Tips and Jottings for the Constructor

Radio Record, Volume V, Issue 28, 22 January 1932, Unnumbered Page

Tips and Jottings for the Constructor

Eliminator Instability. HEN a receiver which is quite stable when working off batteries is connected to an eliminator, it often happens that instability’ results. The most common symptom is that of mo-tor-boating. This: is due to the fact that the eliminator; unlike a battery in good condition, has a high internal impedance. The cure is the introduction of decoupling devices into the anode circuits, and especially into the anode circuit of the -detector. If the eliminstor is designed to deliver voltages not much higher than those for which: the set was originally designed, : however, it may happen that the introduction of resistances in series with the "B" supply reduces the voltages available at the anode of the valves to such a degree that the performance of the set suffers. In such a case it should not be forgotten that it is often practicable. to use a choke-in place of a resistance, and that the introduction of the choke will only reduce the available voltage to a very small extent. Connecting Loudspeakers, S is generally well known,.there is a right and a wrong way to connect loudspeakers or ’phones to a set which does not use an output filter. The lead with the red tracer runnivg through it, or which is marked positive, should be connected to the "B-+-" output terminal. If this is done, the current is flowing in such a way that the field it creates in the speaker winding assists that of the permanent magnet. Connected in the opposite way, the tyro fields are in opposition, and it is only a question of time before the magnet becomes insensitive. Generally, the symptoms of de-magnetisation are decreasing sensitivity and a marked falling off in quality and.clearness. With a well magnetised loudspeaker unit, it may take a long time to reach this state, but. wrong connections are certain to be detrimental. Care of Accumulators. VER ‘discharging ah accumulator will materially shorten its life, Nothing would be achieved by doing xo. because when an accumulator ig discharged its voltage begins to drop very rapidly, and the receiver soon ceases to function. The voltage to which it is usually permissible to drop in each cell is 1.8, and down to this value you wili not notice any appreciable falling off in volume from the receiver. As soon as the voltage drops below this, hovwever, you will find results getting wea ker and weaker, and this should he your signal immediately to disconnect the accumulator and have it charged. Do not wait until results fade right away. An even more sensitive indication that it is charging. time may be obtained from the reaction control. As soon 3%

the accumulator voltage drops even a little you will find that the reaction condenser has to be turned further round to give the same effect as usual. A Drilling Tip. MANY beginners find it. difficult when drilling holes to keep the drill perpendicular, so that the axis of the hole is at right angles to the surface of the material. A punch mark on the surface of your panel ensures’ that the point of the drill will not wander in starting the hole, but when the terminal is fixed in position you may find that it leans to one side or the other. The only cure then is to enlarge the hole so that the terminal can stand upright, and this is a poor makeshift at best, and not by any means a workmanlike job. Until the knack of keeping the drill upright is acquired, it will be found that a small box such as a wooden matchbox placed upright on the panel near the drill is an excellent guide to ensure straight drilling. You will then find it quite easy to look down over the top of the drill brace and to keep the drill parallel with the nearest side of the upright box. If you tilt the drill out of the perpendicular in any direction you will at once detect your error. Safety First. OST amateurs have one or two spare fixed condensers on hand, but not everyone knows that a great many sets would greatly benefit by the addition of a ".001." The idea is that in condenser-controlled reaction sets (popularly called "Reinartz reaction") the reaction condenser should have @ fixed condenser of about .001 mfd. in series with it, to prevent "B" battery shorting. To insert the condenser, first of all examine the connections to the reaction condenser. In most sets you will find that one of its sets of plates is connected to B+ (possibly via reaction coil), and the other set of plates is connected to the filament wiring. All you have to do is to break one of these leads and connect the condenser in series. In other words, cut the lead and take one of the ends to one side of the fixed condenser and the other end to the other side of the fixed condenser, There will be no alteration to reaction effects, ete, but now, if the reaction condenser plates are accidentally shorted, no damage will result from stray anode voltage. Buying "B" Batteries, WHEN buying a "R" battery, remem-ber-that the rated voltage and its size are not the only matters calling for attention. Important as these are.

the question of "internal resistance" must not be neglected. A'battery hav‘ing a high resistance fault will spoil the reproduction of the best set, by setting ‘up coupling effects between the various stages. This usually results in distortion. In addition, the resistance may be variable, and then there are present also all sorts of crackling and -frying noises, making the battery practically useless. It is impossible to- settle the question of "resistance" when. buying the battery, and so only those of high quality should be considered when purchasing. Finally, beware of batteries that show any signs of dampness or of coloured ‘patches, . ~ rs