RADIO AND ITS | RECEIVERS

Sun (Auckland), Volume II, Issue 569, 23 January 1929, Page 14

RADIO AND ITS | RECEIVERS

Conducted for THE SUN by

R. F. HAYCOCK

SAVING ON SETS

GOOD PARTS ESSENTIAL jj ECONOMY AND EFFICIENCY . 1 ; 1 In an endeavour to guard against all ~ eventualities, a set designer often lias , to make his set slightly more complicated or costly than in certain conditions it need lie. He cannot know in exactly what circumstances or by whom the receiver will be operated, and he has to do his best to ensure successful results, no matter to what adversity duplications of his scheme are subjected. CABINETS AND PANELS The constructor who cares to use a little initiative can frequently build a receiver for considerably less than a published estimate. But if he is not quite sure of his ground lie should not attempt economies. There is an old saying familiar to all about a ship and a lia’porth of tar. This applies most aptly to radio-set construction. However, let us examine the business in detail. The first requirement in set assembly is a panel and a cabinet. Everyone will know that the quality of the cabinet has no bearing on the working of a set; it is merely a matter of appearance. The most sensitive and selective set yet designed could be housed in a plain deal* box. But in cases a first-class ebonite panel is an important necessity, and the only alternative is a panel of equally insulative material such as bakelite or glass. However, many modern sets could embody wooden (or cardboard, for that matter) panels without suffering the slightest harm. Especially is this the case when all the terminals are mounted on a strip at the back of the baseboard. The kind of set I have in mind is one where the panel carries only variable condensers, and, perhaps, an L.T. switch and the mounting screws or spindles of these condensers all are connected to “earth” (or E.T.). VALVE-HOLDER ECONOMY A metal panel could be, and frequently is, used in such instances; insulation cannot be of interest here except in regard to a wave-change switch should such figure in the assembly. In a four or five-valve set, four or five-valve holders of an anti-micro-phonic character are invariably specified these days, but it might be interesting news to many amateurs that it is, in most cases, only the detector valve that need be feared as a source of microphonic trouble. It is fairly safe to say that money can be saved by using good but cheap valve holders of a straightforward nature for all the other valves. There is a little risk here, I must admit, but the constructor can use his own discretion. Unless the set is a “freak” the valves are very “dud,” and the room in which the receiver is used is a vibrating sound box, the outfit will be as quiet as could be, more particularly if it is stood on some soft material. I have known sets, and quite a few, too, which gave no trouble even although no “anti-pong” holders at all figured in them. In a portable set “sprung” valve holders serve the dual purpose of protecting the valves against mechanical shocks as well as preventing microphonic noises, so in such the specifications should rigidly be followed. DO YOU NEED RHEOSTATS? Filament rheostats are, in certain cases, quite unnecessary luxuries. Especially is this so when the constructor is going to run. 2-volt valves with a 2-volt accumulator cell; even fixed resistors will be quite unneeded. The modern 2-volt valve (one that is actually rated at this exact figure) will operate at the slightly extra current which will be met with when the cell is fully charged without damage occurring or even without any real effect, on its length of life. And the valve will not noticeably decrease in effiicency when the current falls, as with the discharging of the battery. In any case, a rheostat won’t help one to increase the current when the accumulator is only capable of providing a pressure of 1.8 volts just before being recharged. It is only against the overload one is able to guard with a rheostat or resistance, and this will not be of great moment when one remembers one never gets the full battery voltage across the valve; there is always the wiring in the set and the battery leads to supply some extra resistance. In some circumstances rheostats, or resistors, will be essential, such as when two-volt valves are to be operated from a three-volt source of supply. But it may not always bo absolutely vital for more than one “master”, rheostat or resistance to be used. Seldom can it be necessary to have one for every valve. Sometimes it happens that a set which takes a constructor’s fancy incorporates refinements which are really not justified by the depth of his purse. These could without real ill effect be eliminated. But the problem is for him to be able to distinguish what are refinements. REGARDING BY-PASSING I do not consider a loud-speaker bypass circuit ,a luxurious refinement. Even in a moderately small set it is useful in the majority of cases from a reproduction point of view; it safeguards the loud speaker and it makes extension leads easy to run and safe. This part of the circuit usually consists of an L.F. choke and one or two fixed condensers. But in an average set H.T. by-pass condensers are by way of being absolute gilding. According to the book, these condensers are required for eliminating “battery coupling” and smoothing irregularities in the H.T. I do not find them successful in the first respect

(others may be more lucky! >. while, in my opinion, if the H.T. battery yets into such a bad condition that it develops irregularities, it is time it was changed. Anyway, its voltage cannot be what it should be! Such “by-passing" (in my opinion again) should be confined only to mains units. Certainly I have never had any trouble through not takingadvantage of the scheme. And it is surely significant that it usually figures only in the so-called "luxury multi -valver.” QUALITY OF COMPONENTS So far, I have dealt with eliminations, and it is time I had something to say about component quality. There are certain parts vitally concerned with the actual operation of the set on which it pays to spend as much money as possible and go to the highest possible grade of first-class British manufacture. There are others where a slightly less perfect standard can be aimed at without prejudicing the working of the set. Grid leaks do not need to be expensive wire-wound resistances, as they do not have to carry appreciable currents. A reaction variable condenser seldom need have a slowmotion control or be fitted with a "vernier” dial. A fixed condenser used in a series aerial position can, generally speaking, be of the cheapest variety, but not if it is to be used to break the . earth connection when a mains unit is being used. Then a high quality condenser capable of withstanding high pressures is needed, as with the grid condenser used in !R.C. couplings. USING CHEAP VALVES If you must economise in valves, then make the detector your scapegoat. I have used, with great success, half-crown foreigners in detector positions, but seldom, if ever, have they given results equal to good Britishers in H.3?’. and I stages. Don’t economise in respect of an R.C.C. unit or an L.F. transformer; these items are keystones of a radioset assembly. If you must save more money, buy the cheapest possible tuning variable and fit it with a respectable slow-motion dial. The results may be equivalent to those obtainable when a -first-class variable condenser is used. On the other hand, they may not. Trouble might be experienced owing to vanes’ scraping, completely wrong capacity and. therefore, tuning range, and so on. You see, this economising business is dangerous if carried too far, and I commend it only to the discriminating amateur.

TERMINAL TIPS

When you have finished soldering the wires to the terminals of your receiver you usually have to go round them all and tighten up the nuts. The heating of the terminals has softened th-e ebonite, and nuts which you turned tight home are now quite loose. If you have had to keep the iron on one terminal for some time, there will be a sort of raised ring of ebonite on the face of the panel round the head. The ebonite has bulged with the heat. You can avoid this disfigurement by turning the nuts home only just far enough to keep the terminals steady, till you have done the soldering. Then, when the terminals are cool, go round with a spanner and tighten them as much as you like. Preventing Turning Another way of ensuring that terminals do not turn in the panel when the receiver Is finished is to fit them with locking pins. To fit the pin, put the terminal upside down .in the vice. A block of wood underneath will help to support it and to prevent it from tilting sideways. In the underside of the head drill a hole with a fine drill, midway between the shank and the edge of the head. In the hole put a piece of hard brass wire. This should be a tight fit in the hole, projecting about l-Bin. To mount, drill the usual clearance hole in the panel for the shank, put the terminal in, and give it a light tap with a mallet, so that the pin makes a mark on the panel. Centrepunch this mark, aud drill a hole with the same drill as before. When yon put in the terminal, tap it home with the mallet, and Secure with the usual nut.

AN ETHER LAW-BREAKER

There was an interesting report in the papers the other day about the French police having discovered, not far from the Italian frontier, an unauthorised wireless station for which they had been looking for some time. The station was in an uninhabited cottage near Cannes. The police kept watch at midnight and arrested an Italian and a prominent anti-Fascist who had been convicted several tim>* of political offences. It appears that anti-Fascist broadcasts have been made from this station and the authorities attach much importance to the discovery.

TESTING CONDENSER INSULATION

Fixed condensers are worse than useless in a receiver unless their internal insulation is sound. This is specially important in H.T. battery shunting condensers, and those who have electric supply mains available can test any doubtful condensers in the following way. Cut one of the leads to a lamp, leaving the other lead intact, and connect the ends of the cut lead to the terminals of the condenser. The lamp and condenser are then in series, and when the switch is put on, the lamp should not light if the condenser dielectric is sound. Any fixed condenser should be able to withstand safely the 2’20 volts of the mains, so that no damage will result from the test. In the event of a fault in the dielectric, and a consequent contact between two adjacent plates pf the condenser, the lamp will light, acting as a safety resistance, and preventing a short circuit in the mains.