Radio IN THE HOME

Timaru Herald, Volume CXXV, Issue 18352, 24 August 1929, Page 15

Radio IN THE HOME

(ZL3CJ) There is not a great deal t } comment on during the week. Two stations 4YA and 7ZL (Hobait) are wortny of special praise. *JYA has nw installed the new transmitting apparatus and the increase in volume, and the great improvement in modulation fills a longfelt want. The volume in the daytime is equal to 3YA. Station 7ZL has excellent volume and very steady modulation. The Church of England service last Sunday was as clear as any service the writer has yet heard. IYA and 2YA quite good, but the programmes are not up to standard. 3YA has just concluded a busy week. Volume and modulation quite o.k. Programme fair. 2ZM and 2XF, not as loud as last week, but modulation very good. 2YB and 3ZC, good volume, fair clarity. 4QG and 2GB, from fair to good. 3AR next to 7ZL for modulation. 3LO and 2FC, very good in every way. 2BL not like the old 2BL. The modulation is too harsh, another case of sacrificing modulation for volume. Short wave reception has been more interesting than usual. KDKA, messages to the Byrd Party on Sunday quite o.k. Volume good and modulation steady. W2XAD—Good volume and modulation. W2XAF —Not up to the usual. W6XN—Very fine modulation. VK2ME—Great volume, good modulation. 5SW —Good all round. “Aberdeen calling! The remainder of our programme will take place from 2LO, London. Such was the announcement from Aberdeen last Tuesday. The item heard was a talk on the theory of wireless. The speech was 60 per cent, readable. Other short wave stations have been good, particularly RFM (Siberia). DUPLEX TELEPHONY. During the past two weeks, several countries have been carrying out two way conversations. VK2ME and GBX (Rugby) carried out five most successful tests. The volume from both stations has been quite good, while the speech has been 75 per cent, readable. Although England was slow in carrying out duplex work, she made a success of it. TUNE IN TO-NIGHT. Mr Drayton Venables, of Whangerei, will be singing at IYA this evening. His numbers will be ‘Love of Shadows,” “Bravest Eyes,” “Ninetta,” and “Eentre Nous.” Mrs B. Jellard (contralto will sing “My Ain Folk,” “My Dear Soul,” and “Just Because the Violets.” Other vocalists will be the Behemian Duo, singing Maori melodies. Elocutionary items will be given by Miss R. Speed, and there will be novelty piano numbers by Mr G. Johnson. Bright music will be played by the Studio Orchestra.

2YA has scheduled a very entertaining vaudeville programme for this evening. The orchestral items will include the overture “The ’ Crusader,” a cornet and trombone duet by Messrs Sneddon and Capes, entitled “Watch- > man, What of the Night,” an orchestral novelty “Two Brass Men,” and a musical comedy selection, “Queen High.” The Melodie Four will be heard in songs and concerted numbers, both grave and gay. An item of outstanding interest will be the appearance of Will Haigh and his pal Johnny. This will be a novel ventriloquial turn. Mr L. Ashton, the popular entertainer, will present two humorous numbers. Mrs Mildred Kenny’s Steel Guitar Trio will be heard in several Hawaiian numbers, adding further to an already entertaining evening. A good entertainment of vaudeville type will be presented by and relayed to 4YA. Soprano and tenor vocalists will be Mrs L. 8.. Salkeld and Mr W. J. Trewem respectively. Mr Charles Lawrence will be contributing, also Mr G. Titchener. Miss Elaine Moody’s Hawaiian Instrumental Trio will be welcomed, and Mr Les Grummitt, who has not appeared at 3YA from some time, will play some of his very clever improvisations on the piano. A COMPLAINT. A correspondent has written to “Microphone” to the effect that he can hear an amateur station (ZL3CJ) speaking on his ordinary broadcast receiver, but not during broadcasting hours. Many thanks to this gentleman for bringing this complaint to me. Firstly.—l have never been on during the hours. Secondly.—There is nothing to stop me if I wanted to. Let us continue in straight forward language. It is quite possible for listeners tq hear “key clicks” from amateurs. Such key clicks can be almost eliminated, but if a listener lives within 100 yards or so, all the chokes in the world will not completely cut out these “thumps.” For my part, such thumps will be lessened as far as possible, and also my operating will not be done on Saturday and Sunday afternoons, or between 8 and 10 p.m. Now in conclusion, not only for the benefit of my correspondent, who as stated before is a gentleman, but also

(By “Microphone.”)

for others, it will be well to remember this (and don’t forget it): To receive an amteur on speech say on 80 metres (locally) with an ordinary broadcast set, the latter must be in a state of oscillation. VOLUME CONTROL. In the early days of broadcast recep - tion it was usual to control the filaments of the valves by means of rheostats placed on the panel of the receiver, and these were often employed as volume controls. The reproduction was then so poor that a little extra distortion was not obvious. But constructors have now realised that this methods or toning down volume produces poor reception. If the filaments of the audio frequency valves are turned down, the amplifier will be readily overloaded, whereas, if the control is placed in the filament circuit of the radio frequency amplifier, the stability of the receiver is liable to vary with the volume. For these reasons, the filament rheostat method of control has been abandoned. Many listeners, finding that filament controls introduced distortion, attempted to overcome the difficulty by slightly de-tuning their receivers on strong signals. This method of control has the effect of clipping off the side band of the carrier wave and producing distortion, especially if the transmitter is sharpiy tuned. Furthermore, it was found that, with the increasing number of broadcasting stations operating, de-tuning very often introduced interference from stations on neighbouring wave lengths. Seeking Ideal Control. The next step towards ideal control was the introduction of high value wire-wouhd resistances which could be introduced into various parts of the receiver without producing extraneous noises. These were placed In various positions in such a way as to waste the surplus energy of loud signals. The difficulty is to find the position in the particular circuit 'which will give efficient control without too greatly altering the constants of the circuit. One of the first positions to be tried was across the loud speaker. A variable resistance of about 0 to 5000 ohms connected across the speaker terminals readily controls volume from a whisper to full strength, but the obvious disadvantage of this position is that, if the signals are sufficient to overload the amplifier no amount of control in this position will reduce the distortion. It is therefore, obvious that the control of volume should be placed as near the aerial end of the receivers as possible. S.G. Valve Makes Overloading Easy. Many receivers were equipped with variable resistances across the secondary windings of the audio transformers, but the introduction of the screened-grid radio amplifier has meant that it is now very easy to overload even the detector valve of the receiver so that this position is now no longer suitable for receivers employing powerful radio frequency amplifiers. Furthermore the variable resistance in this position is liable to upset the frequency response of the amplifier. This can be avoided, however, by using a high resistance potentiometer (500,000 ohms) in place of a variable resistance. With the potentiometer, the outside ends of the wire are connected to the grid and grid bias terminals of the transformer, whilst the contact arm is connected to the grid of the next valve.

The same method of control may be employed in resistance capacity coupled amplifiers, in which case the grid resistance is replaced by the potentiometer. In all cases where this method of control is used, the resistance or potentiometer should be placed in the grid circuit of the first amplifying valve in order to miminise the risk of overloading the amplifier at any point. Resistance in H.T. Before the introduction of efficient neutralisation of valve capacity by external balancing and its elimination

in the screened-grid valve, it was the custom to control both stability and volume by means of a resistance in series with the plate supply of the raldo frenquency valves. This method is so little used now that it will not be discussed in any detail". Other methods of control in the radio frequency amplifier have been tried, such as variable resistances across the secondary of the radio frequency transformer, or in series with the grid leads but these all tend to broaden tuning as the volume is reduced, and are best avoided. The remaining positions that have been employed are in connection with the aerial circuit of the receiver. Volume may be adjusted by means of a resistance 0 to 5000 ohms) in series with the aerial lead of the set, but this tends to decrease selectivity. A more satisfactory method is to use a resistance of about 0 to 20,000 ohms across the aerial and earth terminals, or, better still, a high resistance potentiometer, with the contact arm connected to the aerial terminal of the aerial coupling coil, one end of the resistance to the aerial, and the other to earth and to the earth terminal of the aerial coupling coil. Potentiometer Control. A method which has gained in favour recently in single control receivers is to replace the grid coil of the the first raido frequency valve with a variable resistance. By this means a tuning control is avoided, and a volume control provided. Such a receiver may be used on aerials of various sizes without adjustment to the reeciver. The grid resistance can take the form of a 500,000 ohm potentiometer with the sliding arm connected to the aerial, the two ends of the resistance going to the grid and filament of the first valve respectively.

If an amplifier Is to be used with a valve set and its internal connections cannot be checked for any reason, its H.T. negative terminal should not be joined up at all (generally this is not neoessary).