THE WIRELESS WORLD

Otago Daily Times, Issue 20707, 3 May 1929, Page 5

THE WIRELESS WORLD

By Maona Vox.

NEWS AND NOTES.

Items ot local Interest are Invited by “Magna Vox” for publication In this column. It Is, necessary that such matter should reach this office by Tuesday of each week for Insertion on the following Friday. PRINCIPAL WAVELENGTHS. For the benefit of listeners who have not their log always handy, the following list of the principal New Zealand and Australian stations and their wavelengths will appear each week in this column:— Metres. 7ZL, Hobart ,335 3AE, Melbourne .. .. .. 484 41 A, Dunedin .. .. .. .. 463 2FC, Sydney .. 442 2YA, Wellington . , .. 420, 4QG, Brisbane .. .. .. .. 385. 3LO; Melbourne .. .. .. 371 2BL, Sydney .. .. .. .. 353 2AB, Sydney 316 2KY, Sydney . 280 IYA, Auckland 333 3YA, Christchurch .. .. 30G SCL, Adelaide .. .. .. .. 305 ANSWERS TO CORRESPONDENTS. W; J,, Dunedin.—Next week. J. A., Glenore.—l am making .inquiries in regard to the set you mention, as • I have not, as yet, had any experience f X V As soon as I receive some definite information, I shall pass it on to you. Aerial,” Cromwell.—l am afraid you would find a frame aerial'of very little use for such a small set. An indoor aerial reduces the strength ot incoming signals by approximately 75 per cent. F. M. G., Dunedin.-Therc is no cause tor alarm m the heating of the wire in your filament , rheostat, as this device is designed to act as a species ot safety valve which will absorb some of the current from the batterv, so that the amount flowing through « v 1 T, a VQ ■ "will be regulated. Neiitro Anderson’s Bay.—Yes,.you can use the screen grid valve in a neutrodyne if you wish, but the set,will no ; onger be a neutrodyne, because a neutrodyne is .a set which depends on neutralisation for its DX qualities v hereas the S.G. valve does not norm* a y require neutralisation. First of alt, you would have to leave out the noutralusing condensers, and the coil system would have, to be altered entirely. ..Thus, the set would not . be a ngutrodyne, TESTING R.C. UNITS. condenser* should be tested periodically with a pair of phones and a ■ 45-yolt battery. A comparativdy strong click should Ue heard in the phones connecting the anode, resistance in senes w ith the battery ■ and phones A much smaller click should be heard for 1 after the first click, I in tne case of the condenser, no more noises should be heard. If any of these components prove faulty they should be replaced, especially the coupling condenser, , which, if leaking, will give a positive on the- grid of the following valve, ibis not only gives distorted signals, but m time, will ruin, the valve, especially if it hapens to .be followed by either a speaker or transformer. This is the first and most likely fault that, a service man looks for when called in t.n overhaul the set. CARE OF THE ACCUMULATOR. Au accumulator is rather a placid, uncomplaining sort of article. It will stand mis-handling without showing anv visible evidence of such. There is, however, a.disease which can attack it, and which, if unchecked in .the early, stages, rapidly develops and finally puts the battery completely out of commission. This disease is known as sulphating. It takes the Torm of a whitish deposit of almost insoluble substance on the plates. • The symptoms, although very noticeable, arc often neglected, with a result that the battery gets worse and runs down 'quicklj after recharging, even - if only used for quite moderate periods. The causes of sulphating are somewhat numerous—charging at too heavy a rate or -discharg- -- jng at too heavy a rate. Generally speaknig, however, broadcast listeners cause ampliation by nothing more than neglect. Quite, unintentional neglect;, for the most P ar t no'doubt. An accumulator should never be allowed, to run down to a point whe're it will not. operate .the receiver satisfactorily. - If the volume' of yOlll nearest station starts to reduce you should immediately take an -hydrometer reading. If the level of the acid, is below .1180 or 1100. the receiver should be immediately s ' v 'Lebed off,-and the battery placed on charge at the correct charging rate. A run-down battery shofald never be allowed to stand idle for any time, or sulphation will immediatley commence; Should an accumulator not be required for a period of two or three months, it should be given a thorough charge, the acid emptied out. and the plates thoroughly rinsed with distilled water (not tap water). To bring it back again into service, the acid of correct density as recommended by the. manufacturer should be again poured in, and the battery’given a further slow charge. Operations of this-nature can, of course, be left to the charging station quite safely providing you choose a, good one.to handle the work; Accumulator charging-is not the simple task tliaF many people think, •and to place a battery in the care, of a charging station that lias no really qualified man to guard its interest is generally asking, for trouble; Most of the large radio dealers have .now installed' charging plants, where all makes of batteries are charged at reasonable prices, and advice of an expert nature is also available from’ the man in charge of the department. The tops of accumulators should be kept perfectly clean and free from acid. Batteries frequently come back from the charging stations with their cases quitewet with acid. This |s due to the fact that during the final stages of charging gassing of a fairly violent- nature occurs. The terminals should also be kept clean, and a light film of vaseline smeared over them now and then to prevent the acid attacking them and causing corrosion. Accumulators, even when they are protected with wooden-cases, should never lie placed directly on the carpet. The acid creeps in a ferocious manner, and attacks almost everything with which it comes into contact. It has a particular liking for marble, linoleum, carpets, or wooden shelves or table tops, and appears to take a delight in destroying them. Another point is that the funies given off from accumulators whilst. on charge are both corrosive and inflammable. Never take a naked light near an accumulator in order to read the density. Remember, accumulators manufactured for broadcast i reception are not generally provided wjtli i such strong plates as that of a car bat- I tery, hence they need much more care than : the car battery. If you have no hydro- ! meter, buy one to-night; it will repay you ! with the extra service and longer life that your battery will give vou. The hydrometer is the only reliable'test of an accumulator, and you can safely obev its teachings.' RESISTANCE COUPLED AMPLIFIERS. Although resistance coupling is generally looked on as a reliable mans of obtaining pure reproduction in an amplifier, there are points tq.be observed in order to ensure that this desirable -quality is fully secured. The loss of low notes is avoided to a great extent by a choice of suitable coupling condensers and leaks. A more frequent trouble is the loss of high notes. Stray capacities in the amplifier circuit contribute to.the loss of high mote amplificatiou. These capacities are between the plate and filament of the valves, between the plate resistance and its holder, the joint grid-filament capacity of the following valve and across its grid leak and holder, and finally the grid-plate capacity of this valve. The total of these stray capacities has the effect of a bypass condenser of appreciable value, and according' to the frequency of the note supplied by the signal has more or loss effect upon the amplification. This can I be remedied by keeping the value of the resistance as higb as possible—that is, I large compared with the internal resistance of the valve. As the frequency rises, j the stray capacities offer less impedence to the total ■ voltage, so that an increasing proportion is short-circuited or by-passed, so that the higher .the note the less it is 1 amplified. In consequnce of this, an ompli--1 fier, in which the amount of unwanted • capacity is large, will greatly weaken or even tend to cut off the higher note*.

and ( the general effect ■will be low-toned or mellow',” which is often preferred tor musical items. But in the reception or speech the lack of high notes is appreii. «yident, the consonants, particularly the e, being slurred and difficult to distinguish. It is thus seen that if the internal resistance and the plate resistance are increased, the stray capacities remainmg_ at their original value, then amplification of high notes will be further decreased, and any unavoidable increase of stray capacities must be balanced by a decrease of the plate resistance and the internal resistance of the valve. It is usual to recommend a .1 megohm resistance for all plate couplings, but in many cases this value will be too high for the resister coupled to. the detector. If less than 180 volts B is available, then this ralue may' be as low as. .02 meg. The best way to obtain maximum efficiency in this position is to use a variable rsistance •n place of a fixed one. Similarly, the -3 meg. . usually recommended for the grid circuit of the power valve is higher than necessary, except in the case of the 112 and similar types of valves. A convenient way of finding the best value is by using a variable resistance of good make, capable of carrying the comparatively heavy current without noise or* change of value. Generally speaking, it is best to try from 25,000 to 100,000 ohms by means of variable resistances.