A Corner for Beginners

Radio Record, Volume II, Issue 13, 12 October 1928, Page 11

A Corner for Beginners

By

Observer

The Speaker Terminals. [z sometimes happens that loudspeakers are brought in to the dealer because they have refused to function or have become weak. This is not infrequently caused through the continuous connection of the positive terminal on the set, that is, the one "Joined to the B battery, to the negative pole of the speaker. This reverses the poles of the magnet, and if allowed to stay thus connected is liable to cause the magnet to become demagnetised. Phones similarly have to be carefully treated, while a fall is very liable to injure both pieces of apparatus. These are delicate instruments and need particular care. It may be necessary to add that the positive pole is usually either coloured red or indicated with a -++ while the braid of the negative is usually black and the terminal marked -. Grid Bias.

OST listeners are more or less conversant with what is known as the grid bias or C battery, but few realise really how important it is. When distortion occurs, that is bass or high notes either not reproduced or altered so as to give an unpleasant sound, it is fairly safe to assume that ¢+he ( hatterv is at fault.

The current passing from the plate to the filament causes the grid current to swing, that is, the voltage rises and falls. This swing may be as low as two volts in the first valve in the circuit or about thirty volts in a modern power valve in the last stage. If this swing cannot take place without being bent, distortion occurs. The correct erid voltage is necessary to keep this swinging movement within the preseribed limits. The grid bias is different for each valve in the circuit, but constructors arrange the layout so that only a minimum of © batteries are needed. These should be kept at their correct voltage. When a valve is changed care should be exercised so that the correct grid bias is put on to the new valve. Beeause the old valve worked satisfactorily is no guarantee that the new one will work satisfactorily under similar conditions. A power valve may need as much as 80 volts grid bias: obviously if such a valve replaces one designed to take fifteen volts the result will be far from satisfactory and the chances are that the new valve will be condemned. Spring Cleaning. HIS time honoured custom is not without parallel in radioland, especially when big events are likely to take place. Nothing is more disconcerting than to find the receiver failing to function at a critical moment, but like a

ear, it cannot be relied on unless kept properly clean. No doubt the cabinet gets polished regularly, but how long are the terminals, battery connections. and insulators left without being cleanel? Our advice has been to leave the set alone, but this does not prevent such items as those enumerated receiving constant attention. Seraping the valve prongs is worth while, but care must be taken when valves are removed from the set. Corrugated cardboard, such as comes round bottles, can be utilised to form an effective "safe," but it is advisable to do one valve at a time in order that they may not be put back in their wrone positions.

When the lead-in is not soldered to the aerial, corrosion is certain to take place, causing indifferent reception. Joints such as these ought not to exist. A soldering outfit is quite inexpensive and is easy to use. All joints should be soldered, and those not capable of being soldered (battery connections, valve prongs, switch and rheostat connections) should be periodically scraped. As a general rule connections with batteries need far more attention than all the other joints. Last, but by no means least. test the batteries (A, B, and ©) regularly.

A Suggestion. OR the person who has a set installed by an expert, who wants to know nothing except where, on his dial, he ean get certain stations, the following suggestion may be of value. When purchasing an outfit have included a voltmeter and learn the names of the batteries. Ask the dealer to write down the minimum voltages on all batteries that are necessary for perfect reproduction. Keep them tested, and when they fall too low replace them. It is certain that if this were done the dealers would have fewer "wild goose chases," and "Our Mailbag" fewer complaints. The Aerial and Ground. LISTENER had a somewhat puzzling experience, resulting

from what finally proved to be a very simple cause: The set, which had been in perfect order, began to cause annoyance through reproducing the signals only intermittently. The batteries proved to be in order, the valves and connections were perfect. An expert was summoned, A few minutes revealed the really obvious origin of the trouble. The lead-in ind ground wire passed through a single hole in the wall (which, by the way, is not the best method to effect communication with outside), crossed one another and went to their respective destinations. It was noticed that the insulation had worn off both leads and was causing a short so that the energy from the aerial was running to the ground instead of entering the set. This trouble can be easily avoided by keeping the leads separate and examining them occasionally. Where the

wires pass through the insulators, the insulation is likely, after some time, to break down, and may result in a short, especially in wet weather. Interference from Electric Mains. [tz quite often happens that the city listener-in is troubled with continuous static noises, despite the weather conditions, and the reception of his neighbours. Before the set and radio in general are condemned the relation of the power-lines and the aerial should be investigated. These

high tension lines cause magnetic fields which are liable to be picked up. by a nearby aerial. These fields are relatively weak and are given off along the entire length of the wire. To erect the aerial parallel and close to the lines is simply asking for trouble. If an aerial is erected on a corner section around which the lines run a difficulty is encountered. However, by arranging the aerial, even though necessarily short, as distant from the lines as possible, satisfactory results should be obtained. Usually a satisfactory aerial can be erected at the back of the section; this would be at right angles to the nearest lines and distant from those to which it was parallel.