Questions and answers

Radio Record, Volume III, Issue 44, 16 May 1930, Page 28

Questions and answers

"Hi P." (Blenheim) makes reference to ae a query sent in my "Mac", Johnsonville, concerning 4 .wet "B" battery deseribed by "Megohm." He asks where -he might obtain details of this battery. A.: The revised version will appear in the 1980 "Radio Listeners’ Guide," which will be out late this week. " A J.W." {Mangawai) asks if we can ¢ suggest a reason and cure for the static on the YA stations. Reception has been all right in the afternoon, but the static drowns out the music. during the evening. A.: We regret that it is impossible to -eut out static. By using a small aerial or a loop of wire around the room static may be minimised, but only a few of the more powerful stations can be received. " A M." (Gisborne) writes stating that ¢ he wishes to know the types of lamps to use in a D.C. eliminator to reduce 230 volts to 150 volts, 90 volts and 22 volts. A.: The use of variable resistances or a voltage divider is better. The following values will be required :-From 230 to 150 supplying 20 milliamps, 2000 ohms from 150 to 90 supplying 5 milliamps, 8000 ohms; from 90 to 22 supplying 1 milliamp, 4500. This allows a waste current of 14 milliamperes. A good circuit and working particulars are given in the 1980 "Radio Listeners’ Guide." ‘CO P." (Auckland) asks if it would be ® feasible to use a screen grid valve as detector followed by a stage of resistance capacity coupling. A.: Yes, this system is used in modern A.C. sets. 2: Would 20 turns on the tickler be sufficient? A.: You will probably have trouble in eontrolling oscillation if the screen grid ‘valve is used as detector. 20 turns would be about the right number. "R J.B." (Shannon) enclosed a diagram ° of a short-wave circuit and states that he cannot reduce hand capacity, and threshold howl is bad below 250 metres. A.: We are returning your diagram with a few suggested alterations. Other than these we suggest :- 1. Connect an R.F.C. choke to the fixed plates of the reaction condenser and P of the first audio transformer. 2. If condensers have no pigtails it will be necessary to solder a connection from the moving spindle to the frame of the condenser. 8. Use proper double spaced short-wave condensers. @ 4, If hand eapacity is still persistent place two radio frequency chokes in the speaker or phone leads in the output of the set. 5. Put 1 or 4 megohm potentiometer across the secondary of the first audio

to stop fringe howl. Insufficient A battery causes most howl. Advance the rheostat. Do not use a ground. "M. H." (New Plymouth) is also trou- * bled with fringe howl, and has placed a high resistance across the secondary and a 1 mfd. by-pass condenser across the B battery. A.: The grid leak is téo high. It should be at the most 1 megohm and usually 500,000 ohms, A. potentiometer across the first audio secondary is sometimes quite effective. Increase the A battery to the maximum stipulated by the valve manufacturers. Incorrect filament voltage or reducing this only causes more howl by sharpening the tuning. . 2. An acquaintance made up the same cireuit and could get only a loud hum with bursts of oscillation. A.: The grid condenser is either damaged, the grid leak connected round the wrong way, or the grid return broken. Look for an open secondary or grid return. This will cause hum similar to A.C. Cheek from the fixed plates of the tuning condenser to the grid condenser, then from the condenser to the grid of the valvefi and grid leak to A+-. ‘The grid leak has probably been connected on the other side of the grid condenser and A+. "QHORTY" (Stratford) is having trouble with connections from audio transformer. He states: The audio transformer is marked F, P, G, B, while the low-frequency transformer is . marked B-+-P, C-, G. How must the low-fre-quency transformer be connected to conform wi®h the connections of the audio transformer? A.: The terms "low frequency" and "endio" transformer are synonymous. An audio transformer is a low-frequency transformer. The connections are as follow :-F=-C-, G=-G, P=P, B=B +. You will probably have to turn one transformer round slightly to get it in the set if the lay-out is marked for the other. "NEW CHUM" (Dunedin) asks if he can adapt his set to bringing in London and the American stations, by using ‘short-wave coils in his set. . A.: Using short-wave coils in a broadeast set is not as satisfactory as using a special short-wave set or adapter. It would pay you to make up the short-wave described in the 1929 "Radio Listeners’ Guide," omitting the 1 mfd. by-pass condenser shown in the lay-out. An A.C. adaptor will be described in this 1930 "Guide." "TNOUBU" (Hokitika) asks the diameter for the former for the wavetrap described by Mr: Keith in our last week’s issue-3 inches. 2, A well-known kit-set lay-out shows the transformers end to end-should they be ‘at right angles? A.: This is not of paramount importance with modern transformers, as they are usually well shielded. An improved lay-out for this set will be found in the 1930 "Radio Listeners’ Guide." "J ©." (Ongarua) asks if it is possible ‘Js to wind. broadcast coils on valve bases for the. "TIiisterers’ Guide" "‘Screengrid Three." . A.: Yes, from 200 to 350 metres, secon: dary 28 dis.c.. unspaced wire, winding length 2in. Wxtend the base with celluloid if necessary. Primary about 1-5rd

of this, reaction the same as primary. 850 metres to 600, 30 D.S.C. wire, wound 24 inches without spacing. Primary and tickler in the same proportion as before. "QGHORTY" (Cambridge) wishes to build a short-wave set and submits.a diagram for consideration. A.: An’ excellent short-wave set has been described in the 1930 "Radio Listeners’ Guide" which should give better results than the one submitted. It will probably work better without an earth. The turns ratios appear to be about correct. 3: What is the difference in efficiency between valve base and ordinary coils? A.: In actual performance the valve base is slightly less efficient than the others, though their convenience and durability outweigh any small defects. "Ht" (Huntly) wishes to build a * power transformer with an output of 7 volts 2 amps. A.: The primary is 1070 turns of 32 s.w.g. enamelled wire-the secondary 36 turns of 18 d.c.c., core 14 x 14. particulars of methods of procedure are to be found in the 1929 and 1930 "Radio Listeners’ Guides." "QUBSCRIBER? (Cambridge) has built an exponential horn which is not giving him the results he expects. It has a good tone, but is most insensitive. Recently he tried a B.T.H. unit used in the exponential with a 6ft. horn, and the magnification was only half that of a 8ft. horn. Is this a characteristic of long horns? A.: If amplification is not up to expectations either the material or the design of the horn is wrong. The horn should not be straight, but both sides should curve outward, with a long neck near the unit. It is upon this that the amplification depends. Details of an exponential horn will soon be published. 2: The 6ft. horn vibrates more than the 3ft. one. ; A.: Probably something is not fitting or the whole design of the speaker is wrong. 8: I recently introduced an old receiver unit which has a resistance marked at 200 ohms, and has a non-adjustable diaphragm: 8 inches in diameter. It is not very sensitive, probably requiring a step-down transformer. A.:; Yes, use 2 8 or 4/1. a "Ty S.L" (Rosenedth) states that on his ¢ A.B.C. eliminator he has a spare 20-volt winding with which he would like to energise the pot of his dynamic cone speaker, He wishes to know how to construct a Westinghouse type of metal rectifier for this purpose. A.: Such a rectifier would be unsuitable for amateur construction. One could be quite easily fitted and the method has been described in this year’s "Radio Listeners’ Guide." "TUNDERHEAD" asks the following questions with regard to the construction of the 2 R.F. Browning-Drake: . 1. Is it necessary to have a .00025 variable condenser in the audio circuit? AL? The purpose of this is for regenergtion. The swinging coil could be used, but the condenser was found to be more efficient. Could I do away with some of the rheostats either by using one rheostat for all the valves or an amperite for each?

A.: The rheostats are used as volume. controls. Jf you wish to reduce the number put the radio valves on one and use amperites for the other valves, that is, providing they are used with a battery of higher potential than their filament. 8. Would it make much differenceiif I were to place both R.F. stages ‘ih the game screening box? % A.: It would cause instability. The idea of shielding is to separate one R.F. stage from the other. . "H F.B." (Auckland) asks for informa- * tion on band-pass filter coils for short-wave. He asks what would be the approximate number of turns with 28 d.s.c. wire for the aerial: A .0005 condenser will be used to tune the grid of the .E. eoil. A.: Very little has been done. with band-pass filters on short-wave, and the whole subject is a very doubtful one. The object of B.P.F. is to reserve selectivity, and quality, and to deal with problems that do not present themselves when short-wave transmission is being dealt with. About eight turns spaced on all secondaries, and reaction 10 to 12 turns s.w.g. wire close wound, and aerial 4 to 5 turns of 26 should be an approximate value to start with. , on