RADIO AND ITS RECEIVERS

Sun (Auckland), Volume III, Issue 729, 31 July 1929, Page 16

RADIO AND ITS RECEIVERS

Conducted for THE SUN by

R. F. HAYCOCK.

RENEWABLE VALVES

LATEST INVENTION FILAMENTS REPLACEABLE j Listeners who have had the annoy- : ance and expense o£ buying new ■ ; valves, because by some mischance . j they have burned out, will welcome | the news that a new valve has been j invented which can be renewed when ! the filament burns out. | A valve displayed at a "scent trade ! show in America has a heating j element that can be withdrawn from | the bulb. When the heater burns j out it can be replaced and the owner does not have to buy a new bulb. He buys only the heating elements. | A small screw is removed from the | base of the valve and the base of the heater element is removed to make way for the new one. THRESHOLD HOWL GRID-LEAK CURE Threshold howling, oscillation point motor-boating, call it what you will, jis more than annoying. It is the j DX-reception man’s Vete noir. j It is, of course, that howl or series of clicks, occurring exactly at the ; oscillation point, which utterly pre- \ vents signal reception at that most j critical and sensitive tuning adjust- | ment. Hence the term “threshold” j howl. j Its particular haunt is in leaky-grid detector sets, with a transformer immediately following the detector stage, and also in ultra-short wave sets of almost any type. What is particularly annoying is that it generally occurs in receivers which de--1 mand the best use of reaction for seni sitive working, and the presence of the threshold L.B\ oscillation prevents one from working anywhere near the i oscillation point. | The cure is generally fairly easy to j effect. In very many cases too much | H.T. voltage will be found the root of the trouble, while if a mains elimin- : ator is used it is possible that failure of the chokes and resistances to pass ! enough current for the whole set requirements can set up the L.F. howl. Another remedy is to raise the grid- ; leak value. For instance, threshold howling is rather prone to manifest ■ itself in a detector stage having a 30,000-ohm impedance valve of the “HL” type, a .0003-microfarad grid capacity and a 2-megohm leak, the ; whole followed by a generously-wound ! general-purpose 3 or 4 to 1 L.F. trans i former. The voltage will be about 60-100, and with the foregoing conditions it is quite possible that H.T. changes over quite a wide range effect no cure without diminishing strength. Alteration of the leak value to 3 to 5 megohms, however, will almost certainly remove the threshold instability.

RADIO AND ’QUAKES

VALUABLE BROADCASTS J MISSING FRIENDS FOUND £ One cannot help being especially J impressed with the great value of . wireless broadcasting when the dif- ] ficult conditions associated with a severe earthquake break down all ordinary means of communication. Every evening after the earthquake special messages were put on the air from 2YA and 3YA, which were of the utmost importance to some individual or section of the community. One of the three passengers in the service car belonging to Newman and Co. which was reported missing after the earthquake was the manager of the Canterbury Auto Eleqtric Battery Service Station, Gloucester Street, Christchurch. The first intimation his family had of his safety w r as received by wireless broadcast from 2YA. A number of similar announcements were made and must have brought comfort to many homes, -which, except for wireless, would have passed through a much more trying period. SCREEN-GRID VALVES NOT A PASSING FANCY The screened-grid valve Is not a passing fancy. Radio valve engineers say that the screen-grid is here to stay, because it Is a powerful amplifier; it is very sensitive and affords extremely sharp tuning so that stations do not overlap if the circuit is properly designed; It can handle great volume without distortion; it simplifies construction, thereby reducing the initial cost of sets; it is immune from line-voltage fluctuations; it does not require many seconds to heat up when the set is turned on. TELEVISION WHO WILL SOLVE PROBLEM? “What is the most interesting question with regard to television at the present time?” asks a writer in the English “Wireless Magazine.” “I am inclined to think it is this: Will the problem of television be ultimately solved by one of the world’s best-known television inventors, or will it be solved by some scientific ‘dark horse’ of whom we have heard nothing at all to date? “It is very wonderful how the world’s television inventors go on working patiently day in, day out, on what must be one of the most difficult' problems scientific man has ever attempted. Don’t you admire the courage and persistence of these pioneers of a new science? “Television experimenters seem to be attacking the problem of television in one of two ways. Either they are trying to transmit the moving image direct, or they are trying to accomplish the same purpose via the medium of a film, similar to that used in the cinema camera and projector. “From all accounts, television is still in a crude state. The received pictures are small and are lacking in detail. There is also a very restricted choice of subject since the objects to be televised must be placed very near to the scanning disc of the transmitter. “Here in England, the latest development has been the use of 2LO and Marconi House for simultaneous television and speech transmissions on different wave-lengths. This may sound a little like progress, but I think most of us for the time being will look upon the purchase of television receiving apparatus as the wireless equivalent of buying a pig in a poke.”