THE WIRELESS WORLD.

Otago Daily Times, Issue 20643, 15 February 1929, Page 5

THE WIRELESS WORLD.

NEWS AND NOTES. By Magna Vox. Items of local interest are invited by “'Magna Vox ” tor publication In this column. It Is necessary that such matter should reach this office by Tuesday of each week tor insertion on tlio following Friday. ANSWERS TO CORRESPONDENTS. R. B. L., Dunedin. —In all probability, your variable condenser is improperly connected. This would cause the signal to fade when the band is removed from the dial, duo to “hand capacity.” Connect the fixed plates of. the'condenser to the grid circuit. “Eliminator,” Balchitha.—\cs, hut when using a battery eliminator always turn off the H T before the low tension or filament supply and turn on the low tension before the high tension so as to avoid excessive stress being set up across the smoothing condensers. * “ Squealer,'’ Roslyu.—The noises yon complain of in the amplifier circuit arc probably caused by run-down batteries. Test your B batteries with a voltmeter, and if they have dropped to two-thirds of their rated voltage discard them. If storage “B” is used recharge it. If dry cell "A” is used discard old batteries and replace with new ones. If wet A battery .is used recharge if the hydrometer gives a reading below 1.170. G. F. L., Musselburgh.—What you think is static is more probably the result of dirty terminals on your storage battery. These are easily cleaned by washing with a weak solution of baking or washing soda. The terminals should he swabbed with this solution until effervescence ceases. The terminals and battery top may then be wiped perfectly clean with a rag. Besides cleaning the battery, the soda stops corrosive action by neutralising the acid. Ordinary washing will not do this.

A LOUD SPEAKER HINT. If you Lave a powerful receiver and operate a largo loud speaker vou may ■have been troubled from time to time by slight harshness creeping into the reproduction. Perhaps you have put this dpwn to overloading the valve or to some other trouble in the set itself. At the same time, it is quite possible that resonance in some article in the room may have caused it. A cigarette box may be standing on the top of the set and may be sympathetically resonating at its own frequency every now and then. There are all sorts of objects in a room (and not necessarily very close to the set) which might vibrate at certain frequencies from time to time. As a matter of fact, it is highly possible that there will be dozens of articles, large and small, vibrating, although it is only in exceptional cases that these vibrations will be great enough to be really noticeable. Vases and other mantelpiece objects are prone to sympathetic resonance, as also are fire irons, and small panes of glass in bookcases or china cabinets. If you have fairly sensitive fingers such vibrations can be easily discerned by lightly touching suspected objects. In some cases the only cure may be to remove the objects entirely from the room. To obtain perfection of results. many more points besides this one of resonating objects would , have to be attended to in the acoustics of the room, and although many amateurs may not feel inclined to go to the trouble of a patient investigation of all such effects, they will find it of interest to .feel some of the objects immediately adjacent to the loud speaker. They will be surprised to find how much resonating energy can be developed.

. .VALVES AND THEIR LIFE. Many listeners believe that the only way in which a valve can be put out of action is by a filament burn-out, and that, conversely, so long ns it lights ft must be in good order. Actually, while the fact of a valve burning represents definitely the end of its life, this is only one of the things that may happen to it. Amongst the “ dull-emitter ” or low consumption valves which are used now almost universally “ loss of emission ” is most frequent. One or two types can bo rejuvenated by burning the filament, without any plate voltage for some time, but in the majority of valves now made loss of emission means the end of the valve’s useful life just as decidedly as does a burnt-out filament.

Sometimes the elements inside become disarranged and touch each other. The most frequent occurrence of' this sort is for the filament to sag until it touches the grid, which, of course, puts the valve out of action. In this instance, nothing more serious will happen; but should the plate—which carries the high voltage from the “B” battery—come, into contact with one of the other electrodes, it is quite possible that all the valves in the set may be burnt out. However, this fortunately seldom happens. On equally rare occasions the vacuum in the valve breaks down, due to a slight leak in the glass, or to foreign gases having been enclosed in its manufacture, and the valve loses its efficiency slowly. This, too. is fatal.

Care should be taken in removing a valve from its socket that the base and not the valve itself is grasped and pulled or twisted, as the case may bo. Otherwise the base may become loose, with every possibility -of the leads to the leads coming into contact, causing a short circuit. With a little careful manipulation this can often be adjusted, but in the meantime serious damage may have been caused.

One of the'most important services the development of the dull emitter type of filament did for wireless, apart from the’ enormous, economy in current consumption it achieved, was greatly to increase the life of wireless valves. In the early days of broadcasting, when the old bright filament valve was the only type available, the effective life of the filament, if it escaped accident, was about 1000 hours. Owing to the fact that it was necessary for the old filaments to be operated at a temperature dangerously close to the melting point of the tungsten from which they were made, it was exceptional for a valve to “ die of old age.” The tungsten filaments were very brittle, and if they escaped burn-out through a slight current overload, the filament was frequently fractured by slight shocks. It is probable that the average life of a valve used for broadcast reception was not very much more than 600 hours. Since the development of dull emitter filaments, however., the average life of a valve lias been extended to the life of the activating material in the filament. The filaments themselves are much tougher than the old bright filaments, and fractures very seldom occur. The very low-operat-ing temperature eliminates the danger of burn-out, except by an accidental shortcircuit of the high-tension battery. It is probable that the average valve used in a ! broadcast receiver now has a life of from 3000 to JOOO hours, while cases in which valves arc used for 6000 hours and 7000 hours are not uncommon. It is interesting to note that there has also been' a steady improvement in th'e quality of bright filaments used in transmitting valves. The case is known of a large transmitting valve which was purchased more than three years ago which has been in constant operation, frequently under heavy overloads, ever since. Its filament emission and other operating characteristics arc still quite normal.

TRACING TROUBLES. One of tiic mosl difficult faults to run to.earth, though fortunately p is of-rare occurrence, is that due to an internal disconnection in a fixed condenser (states the Wireless World). Should tin plates become disconnected from the terminals or soldering tags there is no externa! indication of the fact, so that the most careful examination of the wirin ' of the receiver cannot disclose the fault, Though it is easy enough, if proper tests ■are applied. to detect the short-circuiting of a condenser, there is no possible test that can be made with direct current instruments that can reveal a disconnection of the type described. Then arc only two nays- in practice in which such a fault is likely to bo detected; one is by reasoning based on the behaviour of the receiver, and the other is by the troublesome process of the substitution of a condenser known to be in good order.. The writer has encountered this trouble on two occasions: the first was in connection with a portable receiver employing the Hart’yv circuit. When first built it behaved admirably, but after it had been in use a few weeks it suddenly refused to oscillate or to give any appreciable sign of life. Practically every component in the set was either tested carefully or replaced by one known to be sound,

but with no result. After some hours’ work in this way the writer drew on paper the circuit of the receiver, and ticked off one by one, every component that had been shown to be in good working order. At die end of this progress the grid condenser alone remained unaccounted for; it had not previously been tested or replaced simply because it was of first-class make and appeared in perfect order, and it was realised that the fault must inevitably lie there. On holding another condenser in parallel with it the receiver made an instant recovery to normal health, so tjjot the presence of the disconnection was immediately proved. The grid of the valve had been completely isolated from the tuning circuit, so that the absence of signals and of -eaction effectS'was very adequately accounted for. Similar effects, but o lea? magnitude, would occur in a receiver employing magnetic reaction by swinging coil if the condenser by passing the telephones or transformer primary should be similarly faulty. nl this case reaction affects would no longer be obtainable, for there would be no path for the high-frequency currents in the plate circuit of the valve, but any station for which reaction was not ‘normally used would be received as usual. In the other case "eterred to above, the condenser in which the disconnection occurred was the shunting condenser acres? the anode, resistance in the plate circuit of ai. anode' rectific-i. The effect this time way that signal strength dropped to perhaps one-tenth of its normal value, and the quality deteriorated badly. It is difficult to describe the sound music uncl&r, these conditions, but in listening to speech the impression was gathered that the speaker was being throttled, ns though ho were wearing a collar, i-eyeral sizes too small for him. Once again a good deal of time was consumed before the fault was located. In b< h cases Mic difficulty of diagn >sis was enhanced b- the fact that the disconnection was intermittent, each of the receivers recovering once or twice during the process of testing, which gave rise to the belief that the his‘ alteration, made had'cured the fault. Results different from those described would follow upon a failure of a fixed condenser in other parts of a set. but enough has been said to draw attention to a form of fault which is both difficult to trace and unlikely to be suspected.