THE WIRELESS WORLD.

Otago Daily Times, Issue 21359, 12 June 1931, Page 5

THE WIRELESS WORLD.

NEWS AND NOTES.

By Magna Vos.

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 Tuesday of each week for insertion on the following Friday. j Metres.

ANSWERS TO CORRESPONDENTS. “Reaction,” Dunedin. — (1) The spacing from the tuning coil of the reaction coil and the number of turns on the latter, will be found to be the principal influences bearing on the degree of reaction employed, The greater the turn number, the stronger the reaction, but the greater the distance between the coils, the weaker the reaction. (2) The periodical you mention can be obtained from Driver’s bookshop, George street.

“ Detector,” Dunedin,—lf there happens to be a leak across the valve holder, or on the base of the valve, your detector will appear to work just as well without a grid leak as with one. ,

“ Aerial,” Port Chalmers. —Although the majority of frame aerials are used mainly with the idea of making a set Eortable or transportable, some are used ecause the situation of the premises in which the set is installed precludes the erection of an outside aerial, or because there is some objection to an aerial outside on the score of unsightliness. There is rio reason why an earth should not be employed in such cases, as the question of a permanent connection to the set does not matter. Moreover, an earth connection can be tucked away out of sight, and cannot be objected to because it does not look well. The use of an earth will improve considerably the results from a frame-aerial set.

“ Five-valve,” Queenstown. —I should say that your receiver is operating close to oscillating point, and that the variations of voltage caused by change of load in the power' circuit put it into oscillation. The initial occurrence is probably due to the fact that some time is necessary for it to reach a steady stage. For local reception it should not be necessary to apply much reaction, so that a general overhaul of the r.f. and a.f. circuits should be made. You should also make sure that the grid bias is in good order, and that there is no audio regeneration due to common plate current supplies.

4ZO CHILDREN’S HOUR. WELCOME. The following la the thirty-seventh list of new members who have joined Uncle Ben’s Happy Radio Family !—-Joyce, Eileen, and Robert James, Arthur Saunders, Lily Barnes, Jack Fisher, Alice M'Beth, George Brown, Robert Gordon, Alexander Carruthera, Mary Purvis, Gordon M’Donnld, Betty Francis, Grace Thomas, Hugh Barsdale, Peter Hudson, Muriel Hudson, Dick Harris, Elaine M’Queen, Robert Burns, Amy Peters, Vincent Ralston. CHILDREN IN HOSPITAL. Uncle Ben makes an appeal for comics and toys for the children in hospital. Any member desirous of helping In this way please forward articles to Uncle Ben, care of 4ZO, Octagon, Dunedin. LETTERS RECEIVED. The following members have the thanks of Uncle Ben for letters received: —Peter and Lucy Tucker, Robert James, Mary Bain, Lily Stevens, Alexander Bain, June and Albert M’Leod, Robert Burns. , Kia ora, everybody. UNCLE BEN. RADIO WEATHER CHART. • A new weather information Service, in the form of charts which will be sent by radio, is shortly to be introduced on transAtlantic liners. Such charts will reveal high and low pressures, barometer and readings, direction of winds, fog banks, and other information valuable in navigation. Arrangements have already been made for two charts to be issued daily from London and New York. The London charts will be intended for the guidance of vessels from the United States to Europe, and the New York charts will be transmitted to ships sailing from Hamburg. Southampton, and Cherbourg. Ah endeavour is also being made to develop the service so as to permit of the reception in mid-ocean of replicas of newspaper pages published in London and r«ew York during the voyage. WIRELESS TYPEWRITER. A demonstration with a yvireless typewriter was given at Detroit recently. The typewriter, invented by Mr Glen W. Watson, and known as the “ Wateongraph, enables an operator sitting at a typewriter to type out a message which is automatically and simultaneously conveyed by shortwave wireless to another typewriter at any distance. The receiving, typewriter automatically types out the message as sent. A transmitter for ordinary commercial use can be made to weigh only about 201 b. Speed of transmission is limited only by the skill of the operator, since the typewriter can record 1200 letters a minute. Any receiving typewriter, synchronised with the transmitter, could be tuned in wherever the receiver might be, and the message could be typed simultaneously *on several receivers. If secrecy is desired, it can be arranged that only the transmitter and the required receiver are synchronised. The owner would thus be independent of all public means of communication. The inventor pointed out the usefulness of the typewriter for police work; it could'be carried on patrol cars and used for sending messages to headquarters in absolute secrecy without code.

CHATTERING CONE SPEAKERS. Good as they are, balanced-armature loudspeaker units occasionally develop a form of “ zizzing ” or chattering, the cause of which may be exceedingly hard to track down. One of the most obscure faults which may cause this extremely unpleasant accompaniment of speech and music is a defective joint. In all units the armature takes the form of a strip of soft iron to which the connecting rod '- s fixed either directly or through a lever mechanism. Now lightness is essential in the armature and its associated parts, hence makers usually solder the rod or the lever to the armature. Iron is difficult material to solder well, and what is known as a “ dry ” joint may easily escape detection at the factory. The joint looks all right, and at first feels all right. When the componefit has been in use for some little time the vibration to which the armature is subjected causes the joint to give way, with the result that the lever of the connecting rod itself becomes loose in its seating. “ Zizzing ” then sets in, growing steadily worse as the looseness increases.

HUM IN A.C. SETS. A cause of hum in an all-electric set may be a defective rectifier tube or unit. Where a separate B eliminator is used, hum is often caused by locating this accessory too close to the set. In many cases where hum is due to a poorly filtered eliminator, the addition of extra filter condensers externally will greatly improve matters. Having electric lamps and wiring too close to the set sometimes causes hum, as would a defective dynamic speaker, which is usually remedied by adding an A filter condenser to the field coil of the speaker. If the aerial is too large, stations will conflict with each other. Make the aerial as short as will provide you with satisfactory volume from the stations you ordinarily wish to receive, Where the volume is too great, tone quality will not be good. Another remedy for this is to use a series condenser between the aerial post and the aerial. Where you live within a couple of miles of a powerful broadcasting station you will have'considerable difficulty in listening to other stations. A wave trap will be needed in such cases. Often the broadcasting company recommends these devices, especially within a limited area of the transmitter, so that such listeners may enjoy better radio reception. The improved band pass filter type of wave trap, •with two tuned circuits, is especially effective.

THRESHOLD HOWL. A very common fault in short-wave sets is what is known as threshold howl. Here the set cannot be kept just short of oscillation, for, it is made to oscillate and the reaction is then slightly reduced, it misbehaves in the most unpleasant way. Sometimes there is an actual howl; some-

times a kind of grunting noise is heard; sometimea the sound is more of a chattering noise. The reason in all cases is the same —the set is wobbling into and out of oscillation. If the wobbling is very rapid, a howl results; less rapid wobbling leads to grunting or chattering. One of the prime causes of threshold howl is- an unsuitable positive potential on the grid of the detector valve. Actually the amount of positive bias required for the most effective rectification is usually not the same as that needed to obtain the smoothest reaction control. One has to effect a compromise, so adjusting matters that the best rectification is obtained consistent with smooth reaction control. The simplest way of doing this is to make use of a potentiometer. - The grid leak return is taken not to low-tension positive, but to the slider of the potentiometer connected across the filament supply, and a condenser with a capacity of about 0.002 mfd. is wired between the slider and earth. The use of a rheostat for the filament control of this valve is also desirable. Modern valves are not so finicky about their filament potentials as were the old bright emitters, but when they are used as detectors in short-wave sets a means of adjusting the filament potential to a nicety is, as a rule, a very distinct advantage. If the set suffers from threshold howl make use of the potentiometer and the rheostat. Start first of all with a comparatively low plate potential for the detector, and with the potentiometer slider near the negative end of its travel. A little experimenting will show the filament temperature which gives the best results, and the listener will be able to discover by gradually raising the plate voltage and making use of the potentiometer what conditions allow the valve to work most effectively. If reaction is fierce or erratic, or if threshold howl cannot be cured by the methods already suggested,, then another cause is at work. High-frequency currents are making their way through into the note-magnifying end of the set. This is what happens: The B battery is common to the plate circuits of all valves. This battery has a certain resistance, and across this resistance high-frequency potentials will be set up. Back-coupling is produced, therefore, between the plates of the valves. To make matters worse, the telephones are in the plate circuit of the last valve —and the telephones are worn by the operator. He may thus find that any movement of his head near the set will affect the tuning. The reaction control also may become fierce owing to the fact that there is a secondary reaction effect due to the backcoupling. There are indicated two methods, which, either singly or in combination, are effective in preventing highfrequency leakage, and back-coupling effects. The first method is to use not one but two high-frequency chokes in the plate circuit of the detector valve, and to wire a condenser, which may have a capacity not greater than 0.0003 mfd., between earth and the junction of the two chokes.

7ZL. Hobart , .. 535 3AR, Melbourne .. ., 4YA, Dunedin . .. 463 2FC, Sydney . .. 442 2Y A, Wellington .. . .. 420 4QG. Brisbane .. .. . .. 385 3LO, Melbourne .. .. , .. 371 2AB. Sydney . .. 310 2KY. Sydney .. ., . .. 280