THE WIRELESS WORLD

Otago Daily Times, Issue 21543, 15 January 1932, Page 3

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. 2BL, Sydney.—Bss K.C., 353 metres. 2FC, Sydney.'—66s K.C., 422 metres. 3AR, Melbourne. —620 K.C., 484 metres 3LO, Melbourne.—B3o K.C., 371 metres SCL. Adelaide.—73o K.C., 395 metres. 4QQ, Brisbane.—76o K.C., 385 metres. IYA, Auckland.—B7o K.C., 333 metres Every day except Monday. 2YA Wellington.—72o K.C., 420 metres Every day except Wednesday, 3YA, Christchurch.—9Bo K.C., 306 metres Every day except Tuesday. 4YA, Dunedin.—6so K.C., 463 metres Every day except Thursday. 4ZM, Dunedin.—loßo K.C., 277.8 metres Tuesdays and Saturdays. 4ZO, Dunedin.—loßo K.C.. 277.8 metres Monday and Friday. 4ZB, Dunedin.—loßo K.C., 277.8 metres Wednesday, Thursday, and Sunday morning, ANSWERS TO CORRESPONDENTS. J. A. H., Kingston.—l am afraid you have no redress. You might, however, write to the wireless inspector, Dunedin, who perhaps could offer some solution. S. G., Dunedin.—lf you are using a screen-grid set and have a variable highresistance on hand, try connecting this in the B plus lead to the screening grid, as very often a fine variation of the voltage to this point gives improved results. J. R. M‘B., Dunedin.—The filter choke and condensers, and the voltage regulating devices could be used with any rectifier, but as the new rectifier requires filament current, you would need a winding suitable for this purpose, It would probably be difficult to provide this on the present transformer, which leaves you the choice of a new transformer or a separate transformer for the filament. The existing filament winding could not be used.

“ Voltmeter," Mosgiel.—(l) You should have made your test after the battery had been working for a little while. A B battery has a tendency to recover a little when it is not in use, and your reading would probably be a little misleading. (2) When connecting up voltmeters, ammeters, etc., you should see that they are connected the right way round. They are marked for polarity, and this should keep you on the right track. CONNECTING THE POTENTIOMETER. Many present-day sets use potentiometers for controlling reaction, and for adjusting screen grid potential. It is not widely known, however, that there is a right and wrong way of connecting one up. It should be connected in such a way that the battery switch throws it out of action when this is turned to the “ off ” position. Quite often it is so wired that it is not cut out of the circuit when the switch is so turned. A 300 ohms potentiometer across a six-volt battery passes 20 m. amps continuously. The drain is small, but in time it will run the battery down. Matters are still worse where a potentiometer is connected wrongly across the B battery for adjusting screen grid potentials, for a few mills continuous drain will soon flatten the heaviest type B battery. CURING INSTABILITY. An often unsuspected cause of instability in a receiver is the undue proximity of certain connecting wires; for example, grid and plate leads. This is quite likely to cause uncontrollable oscillation. Sometimes it is found that the wiring cannot be spaced further owing to the disposition of the components, and it may be thought undesirable to alter the layout. In such a case, if some of the more important wires, such as the grid and plate leads mentioned above, are cased in earthed metal tubing the set can often be made completely stable. This tubing should, of course, be of fairly large diameter, or otherwise its

presence will throw a large unwanted capacity across the tuned circuit and render ganging, if this is used, difficult. In this case the internal diameter of the tubing should be at least twice the external diameter of the wire, including its insulating sleeving. Soft copper or brass tubing may be readily obtained for this purpose, or as an alternative metal braiding may be used. For a temporary substitute, ..ordinary tinfoil may be wrapped round the insulating sleeving of the leads which. require screening, and held in place by binding with a length of thin bare copper wire, the free end of which could be connected to the nearest point of the earth potential. OVERHAULING THE AERIAL. When overhauling the aerial remember that the lead-in tube should also be inspected to make sure it is in first-class working order. (Only perfectly clean contacts should be tolerated.) If you use an outdoor aerial it should be fitted with an earthing switch, so that the aerial can be connected direct to earth outside the house when not in use. On no account should an aerial with a condenser wired in series with it be left for long periods “ unearthed ” when the set is out of use, as even in winter an insulated wire exposed out of doors is liable to gather an electric charge (from snowflakes, rain, etc.). If you have a compression condenser on hand and not in use, remember that inserted in series with the aerial lead it is often a great aid in improving selectivity. One disadvantage of using an aerial behind a picture-rail is that it is too close to the wall, the ideal arrangement being an aerial well spac.ed away by stand-off insulators. An excellent means of reducing the damping of an aerial for shortwave work is to connect _ an ordinary neutralising condenser in series with it. By arranging that the down lead from the aerial either dips below the lead-in point, a good deal of leakage due to a wet lead-in, etc., can be overcome. SOME USEFUL HINTS. Correct grid bias is particularly important for _ the_ last valve in a set, and too little grid bias will inevitably shorten the valve’s life and introduce distortion. Although no current is actually taken from the grid bias battery, its paste electrolyte tends to dry up after six months or so, so that the battery should be replaced when distortion appears or its voltage drops. With valves of the indirectly-heated variety, it is usual to obtain grid bias by means of a resistance connected in the cathode lead. Bias obtained by this method is often termed “ automatic bias.” 0n« of the advantages of an anti-

motor-boating device is that it can be fitted externally, if the set is of a type in which it is _ difficult to interfere with the internal wiring. A possible but often unsuspected source of distortion is the allowing of grid bias flexible leads to run close to other leads in the set. Be extremely careful never to connect up a grid bias battery the wrong way round. Even the momentary application of positive bias may do considerable ham to a valve. Programmes can_ sometimes be prevented from clashing by connecting & .0001 rafd., or similar small fixed condenser, between the aerial terminal and the aerial lead-in. If you are using a permanent magnet moving-coil loud-speaker, be careful not to put a delicate watch near it, for you may magnetise the movement and spoil its accuracy. When one of the loud-speaker leads is connected to earth or filament and the other to a condenser forming part of a choke output unit, this condenser will have practically the full H.T. voltage across it, and therefore it should be of good quality, able to withstand such & strain. Do not run long leads from your set to distant loud-speakers unless you employ an output filter circuit to prevent H.T. wastage. . If two condensers are used in an ordinary choke-output circuit, remember that they are in series with one another, and that this will greatly reduce the total capacity of the arrangement.