The Diagnosis of Radio

Radio Record, Volume V, Issue 11, 25 September 1931, Page 16

The Diagnosis of Radio

By the Technical Editor

~~ iN previous instalments we Kien | Dave outlined the functions of the various components in ; a radio set. This time we Must go a little farther and place some of those components together to build up a circuit. Look for a moment at diagram 1. You will instantly recognise the components. indicated by the theoretical or schematic symbols. There is an aerial, a coil, and the earth. The aeria} is ‘not brought in to.a fixed tapping. but is varied; that is, it can be taken to any tapping upon the coil. We go a stage further with diagram 2, where the aerial is brought to a fixed tap on the coil. Connected across the coil is a variable condenser. Now we know from our previous talks that, when a coil and a condenser are used together in this manner they can be made to accept or reject certain frequencies. If they ave at a value, say, that resonates on 2YA’s frequency, they cannot receive any others, or rather. they offer a high resistance to all other frequencies. You will notice in diugram (8) a dotted line near the earth symbol. If we did away with the earth and took a _connection along that dotted line to our set, we would have a wavetrap. After a moment’s hesitation you will see exactly what a wavetrap does. We adjust the coil and condenser until they offer a very high resistance to the-sig: nals from the station we do not wish to receive-in other words, that ar rangement will pass all other signals except them,,the signals from the in-.. terfering station being absorbed in th rejecter circuit. The next diagram introduces a third coil. We have a coil between the aerial and the earth and near it is a larger coil. This is known as the secondary, for it is not connected witb the primary, or, we should say, nee‘] not be connected with the primary, for some secondaries are connected to the primary at their lower end. Consider ing these two coils, then. we have sketched out a species of transformer As there is no iron in the circuit, the eoils are merely placed close to one another, and this is known as a radia frequency transformer. The coil up at the top is very similar to the aerial coil and is usually placed at the other end of the secondary coil Its function is to take back high fre. queney current into close proximity with the secondary coil. Varying the coupling between this coil (the tickler) and the secondary, we can allow a

greater or less amount of high frequency current to go from the plate of the valve back into the secondary coil. This high frequency current can then be passed through the valve and mag nified. This is known as regenera tion. You know now fhat, when you see three coils, as drawn, a primary and secondary coil is indicated, the secondary being tuned to admit of any

one station, and a tickler coil, whic provides for regeneration. The circuit in question must now be termed nu regenerative circuit. All of the cir. cuits we have shown, provided they are variable, are classed as aerial tun ing circuits, comprise the first section of any receiver. Crystal Sets Wrz are now able to tune the signals to any desired station. It will be remembered, however, that even if we put a pair of phones in this circuit we could not hear anything, because the high frequency current is pulsating too rapidly for the iron of the receiver to respond. It must be slowed down or rectified, and a simple manner of doing this is to insert a crystal in the coil or in an extension of it. By extending the coil to take in the crystal and the phones we are completing a circuit, and the current can now get somewhere. So we have our first simple crystal set. There is a larger variety of crystal sets that all work on the same principle. The difference lies only in the method of coupling the aerial, or, to use technical terms, now that we understand them, in the aerial tuning circuit. Jf the aerial is connected to a fixed tapping, usually a third from the earth end (it is now auto-transformer), complete the circuit ,ig the usual way, we will get the

strongest circuit, but the cireuit will be very broad.in tuning, that is to say, the local station, although it has a definite peak, will come in when the condenser is turned away from this point. {f there are two stations operating in the same town, they will both be frightfully mixed up, so we have to introduce certain losses in order to bring about selectivity (the ability to discriminate between the stations). One method is to use a separate primary coil, or even introduce a third coil, which is connected with the crystal, while the main secondary coil is merely tuned to the condenser, There are various manners in whic): the desired effect can be brought about. In a very simple circuit we could do without a condenser by varying the number of turns in the circuit. This can be done as is shown in figure one. Now that we have outlined the types of simple aerial tuning circuits, take a pencil and paper and draw the following without reference to the diagram. Aerial tuning circuit employing a separate primary and secondary coil, the secondary being tuned with a variable condenser. Now draw a crystal set employing an nerial circuit that does not involve the use of a variable condenser. Draw a three-coil tuner, the secondary coil tuned with a condenser, and having a midget condenser in series with the aerial. When you have done these, look up the diagrams and check "Ip your sketches. Valve Circuits HAVING now thoroughly understood the aerial circuit, let us introduce a valve. We might remark here that in any radio set there are four different circuits, the aerial tuning circuit, the high frequency circuit, the detector eir cuit, and the low frequency circuit. We might re-name two and four, calling them the radio and audio circuits respectively. Two, three, and four differ mainly in the use to which the valves are put, We will deal with number three first, for it is most essential, and, in a simpie circuit such as the one-valve set, or even a two-valve set, follows the aerial tuning circuit. We have traced ihe pulsating current to either end of the secondary coil. We know that if we complete that circuit, passing it through a erystal, we can hear the signals. Now we shall pass it through a yalve_ instead. The top of the aerial coil is taken through a grid-leak and condenser tr the grid of the valve destined to be the detector.’ The grid-leak and condeuser are essential if detection is to take place. We shall not pause at this stage to discuss exactly how detection is carried out, that is rather involved, but, whenever you see a grid-leak condenser hetween the top of the secondary coil and the grid of the following valve yor will know that that valve is the deter tor. If we take the plate of the valve ana continue our circuit as if we had a crystal now instead of the valve, we

shall take it through .the ’phones and to earth, and to the bottom of the sacondary coil. Theoretically the set should function, but in order -to make our valve work we must supply current, to cause electron flow. A battery is put in the circuit between the ’phones and the earth. The presence of the battery will not make any difference to the high frequency current completing its circuit. It will go through the battery as if it were not there, Not only do we need a high voltage battery to energise the plate of the valve, but we need a low potential buttery between the filaments in order io cause the electrons to flow fron Lhe filament to the plate, through the.§rid und so bring about magnification of the signals. The battery is connected ay js shown, and it is usual to connect the negative terminal of the B battery to the negative or positive of the A battery to complete the circuit. These points have already received attention when we dealt with the valve, Now we have our one-valve set complete. We have an aerial tuning cir-

cuit. which collects the signals, we have a detector and phones. Because magnification takes place within the valve, the signals received in this @nevalve set will be stronger than thase from a erystal set. If we want slightly stronger signais

we can use regeneration. You will remember.what regeneration is--the feeding back of certain of the high frequency currents running in the plaic circuit of the detector into the secondary coil. This is brought about by a third coil near the secondary or grid coil. There must be some method of controlling the amount of energy sent back. We can either vary the relationships of those coils. known as the swinging coil method, use a variable condenser, and vary the voltage on the detector yalve.. These three different methods have their own particular aé vantage. \@here are other methods of causing, 4 valve to detect. One, the anode bend ar power detector, is possibly the next best known to the grid-leak and covwdenser. Detection is brought about by applying a negative voltage to the grid of the valve, instead of by putting a grid-leak and condenser in series with it. In the low frequency circuit a third. or grid, voltage is applied to the valves and we employ exactly the same circuir for the detector except that a ver) much higher grid voltage is used. The { circuit is illustrated. A third method of detection, onc which seems to be coming very popnlar, and which possesses many advantages, is the power grid type. This is really a combination of the power detection and grid-leak detection, incorporating certain variations made in the constant,

The Low-frequency Circuit. [ we wish to strengthen the signals which have come from the detector, we can do so by employing more valves. It will be remembered that the current flowing in the plate circuit of the detector is of two kinds, direct from the battery’ and audio, or slow pulsating current from the valve itself, that is, the rectified a.c. This rectified a.c. current must somehow be got to the grid of the next valve. Now there are several ways that we can get at this. In the first case, unless we employ a special circuit, we cannot allow-a (direct current flowing in the plate circuit to get on to the grid. That would cause a frightful hum and nothing else, so we must uSse‘a combination of stoppers. Probably the most popular method is by using a transformer. As the current is pulsating slowly for audio frequency current, an iron core transformer can be employed. The transformer has the advantage of stepping up the voltage as well as supplying an excellent means of coupling. The primary coil of the transformer is connected in the plate cireuit, and takes the place of the phones. If another coil is placed near this, the pulsations will be picked up.

if the second coil is larger than the first they will be magnified to a certain extent. This is exactly what we want. There will be no direct current in the second coil, but audio current only. [f we connect one side of this to the grid, and the other to earth, or to earth via a small battery, we shall complete the grid circuit, and be getting the signals on to the grid of the first audio valve.

Another method of coupling the detector to the first valve is by resistances and chokes. In the first place a blocking condenser is placed between the plate and the detector valve, and the grid of the audio valve. The blocking condenser will pass the audio frequency current, but stop any direct. Now the direct current must reach the plate of the detecter valve, but audio current must be prevented from leaving the plate circuit by this path. This is simple enough, knowing what we do about the use of chokes. If we put in an audio frequency choke, the direct current can pass, but not the audio current. So, the direct current reaches the plate, and _ the audio current, the grid of the next valve. The circuit must now be completed. Inside the valve the electrons eross the vacuum, but externally connections must be made by some other means, and a circuit somewhat akin to the plate circuit of the preceding valve is employed. We must stop the audio frequency current leaving the grid, but must pass what little direct current flows in the grid circuit. This can be done either by using a very high value resistance or a choke. ‘This method of coupling is usually referred to as resistance capacity.

High Frequency Circuit WeE have explained how magnification takes place after the detectur valve. An even more satisfactory method is to magnify the signals before they are detected, that is, while they are pulsating at a tremendous rate (at the radio frequency). A valve is employed in almost the same hookup as for the audio stages, but seeing the current is at radio frequency iron core transformers cannot be used, and go they are replaced by radio frequency transformers, or ordinary coils. We have our aerial tuning circuit connected with the grid of the first valve. Notice there is no grid-leak and coadenser in this cireuit and no high grid bias. High frequency pulsations will jump from the primary to the secondary coil and so on to the grid of the following valve, which may be another high frequency stage, or the detector. Thus it will be seen that the principle of operation is the same as in the Nudio stages, . Analogous to resistance capacity coupling is the tuned anode system, -hown in the accompanying sketch. It will be seen that the direct current gets to the plate of the valve through a radio frequency choke. The eurrent cannot pass through this choke, but can pass through a condenser and so on to the grid coil of the next valve. This method is particularly suitable for the sereen-grid valve.

In connecting the screen-grid valve in any one of the circuits mentioned the usual thing is to connect up all the four terminals we have previously commented on to grid, plate, and the two filaments in exactly the same way. The extra terminal is the auxiliary grid. This must be connected to a suitable "B" pattery supply. By-pass condensers and chokes are used usually to restrict radio frequency current to their proper circuits and prevent them from wandering around the circuit and causing instability.

We have now studied the four main portions of the set and explained the functions of the principal components. Others can be added as refinements. For instance, we can use a variable resistance to control the amount of current reaching the filaments of the valves. This is known as a rkeostat. It is employed because some valves work better with a little less voltage on them than is delivered by the battery. oo Space does not allow us to deal with much more in this series. In the next instalment we hope to talk a little about the refinements of radio, to ¢xplain the meaning of push-pull, and

to comment briefly on the power pack and a.ec, circuits. If before this time anyone has any further points they want elaborating we shall be pleased to hear from them,