THE WIRELESS WORLD.

Otago Daily Times, Issue 20317, 27 January 1928, Page 5

THE WIRELESS WORLD.

NEWS AND NOTES. By Magna Vox. Items of local interest are invited by •• Magna Vox ” for publication in this column. It is necessary that such matter should reach this office by luesday ol each week for insertion on the following Friday. ANSWERS TO CORRESPONDENTS. ‘Aerial,” Lawrence. —The cords connecting an indoor loop aerial to a receiving set should not be more than 311 in length. They should be kept well apart. J. 11. 8., St. Clair.—Connect to an insulated wire stretched on the ground, and you will find this a much more satisfactory earth connection for a portable receiver than a poor or metallic ground such as a rod driven into a wet spot. “ Current,” Mosgiel.—lf you wish to keep the current filament low, I suggest that you use the point one type of valve which can be purchased with four or sixvolt series. Power valves cost_ a little extra, but they are well worth it. R D. O’B., Oamaru. —The approximate BG- battery drain of 10 valves of the type you mention is 50 to 60 M.A., and is tar too great for small Lelanche cells. X would suggest your using a B battery accumulator. There is absolutely no ii xsattei'y drain as soon as the filaments are switched off. The fact that you cannot use more than seven valves proves conclusively that the batteries have not sufficient capacity to supply more. HINTS AND TIPS. The most common and ruinous abuse of the battery is to let it go “ dry.’ The electrolyte solution must always completely cover the plates in all cells. Regular inspection will show the condition of a battery. If it is undercharged, the specific gravity of the electrolyte 'fill be 1250 or less; if it is overcharged, it will be over 1300 and it will show signs ox excessive heat and spraying. The best test for a new valve is to put it in a regenerative receiving set oil oscillator and determine whether or not it will oscillate. This is true, because a valve that will oscillate will also detect and amplify (the two other functions of a valve). This is the test often used by manufacturers and dealers. A valve that will not oscillate is practically worthless for any use but rectifying. To obtain the best results, an indoor aerial should be as high as possible. The highest indoor position in any house is immediately beneath the roof tiling. An aerial consisting of six or eight parallel wires, attached to insulators, screwed to the under-side of the roof framing, is the best possible arrangement. In actual practice, such an aerial gives excellent results, especially if used in conjunction with a fairly sensitive valve receiving set. All home builders of crystal sets should try placing a small fixed condenser between the two ’phone terminals. This generally results in rather stronger reception. A fixed condenser of .001 microfarads will, as a rule, prove the best. Fading of signal strength to a degree bordering upon inaudibility may puzzle the uninitiated listener until he examines the aerial. Spiders’ webs mysteriously collect across the insulators. Whilst the air is dry no material difference may be noticeable, but dew will gather on the webs and effectually bypass much of the precious high-frequency current to earth. A great advantage in using a variometer for the tuning device of a crystal set is that there are no “'dead-end” effects in the variometer. No tappings or inductances are necessary; there is also smooth and infinite adjustment between minimum and maximum positions, and the variometer is a robust mechanism. But get the correct-sized variometer for most efficient results. When mounting a crystal in a cup of the type in which the top or cap is screwed down to hold the crystal in place, yos will sometimes find, especially if you are using a small fragment of crystal, that the latter cannot be fixed tightly even when the cap is screwed home. To get over this difficulty, take, a piece of tinfoil and crumple it up in a small pad, place this in the cup first, to raise the crystal up little, and then screw down the cap. Add more foil if necessary. If the hole in the cap is too large, roll some foil into a little stem, and bend this round into a ring, and lay on the top of the crystal. Or a brass washer having a smaller hole than that in the crystal cap may be introduced upon the top of the crystal. SHIELDING. A question which exercises the minds of many people to-day is the exact use of “ shielding ” or the placing of sheet metal round individual stages of radiofrequency amplification in such a way as to envelope completely the complete stage. Shielding was primarily introduced for the purpose of preventing out-of-balance effects between successive stages of amplification when the orthodox five-valve set of last year was increased to six or more valves, necessitating four or more stages of tuning. For technical reasons, in order to still maintain relative simplicity of control, with multiple stages, shielding was introduced and immediately became the fashion, with the result that an erroneous impression has got abroad that no set is complete without it. For six or more valves this impression is well founded, but for five or less valves it is wrong. All manner of wonderful things ,are attributed to shielding incidently, and most of them are purely mythical or merely sales talk. Actually shielding causes inefficiencies which are only warranted in six (or more) valve designs because more is gained in the end through the ability to use more valves with stability of operation and thus obtain a greater ultimate amplification. With five valves or less total shielding causes the same inefficiencies, and these outweigh anything picked up by virtue of •shielding having other virtues. WAVE LENGTHS. Broadcasting is a tricky business (says a Melbourne exchange). It is subject to so much control with consequent regulatory changes. It is unstable enough in the effects produced by the rapid development in the technique of receivers which operate against any trader safely budgeting a year or so ahead on any particular line. The danger of obsolesence is ever before him, and he must stock carefully with an eye not only to demand, but to new ideas in set design. Therefore, it is most desirable that the necessary changes should be as few as possible. But we find that it is proposed now to alter the wave lengths. It isn’t as if they have not been altered already. And in all truth it isn’t as if they were not in need of alteration a long time ago. This tardy recognition of the realities of the position is better late than never: but it is a pity that it is so late. Any comprehensive alteration of the wave lengths such as is apparently contemplated is a serious matter. The listeners have been subjected to the annoyance of changes before, and the radio trade is likewise embarrassed. The change from the long to the short wave lengths a year or so ago was the cause of considerable complaint. When 3LO, Melbourne, started on 1730 metres people purchased sets with a wave range suitable for that wave length. The home-assembled sets —and there were many of them —were put together on the same understanding that there would be no need for any drastic alteration in the tuning elements of the receivers. Whatever was the cause of it, the “ long ” wave length did not last long, and the agitation for and against the alteration was widespread, but the effect of the alteration was generally satisfactory. Now that we are to have another change we can look for some heart burnings, especially if there is to be any radical change in the principal stations. While it is admitted there is abundant reason for altering some of them, it is to be hoped that no wholesale changes will be made. Many of us now are interested in the transmissions from “B ” stations. It looks as if we must have our sets altered if we are to pick them up in the future. If the wave lengths of those stations drop far below 250 metres a good many listeners will not hear thsir class “ B ” stations unless their sets are altered. Thus we see that any alteration of the wave lengths brings in its train some troubles for the listeners, the traders —in fact, for all concerned. Knowing that, one must assume that the authorities have good reasons for the alteration. But wo can in all reasonableness ask that the alterations should be complete and final—■ at least for, say, five years or so. One cannot safely look beyond that distance in anything concerned with broadcasting.

A LITTLE ABOUT BATTERIES. In a short; space of time I have had brought under my notice dozens of letters

concerning faults in broadcast receivers. 1 do not exaggerate when stating that of the difflcutlies described as poor operation at least 60 per cent, were due to battery troubles of some sort. Headers should definitely remember that it is impossible to operate any valve receiver at its highest efficiency unless all batteries—A, B, and C —are in the very best of condition—not in fairly good condition, but definitely “ up io the mark.” The general lack of appreciation of this fact is probably duo - to the misleading symptoms of battery failings. These are such as to suggest, even to the average expert, trouble in any part of the receiver other than that actually responsible for the difficulty'. When the A battery becomes half-dis-charged trouble may be evidenced in several deceptive symptoms, particularly when the battery is connected to a receiver consuming a fairly high amperage, through high resistance leads (long leads, small wire or both). The potential drop, often quire unnoticed as far as change in filament brilliancy is concerned, will increase the plate filament impedance of the valve, with effects seemingly unrelated to A battery deficiency. A loss in the bass notes may be the only effect, and during the latter half of your A battery discharge reproduction will be tinny, lacking the round fullness of tone output that a good receiver under ideal conditions is capable of. It is probable that the C potential—B potential balance—will be unbalanced and distortion will be noticed on powerful sustained notes. A harshness or rasping that will probably be attributed to blasitng of the loud-speaker. Reception will become noisy. A step on the floor or a pencil droped alongside the receiver will often set up a recurring series of rasping sounds. Loud notes of a particular quality will induce similar sounds during amplification, which an expert will often attribute to rattling in the loud-speaker. A highpitched squeal, subject to modulation by touching different parts of the amplifier, is usually a good sign of a slightly low A battery. To obtain the most useful and satisfastory service from an A battery it should be wired to the receiver with a good quality heavy flexible wire. A semi exhausted B battery will effect a series of similar difficulties. Squeals, progerssing to a state of unqualified howling, commonly accompany the fast-ebbing life of the B battery, but are seldom traced to them correct source. These are caused through the increased resistance of the decaying cells, which provides an effective coupling between the audio stages, with the resulting and all-too-audible interaction. Distortion of the rasping, blasting type, along with weakened volume, will often precede the noisy stage, to be blamed on the loudspeaker or transmission. At the first suggestion of defective reception, the loss of roundnesa of tone, or occasional blasting, the batteries should be the primary consideration of the dissatisfied listener. Place the A battery on charge unless the hydrometer reading shows that the density is above 1190. The B and C batteries should receive the immediate and intelligent application of a voltmeter. Do not blame your receiver, your loud-speaker or the station until you are perfectly sure that all of your batteries are in tip-top condition.