugging on the nerial, The pulley wheel must, of course, run freely. , Battery Potential for the Carborundum. S$ explained in the "Corner" previ- ’ ously, a permanent detector is much improved by the suitable application of a current of electricity. Spe- cial units are procurable for this purose, but their price rather precludes : heir use except to the A "fortunate few. However, by the following method the same effect can be obtained for the expenditure of a very small sum, that is, for the cost of a two-cell torch battery and a potentiometer; the latter is a high resistance rheostat with three tappings. Just recently the writer saw these on sale for a ridiculously low figure, a few pence, but the usual price is a matter of a shilling or two. The usual method of applying voltage to 2 earborundum-steel combination. such as those obtainable in cartridge form, is shown in the diagram. The ordinary tuning cirenit is shown at the left, the crystal in this case being shown connected to a tap on the tuning coil, but may be connected to the nevial end if found an advantage. The circuit is taned by a .0005 variable condenser, with about 56 turns on the coil, or .00025 with about 78 turns, the

wite being better spaced about half its own thickness. The diameter of coils is 3 inches. The foregoing particulars are given for those who wish to construct a complete receiver, but any existing crystal receiver may have the ecarborundum cartridge put in and the biasing arrangement added. The biasing is accomplished by means of a potentiometer of 400 ohms resistance. The arm is connected to one side of the ’nhones, and the two other contacts

to the negative and positive, respectively, of two small flashlight dry cells. A connection is taken from between the two cells to one end of the crystal and to a fixed condenser of .0005 capacity, which functions as a by-pass. A switch should be provided four each battery us shown, and this may conveniently be a double-pole _ single throw, so that both are switched off together when not in use. It is immaterial which end of the detector is connected to the central point of the battery, as the potentiometer allows the application of a voltage of positive or negative from 0 to 1} volts. If one cell is omitted its position is bridged over with wire, and it is then necessary to find out the correct position of the detector by reversing it. Only one side of the switch would then be required. Weak Signals. {[t often happens that signals become very weak, and the erystul owner is at a loss to find the cause. Aerial aud earth may seem all right, yet the signals are decidedly weaker than they used to be. Where shall we look for the trouble? If it is an adjustable crystal that is in use, then one cause of trouble may be that the cat’s whisker may be rusty, or, perhaps the crystal surface may be dirty. A snip with a pair of scissors will cure the former. and the crystal itself may be broken to form new surface, or replaced. In any crystal set, the crystal should be kept bright and clean, but don’t handle it with the fingers. A small pair of forceps should be used whenever it is to be handled. To clean the crystal, a few drops of methylated spirits should be applied. and the crystal put out in the sun te dry. Lowering the crystu] into a smal) quantity of spirits would be more effec tive, but cate must be exercised not to allow it to touch the bottom of the contniner, lest it should pick up more dirt than it originally possessed,

Always keep the set clean, Dust should not be allowed to accumulate under any circumstances, for this reason it is advisable to construct a cabinet, no matter how simple it may e. Dust and Dirt. ANOTHER possible cause of troubleis an accumulation of dust and dirt on the diaphragms of the telephones, This can be removed by any careful listener, with a piece of soft, oiled cloth. Dust will settle between the contact plugs, and can easily weaken reception. The trouble may also lie out of doors in the form of soot, or dirt, of the insulators, or a dirty contact at the lead-in, this being especially true of sets where no valves are used. Another cause of weak signals is a poor earth connection, but this was fully dealt with in our special issue of last week. Faulty Phones. SOMETIMES the cause of weak signals lies-in the telephones or the loudspeaker, even although these do not appear rusty or damaged in any way. But if they have been dropped to the floor or otherwise roughly handled, it is quite possible for such treatment to have demagnetised them (especially if they have been purchased a year or so ago), or connected to the amplifier the wrong way round for some time. If this fault is suspected, however, it can be easily chetked by borrowing a pair of good ‘phones for a time and trying them out when the improved reception will show whether the ’phon : are at fault.

Another common fault is the fluctuating of programmes, Le., when the set has the habit of "going off." If an amplifier is in use the first thing to look for is the accumulator connecting bar, which may be making a loose or dirty contact and thus causing the filaments to be starved. *Phone leads or, indeed, any flexiible lead-in may give rise to this fault, the wire inside the insulation having become frayed, thus making intermittent or loose contact. In both the above eases the remedies are obvious. Another cause of this kind lies out of doors, for it may be that the aerial or the lead-in is swinging against some obstruction. If the wire happens to. touch a gutter pipe or some sueh thing the signals will go to earth instead of through the set, so do not run the leads too near buildings, walls, etc., but have them sufficiently tight to make sure there are no stray contacts. Dust and dirt between the condenser plates is another cause of scraping, but is casily located because the scraping noise corresponds to the movement of the condenser, Blind Spots. ANOTHER puzzling fault is so-call-ed "blind spots" in a erystal set. Actually blind spots cannot exist in 4 crystal set, yet the owner of a crystal may be puzzled at the disappearance of his signals at one particular reading. An acquaintance of the writer was for some considerable time puzzled with this annoying fault, and he was at a loss where to look for the trouble. Finally the condenser was critically examined and the trouble laid bare. He had been making a few alterations and had been using the soldering iron to join some wires to a lug on the condenser. Some of the solder had overflowed and a small portion lodged between a fixed and a moving plate. The moving plate was not quite straight, and in one particular place touched the fixed plate, and so the "blind spot."