A Diagnosis of Radio

Radio Record, Volume V, Issue 5, 14 August 1931, Page 15

A Diagnosis of Radio

(By

the Technical Editor

(Continued from previous issues.)

Resistances BE all know that the word "resistance," whether applied to radio or not, means that something is hindering something else. We know, too, that iZ we connect certain wires and radio eomponents with a battery, or with some source of current, that current will flow through those wires. If we introduce something that will hinder

the passage. of: that current, we are. introducing a resistance and limiting the amount of current that will flow. Now

we know that if a water pipe is dirty, or blocked, i.e,, if there is a resistance ~ to the flow of water, not only will less © water pass, but it will have less pres‘gure, some of this latter quantity being absorbed in the resistance itself. Now that is exactly what happens in the radio cirenit. j The resistance causes not only a hd decrease in the number of amperes _ which term indicates the amount of ’ gurrent flowing, but also causes 3 certain drop in voltage, which measures the pressure. Everything through which current flows has some resistance, but ordinary wire and certain components have a

very low resistance, and we can disregard their .effect. Other components have a high resistance. here is another class of resistance ch we are going to consider now, and that-is the special ones. These, as the name implies, are devices used purposely to break down the voltage and limit the flow of current in certain parts of a circuit. We divide these into two main classes, fixed resistances and ‘variable resistances. The fixed resistance can be rated at a certain value, whereas the value of the variable resistance can be changed within certain imits. A resistance is rated in ohms, and the number of ohms jn any resistance is given by the amount of current flowing through it, _ divided into the voltage that the resist"anee does away with, or to use a more technical term, the voltage that the resistance dissipates. ‘If a resistance, passing. one ampere of current:breaks down the voltage by one vor its resistance would be one ohm,. In-.other words, R = BH ~- I,

where E is the emf. or voltage dissipated, and I is the current passed. If we have a resistance of a certain number of ohms and pass through it small current, the voltage dropped wiil be correspondingly small, but if we pass a greater amount of current through it there will be a greater drop in voltage. . Now this brings us to an important point. The material of which a resistance is composed carries only a certain amount of current, and if we attempt to pass more current through it than it is designed for, it will become hot and will probably burn out. For this reason every resistance is not only rated in ohms, but also in carrying capacity, and it is important. to see that each resistance is not given more work to do than it is designed for.

Fixed resistances are used in the power pack to break down the voltage for the different uses it has to be put to; in a.resistance capacity coupled amplifier; as a grid leak in order to bring about detection; as a grid bias register to develop a difference in potential (voltage), to bring about bias; and in the filament supply, to keep this at the proper level. Variable resistances are generally of two classes, the rheostat and the potentiometer. The rheostat has only two terminals, the one attached to the moving arm, and one end of the resistance itself, but we can vary the distance between the movable arm and the fixed terminal, and so put in any desired amount of resistance. A potentiometer (that name, is really incorrect) -has three terminals, one at each end of the fixed resistance, and the third is the variable tap. This can be settled at any point between the two. It is arranged so that there is a constant number of ohms between the two outside terminals.. The potentiometer is used across filament supply, so that the "B" current can be brought into the centre tap: as volume control; and to provide a yvuriable return in the detector circuit, Switches A SWITCH. is a very simple piece of radio apparatus, its function being to make and break a circuit. A switch may be one of several types.

(1) Single pole, single throw. This ig a switch which can be operated in one way only, that is, it can either be on or off. (2) Single pole, double throw, in other words, a two-way switch, the set with the moving arm can make contact with either one of two points. Gramophone and radio switches are usually of this type. (8) Double pole single throw. This switch contacts two sets of points at one time without connecting the set together. (4) Double-throw double pole. In one ‘way it connects two sets of two points and when thrown over the other :-way contacts another two points. (5) The multiple stud switch in which a moving arm contacts any one of several different points. These switches were very much used in the early days of radio, but are now not in general use.

Jacks and Plugs JACK provides an easy way of making contact with any circuit.

A jack, as can be seen from the ilustration, is only a piece of metal bent at right angles, called the body, with

a bent spring arranged at a convenient distance from the body, and in such a way that it will not contact it. A plug which provides a means of bringing any outside piece of apparatus in contact with the jack, consisting of a metal cylinder fitted inside a hollow ebonite (Concluded on page 30.)

Diagnosis of Radio

' {Qontinued from page 15.) tube. In the tube are two springs, the wire goes from one to the metal sleeve and from the other to the tip of the jack, which is insulated from the metal tubing. When we put the jack in the plug the tip contacts the bent spring and the body the sleeve, and so, by pushing our phones or speaker tips into the two springs in the hollow ebonite, we can bring them into contact with the circuit of the set. The jack can be arranged so that when the plug is pushed in, two other springs are brought into contact with one another. This is known as_ the single filament jack. The second set of springs cannot electrically contact the first set. A piece of ebonite or other insulating compound pushes the two springs together. If we connected a filament circuit to the two top springs these could be made to act as a switch every time we inserted or withdrew the plug. A single closed jack is such that, when the plug is pushed in, the spring moves away from a third spring and

breaks the contact. This jack conld he used for a gramophone and radio combination. The next type is the double filament Jack. This is really.a very complicated piece of apparatus. When the plug is pushed home several things happen at once, Two circuits are broken and two are closed simultaneously. The use of complicated jacks, such ag these, unduly complicate and hamper wiring. It 1s unwise to use a jack of this nature in order to plug in the first audio stage. If you want to be economical when you are using your set on a local station, arrange the aerial as we have shown in the "Outspan Five" and the "Radiogram" set, so that the radio valve can be cut out. You will find this will eliminate the jacking processes which, by complicating the wiring, quite spoil the set and make it unstable.