RADIO NOTES

Dominion, Volume 24, Issue 105, 28 January 1931, Page 6

RADIO NOTES

By ‘

“ETHER ”

Query Corner. “Fireworks” says: “My power valve lights up with a curious blue glow. Should I do anything about the matter?” Yes, buy a new valve. The present one is soft. This means that the vacuum inside is faulty. Before inserting a new valve itwould be as well to get a radio dealer to see if there is anything wrong with the wiring. Faulty grid bias, or too high a plate voltage is likely to make the new valve go wrong in the same manner. GRID BIAS Pitfalls in New Method dry cells still useful Thanks to the indirectly heated alternating current valve, there are numerous ways of obtaining the necessary bias voltage for the grids of the various valves of a set. As a rule a high frequency; valve requires about i volt negative bias, an anode detector requires about ten volts negative bias, a grid detector anything from three to ten volts positive bias and the audio valves anything from five to two hundred volts negative, depending upon the set. In spite of the craze for free grid bias there can. be little doubt that, so far as an unambitious amateur is concerned, the old-fashioned dry cell bias is still by far the easiest to install and far the easiest to service. Moreover, there is positively no hum. The main disadvantage, in fact the only disadvantage, is that dry cell bias requires to be renewed every twelve months. Admittedly the set draws no current from the cells, but in time they deteriorate of themselves. Anybody owning a receiver that derives its grid bias from cells should think long and seriously before making any alteration. So-called “free grid-bias” is obtained by passing the plate current across a suitable resistance. Ohm’s Law states that if you pass a current through a resistance the product of the current and resistance will give you the voltage dropped across the resistance. This is actually what is done for free grid bias. A resistance is used that gives the desired drop when the normal plate current is being drawn by the set. The matter is not as simple as it seems at first sight. Unless precautions are taken all manner of undesirable back couplings occur. The liberal use of bypass condensers generally cures this. In the case of anode bend detection, however, nothing seems to cure the trouble. It may be accepted as an unfortunate truth that much trouble will be saved by fitting anode bend detectors with their own dry cells so far as bias is concerned. Those who intend to launch themselves into free grid bias for the first time are advised -to copy their circuit diagrams slavishly. - It is important to obtain resistances as specified in the diagrams supplied. Moreover, if howls and other noises are to be avoided the larger the condenser across the various tappings of the dropping resistances the less chance of trouble when the set is connected up for the first time. HOME RECORDING Interesting Invention PRACTICAL SUCCESS Some interesting experiments were recently demonstrated in Wellington in the matter of home recording. In addition to the ordinary four screened-grid valves for radio frequency amplification thp new H.M.V. receivers incorporate in the audio side a device whereby the purchaser may actually make records of his own voice. A switch on the receiver beside the gramophone turntable can be set to four positions: Home recording, radio recording, record reproduction, and radio reception. It is possible with this receiver either to make personal records or to make records of a broadcast item being received. In the case of recording a radio item it is only necessary to place the switch at the correct setting and let the special recording needle do the rest. The same pick-up serves both for recording and reproduction. While a record is being made the pick-up is fitted with an additional weight. A special needle is used both for reproduction purposes and recording purposes. . . When personal records are made it is necessary to speak into a small hand microphone provided for the purpose. The records themselves are about five inches in diameter, flexible, and provided with spiral groves to guide the special needle. Speech from the microphone is amplified by the audio side of the receiver until sufficient power is furnished to cut the varying frequencies in the record. As soon as this has been done it is only necessary to remove the weight from the pick up, switch on to reproduction, and listen to the result. Although clear, speech is weaker than on an ordinary record. If the record is played on a non-electric gramophone results are decidedly weak, but with elec- . trie reproduction results are passably loud. Some little knack is required to speak at the correct pitch into the microphone. Otherwise this home recording invention is perfectly straightforward. ’B/REAMING HOLES Precautions Necessary When drilling ebonite panels for the mounting of components it sometimes becomes necessarv to make rather large holes, larger than the average size of drill used by the amateur. It is occasionally recommended that a smaller hole should be drilled first and that this should then be enlarged by means of a file or some similar tool. This is quite a practical way of enlarging a hole in ebonite, but there are one or two little precautions to bear in mind. In the first place the cutting should be done gradually, not too much at a time, otherwise the ebonite panel may crack. In the second place if one attempts to enlarge a circular hole by means of a flat tool there is a curious tendency for the hole when enlarged to be more or less three-cornered or triangular. If you try to enlarge a circular hole by tnenns of a triangular file the hole tends to come out with four corners or roughly square. This effect is’ more pronounced when enlarging a hole in thin metal sheet such as tinplate, but It also occurs with ebonite. The best way to avoid it is to do the enttin" very gradually nnd not to exert too much force. At the same time the ebonite panel may be gradually shifted around into different positions. In this Way and with a little care it is quite possible to enlarge the hole, keeping it circular and at the same time concentric with the- original position. Incidentally, the taper of the tool should not be too slight, otherwise there will be a tendency for the tool to jam in the hole; on the other hand, if the taper is too pronounced'it will make a tapered hole instead of a parallel one. This latter effect can to some extent be overcome by using the tool alternately on opposite sides of the panel.

RADIO BRINGS HELP Mawson Expedition Incident A report by radio has reached Amalgamated Wireless from Sir Douglas Mawson’s Antarctic Expedition stating

that the wireless operator on Mawson’s ship, the Discovery, succeeded in locating a whaling vessel by means of his wireless direction finding apparatus, when the expedition was in need of coal. The wireless equipment carried by Dr. Mawson’s ship was specially fitted with direction-finding apparatus for just such eventualities as this.

There has been a notable movement lately in the direction of fitting vessels on the Australian coast with this type of gear, mainly in order that the commander of a ship at sea may always ascertain his exact position. The Adelaide Company’s new motor vessel Manunda and the new Huddart, Parker liner Westralia, were the first equipped with direc-tion-finding apparatus. The directionfinder picks up the precise direction of a

wireless signal, so that by taking two or more bearings from different radio stations (or ships), the captain of a vessel can determine his position irrespective of fog, rain or heavy weather. ' Last year it was agreed by the International Safety of Life at Sea Convention, which met in London, that the fitting of direction-finding gear should be made compulsory on passenger ships of over 5000 tons'after July 1, 1933.