THE WIRELESS WORLD

Otago Daily Times, Issue 20701, 26 April 1929, Page 5

THE WIRELESS WORLD

NEWS AND NOTES.

By Magna Vox.

Items of local Interest are Invited by Magna Vox" tor publication In Ibis column. It Is necessary that such matter should reach this office by Tuesday ot 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. i/Aj, Hobart ........ 535 3AR, Melbourne 484 - 4YA, Dunedin 463 2FC, Sydney 442 2\A, Wellington 420 4QG, Brisbane 385 3LO, Melbourne 371 2BL, Sydney 353 2AB, bydney 316 2KY, Sydney 280 IYA, Auckland 333 3YA, Christchurch .. .. 306 SCL, Adelaide 395 ANSWERS TO CORRESPONDENTS G. R. M‘K., St, Kilda. —You have certainly damaged the valves. Try removing the B batteries altogether, and leave the filaments of the valves burning at six volts for three or four hours. _ You should remember that connecting two batteries of equal voltage in series doubles the voltage. Whistle,” Mosgiel.—A flat B battery is probably the cause of the whistling in your set. If, however, the set whistles only on stations, you'are using too much regeneration, which, might be caused through too large a reaction condenser, too many turns on the reaction coil, or too high a voltage on the plate of the detector valve. Aerial,” Balclutha.—Evidently your aerial is not sufficiently insulated from your earthed object, which, when wet, becomes a conductor and earths the aerial. If your coils absorb moisture, place an ordinary lighted electric bulb near them and ( let this dry out the moisture. Ray Dio,” Anderson’s Bay.—Even though the receiver is not connected to any accessories using AC power, it will frequently be affected by AC hum, which is often picked up by a lamp or conductor in the immediate vicinity of the receiver, as it is a common practice to place a lamp on top ot the receiver to illuminate the tuning controls. Naturally,.the lamp and_ the cord supplying the AC power to it are in close proximity to the instruments in the t receiver, which, on account of their sensitive nature, pick up any' interference generated in the field around the conductors. In attempting to eliminate the hum, it is best to examine carefully all lighting fixtures in the vicinity of the receiver.

XEW ZEALAND RADIO HANDBOOK I have to acknowledge receipt of the .latest issue of the New Zealand Radio Handbook. Like its predecessors, this usetul little publication is a mine of information not only for the experienced radio man but for the amateur also. For the man who wishes to build his set. details of several simply-constructed sets accompanied - by dearly-annotated diagrams, are included, whilst the enthusiast with more experience is also well catered tor. An . especially useful . feature is a handy wnre table, which gives in tabufi° rm details of every kind of - he set.-hmlder is. likely to meet with in the construction of coils, HF transformers, etc. As a supplement a time conversion chart and a series of t™ l ma rk cr s accompany the volume, which from the first page to the last is tull of interest, -and which should form a valuable addition to every enthusiast’s radio library.

CARE OF THE TOOLS.: . + hat \ ave keen neglected and I ,1° ™st can be cleaned b- any one of the two following methods, both j ch ?. re , Tery effective. , The first method calls for an earthenware dish ffl ed • ctean water. - In this, should DO mixed a small quantity of chemically, pure su phunc acid. Then-the tools arc wrapped in sheet zinc,, such as is used r lining piano cases, and droppeu into the mixture. They should he left until all signs of rust have disappeared when they should be removed and washed well m running water and then dried in a warm oven. The purpose of the use of zinc in this method is to prevent the acid trom eating away the steel. It is only necessary .for the zinc to make contact with the steel in one place. For the second method prepare a solution ma J innn° m J OO ,P art s of stannic chloride and. 1000 parts of water. Add-to this two ?nnn ° f ! art ? rlc aci <l dissolved in another pa , rts of £ w . a ter. Then add 10 cubic onnn lll6^? 3 °t lndl 8° solution mixed in 200° parts of water. Clean the tools thoroughly in petrol to remove all traces of grease and dirt t and then apply the solution to them with a brush. After a tew moments rub the tools briskly with a moist cloth, followed bv a dry one and then polish the metal if desired, and all rust will be removed. By keeping a lump of camphor or unslakcd lime in the tool chest, rusting can be prevented to a great extent. SOLDERING HINTS. Soldej-ing is really quite simple, the main idea being merely to heat the sutwiVch, have , , to be joined together sufficnently for solder to adhere to them. When they are thus warmed,- solder can „ ~ lun across from one surface to another, and on this cooling it will sot as one mass, thereby joining the two surfaces together. The great art is to keep clean both the iron itself and the working surfaces. First of all the iron must be tinned, which is done by heading it m a clear flame (a gas jet is ideal), till it commences to burn with a green name. When this occurs if can be removed and filed till it is bright, and whilst ?A!} 'S ry hot it should be dipped into a un t < i UX . a bob of solder, in a .tin lid. the meKed solder will then run over the clean surface of the iron and coat it with a bright covering of solder, and then the iron is wanned up again ready for business. Whilst it is reheating the two surfaces can be thoroughly cleaned with a file or emery cloth, till the bright metal shows, touched with a little flux and then brought into contact with the hot iron until a thin coating, of solder appears on tncni, also. (In either words, until thev arc tinned also.) When both the snrlaces have been tinned: they are held together, the iron is heated again, and then it is placed over the two surfaces so that it can simultaneously and equally heat 'them. When hot enough the blob of solder adhering to the iron will readily run over the two surfaces and these should be held perfectly steady until it solidines. which it does a moment after the iron has been removed.. At this, stage, and before the joint has had time to get cool, mpe it over with a clean cloth so o 8 to trT nio , vc any tra ce of superfluous Hux Tim heat will have liquified this, and it can be removed easily,and quick!v. but if.it is left for a little while it will get cool and become greasy, and then Ire exceedingly difficult to remove. If however, the above bints are borne in mind soldering will become a very easy and pleasurable occupation.. ■ » ADJUSTING THE AMPLIFIER. The construction of “B” battery eliminator units for the supply of hightension current to the plates of amplifier tubes has been discussed frcelv. Many set owners on changing from battery supply vire disappointed with the results obtained. This is almost invariably due to laiiity. adjustment of .the bias .values npplied to the grid of,the amplifying tubes (says an Australian paper). It will readily be understood that the application of a high tension supply unit frequently impresses more pressure' on the tubes than is realised. For this reason , u s necessary to use a high-resistance voltmeter to cheek the pressure which is being applied to the detector and amplifier plates. In the case of the detector tube the adjustment of the pressure applied should be made until satisfactory rectification’is accomplished by the tube. In the case of the amplifier tubes, proyided the pressure being applied is within the rated capacity of the tube, no adjust* rn° at of the pressure should be necessary; the adjustment in this case should be made to the grid bias battery. In cases where a bias battery is not used, but ft variable resistance is included in the

negative lead, careful measurement should be made of the drop across this resistnnce ,W.» r - dart 0 be certain that the correct C pressure is applied. Resistances used for these purposes are usually of a variety which will pass the total amount of current which is being used by the receiver. A variable tap is arranged so that it may be slid towards tho positive or.negative end of the resistance. This system differs from the “C" battery adjustment, in that, with a standard battery. the operator can be fairly sure without measurement of what pressure is being applied to the grid. In the case of the resistance, adjustment without a measuring instrument resolves itself into pure guesswork. There are two methods of checking the correct bias. The first is the use of a high-resistance voltmeter, which should be of the order of about 1000 ohms per volt. If the leads from this voltmeter are placed on both ends of the resistance a definite pressure will be indicated on the voltmeter. This reading should bo taken with the full load being absorbed by the receiver. If the positive lead from the meter is moved back along the resistance towards the negative lead the voltage reading will decrease until at a point where the two leads touch the reading will be zero. If the positive lead is now attached to the slider which is attached to the grid bias tcnniiial, a reading-can be taken showing precisely what voltage is flowing in the grid circuit for any particular adjust--incut. This can be checked by first measuring the plate pressure on full load, then consulting a table which shows the correct bias for this pressure and adjusting the sliding tap on the resistance until this pressure is recorded. The second method and perhaps the more accurate, is the use of a plate millimeter in the plate cir-tt?l-,i v -^ vo which is being adjusted. With this meter in circuit the receiver should he tuned to a local broadcasting station oud its behaviour carefully watched. When the bias is absolutely correct there should be no movement of the indicating needle of the meter. If the needle kicks either up or down during the passage of loud voice or music, it indicates incorrect biasing, and it should be readjusted until the needle is stable. If it is found impossible to make the needle absolutely quiet, the bias should be so arranged so that the needle kicks an equal distance up and down from the normal reading. When adjusting the grid bias on power tubes which are passing a considerable amount of current, both the plate current and the filament current should be switched off while the adjustment is being made, if the adjustment requires the opening of the grid circuit, .failure to do this may quite easily min the power tube by overloading its filament.

This will be understood- when it is remembered that the whole o£ the plate current passes through the filament during the course of its circulation through the valve. When the grid bias is removed or the grid circuit opened, the plate current rises enormously, and this, added to the current already flowing in tho filament when it is alight, may ruin the filament emission in a very short time. Iso valve is designed to carry an overload for a very long period, and as the opening of the grid circuit for bias adjustment, purposes may quite easily overload the filament with as much as 100 per cent., those readers who are used to valves will quickly recognise that this practice should be avoided. Of the two methods mentioned the millimeter adjustment is probably the more suitable for adjustment to undistorted output. It is well, however ,’ to read the pressure on the plate circuit, so that the operator knows exactly what pressure is available from the high-tension unit at all times. It should only be necessary to use the voltmeter on the' first adjustment, and this gffords an opportunity to borrow' a meter and return it when the adjustment has been finally made.