Questions and Answers

Radio Record, Volume VI, Issue 2, 22 July 1932, Page 10

Questions and Answers

_that recently publis e J L. (Wellington).-Where could I ob- * tain a circuit for a shortwave adaptor, to suit a six-valve super-het.? A.: Generally speaking, it is not easy to get a home-made shortwave adaptor working in front of a super. het. set. However, in the 1931 "Radio Guide" we described a suitable super. het. adaptor, which you could use. "MicHty ATOM" (Auckland).-We have a "B" battery eliminator and trickle charger. There is a jar containing acid mixed with water belonging to the charger. What type of acid is this? A.: You should have told us the name of your "B" eliminator. If it is a balkite charger a solution of sulphurie acid at a strength of 1200. ‘This holds good. only for this type of charger, however, Our set is a six-vyalve commercial. Could two cone speakers be run off it at the same time, and would they have to be in series or in parallel? A.: Yes, you could run two speakers quite conveniently. The best plan would be to put them in series, with a switch across one so that-you could cut it out and operate from the one speaker only. 83. I have heard that a good three-valve shortwave set will receive all the stations that a four-valver will receive, the only difference being that they will be at less yolume. Is this so? A.: A four-valve set can put them on a speaker, whereas with a three-valve set this in some cases would not, be satisfactory. This latter type of shortwave set is one of the most popular types of shortwave receivers, "PLOTTO" (Cheh).-I wish to build the Multi-mu and Pentode Three deseribed in the "Radio Guide." If using a power pack which gives 220 volts, what would be the altered value in the voltage dropping resistance? A.: Your best plan is to make the set up as described, using the same value resistances and connect the 220 -volts where shown. You will then get proportionate voltages on all the valves. 2..To save space I wish to wind oneinch coils instead of two-inch. Are my specifications correct? A.: Dhe best plan would be to wind the coils as follows: Make two secondaries identical, each being wound with 30-gauge enamelled wire, winding 150 turns, or 32-gauge wire, 125 turns. The tickler, in the first case, should be 60 turns of 36enamelled wire. The aerial coil would be about the same number of turns and the same gauge. "QAPRINGBOK" (P.N.).-I have an allwave commerclal set, but I cannot get such stations as Japan and Paris on shortwave. A.: A great deal of practice is neces ¢ary to tune in shortwave telephony stations, and it is necessary to search somew/fiere around the right part of the dial for them. Have you consulted a food list (such as hed by ourselves)? Tune yery steadily and keep your set just on the yerge of oscillation. They will be there all right, unless something is wrong. ; 2. My dial is marked in degrees. Can you give me any idea of finding out how to log the stations I see advertised as operating on a wavelength of so many metres? . A.: The only method of this is by learning the position of certain stations, and from them obtaining the unknown. For instance, you can find out that 2YA comes in on, say, 80, 1YA on 50, 2BL on 56, and so on. Then you know that the station intermediate in frequency between 2BL and 2YA_ would come in somewhere on the dial between them. We publish a log chart which would help you.’eonsiderably in .this direction. Unfortunately for shortwave you will have co learn the positions of the different stations on your dial, and so get a rough idea. of the calibration. _ ,

TUZZLED" (New Plymouth) : Occasionally music stops on wy set and I have to retune one of the When I do this the set whistles and behaves like an infernal machine. A.: It sounds as though it is not neutralised, or that the dial is slipping. Neutralisation is not an easy process if you do not know anything about radio. Hold the vanes of the condenser behind the dial which igs giving trouble, and see if you can move the dial. If you can, this shows it needs tightening up. If it is tight, then your set will have to be reneutralised, and your best plan would be to get someone to do it. T. F.C. (Carterton): Where can I obtain plans for a two or three-valve short-wave receiving set? A.: One of the best sets we have described is the "Night Owl Three," described in the "Radio Record" of November 138, 19381. "LANK" (Hketahuna) : What B bat«: tery wires do I have to disconnect] to stop the set using power? "2. "Are the following valves suitable for my set: A615, PM6, A609, L610, A615? A.: Providing PM6 is used in ‘the last socket, that 1s, ‘the one nearest the speaker. Locate your two speaker terminals -you can do this by following the cords to the set-then one wire goes to a B-+L terminal and the other goes to one valve socket. This should be the one in which the PM6 is situated, and it should have a 7 bias of from 44-9 yolts, 3. Is the set suifable for dxing? A.: It is an old one, but should give quite good results. ORENZO" (Hawera): Volume is good on the lower wavelengths, but poor on the higher, and if I adjust the variable condenser, results are vice versa. A,: Your set is out of alignment. The best plan would be to have it lined up with an oscillator. 2. Do you think the 224 valves want renewing? A.: This is quite probable, as when the valves become old they often tune broadly and cause the symptoms of which you compiatn. COTT" (Wellington): How can the impedance of a transformer or speaker be found when the dic. resistance is known? A.: It cannot. Special instruments are needed to obtain the impedance. Would the impedance of a loudspeaker winding or coupling device be equal to or twice that of the preceding valve? (Continued on page 23.)

Information Coupon (To be used with all requests for information.) Name of set teonceqcececcesevenacs> Model SSCS ESOHOSERES SH ESEEESEEEEDE Name seo ccen sec osenvesecereseere Address desccconssoccnnsaccascoces SPECT OSHSSHHS OSS SERESHSOSSESESOHREBSEHED Nom de plume ..cevsccccseveess To be kept in Ynguiries Please Note:(1) Be specific and brief, tabuating, if possible. (2 Write legibly, and on one side of the paper. (3) We do not design circuits, (3) We do not design circuits, (4) Limit: three questions, unless 1/- is enclosed. (5) Postal queries limit three questions, Enclose stamped envelope and 1/- fee, . ~

Questions and Answers

(ontinned from page 10)

* Av: That point was pretty well ‘thrash"1 ott in the June "Radio Times," where thé point was nade that for tone, the valve should feed into twice its own impedance, but for maximum transfer of energy, into its own impedance. A ¢ompromise js. often made. 3. In winding a power transformer with the windings, as in my diagram, should the astatic winding bé wound over the "primary or under the secondary? A.: It is immatefial. Wind it in whichever manner is most convenient. K G.B, (Taranaki): Can I add extra *valyes to make my five-Valve commercial set more powerful ?-No. : 2. What should be the value of gridleak and condenser for good all-round reception ? . : A.: The grid-leak about 2 megs. and the grid condenser about .00025. . 8.-The makers say; use 100~feet of aerial. .Would the set be unbalanced if I used 200 feet? _ Ai: No; 200 feet would give you broader tuning, with possibly a tendency to weaken the lower wavelength stations. Why not try. a Beverage aerial, such as was described in the "Radio Guide"? [)*464 (Cambridge) : I have had excellent restilts with the "Outspan Five," and nok want to introduce certain alterations:. Would two, stages of r.c. audio instead’ of one transformer coupled improve signal strength ?-No. ; 2 Ciotild I use 14in. former for s.g. coils. A.: Yes. You would certainly not get greater. strength, bit you would be able to make a more complete set. 8; Would potentiometer grid control be 4n improvement? Av: Yes. you would get smoother oscillation. If you want to get’ still further ‘strength, you could add another 1. stage, ganging the condensets controlling the three r.f. stages, leaving the de"Nector valve operating with a condenser of its own. . [Netée.-The "1982 Outspan" he fully Alescribed in the September "Radio Times.’’] MATHUR (Hpsom): If I raise my -- aerial 15 feet at the lead-in end, will . I get an increase in volume? Much or little? A.: Probably a little extra volume. 2, Is it likely to cause less selectivity or more. noises ?-No. 8. Should I shorten the horizontal part accordingly A.: No, the aerial will not be unduly long. The alteration should be worth te .

N 0.3. (Herne Bay): Is it possible to 4N%e ‘gonvert the "Night Ow] Three" to a.c., and, if so, could you indicate alterations and the approximate cost? A.: In the next issue of the "Radio Times" is the a.c. version of the "Night Owl Three," but we are calling it the "Dx Four," although actually it is a thre¢-valve set. 2. Would a Beverage aerial work in the situation I have shown? "Aa? ss, if. you could get about 500 feet. low aerial is not as likely to ick up ‘noises as is a high one, and for this reason the Beverage aerial would be quieter than your present aerial, Such an .aetial has pronounced directional qualities. IX VALVH (Christchurch) : Your circuit should be quite good, but we ‘egnnot see the reason for three. pentode valves in the output. Two in pushpull should be ample and would require but alight modification. Unless the coil. data given you would have to send to Ame: yiea for the ¢oils. The cost, we should imagine, would be fairly high. Including er valves it may run out to Somewhere R BF. PENTODH (Levin): Is the rf. * pentode more efficient than the sereen-grid valve?-yYes. | 2. Are rf. coils, designed for use with

sereen-grid valves, suitable for usé with the rf. pentode? ; A.: They should work quite well, but to obtain the maximum for the rf. penbode the coils must be especially designed. 8. Your circuit appears to be correct. L B. (Christchurch): I intend. build- * ing the "Multi-mu and Pentode Three." Could I gang the tuning copdensers, and if so would the nutibér of turns on the coils be changed? _ ; A.: Yes; you could gang thie Both secondaries should be the same arid small adjustments could be madé to lire up the coils. A révised version will be published in the Septembér "Times." 2. I am making a power-pack for the Multi-mu Three and. will be using. the field of a dyniimie ‘speaker as choke. Will anéther choke be necéssiry? A.: It depends: on how you lay out your set. If you ate licky you will aot need an extra choke, but, if you ,are building a power-pack and intend using the field choke you must allow extra yoltage. If 40 mils flow through a 2500 ohms field, a 100 volts will be dropped. R §. (Auckland): I resiewed two of * my valve sockets, but I am not certain if I put them back the same why. Does. it niatter?. I am using 201A valves. __A.: Yes, it does. It depends upon which valves you took out, but if the set works as well as it did before, then it is fairly certain that you ate right. 2. Can I put an earth on to my set if I use a condenser? I am employing a dc. eliminator. . A.: Yes, but the voltage of thé condenser must be large enough to withstand the strain. : 1334 (TH KUITI): My set pieks up telephone rings. The aerial passes over the telephone. wires, A.: Hither put a greater distanee between the telephone wires tnd the aerial ot move the aerial to such a position that it does not pass over the wires.

FADING (Sydenham): I havé liad my set for more than three years and am still using the original yalves. They still function and the tone-is good, but when I tune the set to the local station and leave it on for about five minytes the signals fade almost out. When the set is again turned to the, right volume it stays like it and then it does not fade. Is this being caused by the valres? A.: We ate inclined to suspect the valves, particularly the réctifying valve. but there may be other causes. DWCOMER (Wellington): Could an a,¢, shortwave adaptor be used on a seven-yalve broadcast receiver without having to resort to the use of batteties? A.: Yes, there are several converters available which can be used between the aerial and the set without batteries. An a.c. converter was described in the 1931 "Radio Guide," t is rather difficult, however, to get one’ to work with a superheterodyne set, dnd your best plan would be to buy. a special adaptor. W 8.0. (Wellington): Can a 5-~1 * audio transformer be altered so as to male the ratio 3-1? A.; Yes, but’ care would be required. 2; Using-a crystal, an W415, two 406’s in pushpull, where is the-correct positian for a tone control? .. A.: Between the plates of the valves in pushpull. Use a .04 condenser in series with a 100,000 ohms variable resistancé, connecting one end of the resistance to one plate and the cther side of the condenser to the ‘other plate,

SPARKS. (Hawera): Results with the ; "Sparrow Hawk" One have been disappointing, Amateur stations are reedived on thé 40 metres coil, while on the 80 metres band stations from 200300 metiées come in, Why is this? A.: Are you certain you are using the . series condenser which reduces the effective capacity of the tuning condenser? Check the arrangements and make cer= tain that thé nioving plates of the tuning condenser are coniected to one side of h .001 fixed condenser, and the other side of the .0001 fixed condenser is connected with earth. Unfortunitely some diagrams in which the two condensers | are shown in parallel are in circulation, This would throw out your twning. GB. (Paranaki)-My eliminator , *. has ample power on the lowest terCan adjust the current between it-and my set? A.: If you wish to use a lower voltage use ‘a lower tapping on your eliminator. You will not be doing any harm by léaving the higher one free. 2. What are the best valves for my neutrodyné battery set? A: Use 221 valves in all stages except the last, where you should use a BG605 class with 12 to 15 volts pias. 3. How can I re-neutralise my set? A.: By adjustment on a coil in serias with the movable coil. We no not know how your set is laid out, nor can we destribe this particultir unit to you. The method of adjustment is by tuning in a station at about the céntre of your dial, and then cut out the filanient sypply of the third rf. valve, that is the one nearest the detector. This can be done by wrapping a piece of round one of the filament legs. Adjust the neutralising dévice uncil thé signals dre at their weakest. Repeat this procedure for the second and third yf, ydlve until the signals are at their weakest, and. the set is now. neutralised. P=®tECTOR (Auckland) -For a copy of the Regulations write to the Registrar, Radio Seryiceman’s "Examination, No, 8 The Terrace, Wellington. He would also give you particulars concerning the examination. Dp*"4w (Wellington):--I am troubled with electrical ‘interference. -If I insylate my lead-in with insulated tape would’ I quieten a certain amount of the noise? . _.A.: No; interference is a problem. You can minimise it slightly hy having your aerial at right angles to the source of the noise, by using a lead-in of armoured cable, grounding the armouring of the cable, and by using a line noise suppressing device. 2. I propose to make a good twisted joint, covered with insulated tape. Would this be just as good as a soldered one? No; it would not be satisfactory. as corrosion would take place after a certain. time. 3. The base of my dynamic speaker becomes quite warm after about 10 minutes’ runhing. Is.this a cgmmon:-thing?-yYes. 4 I can receive KEI but no other American station. Why? A,: That is probably due to your locality, or it may be that your set is more sensitive of the longer wavelength than on the ‘short, although this is not the accepted rule. ;