Questions and Answers

Radio Record, Volume III, Issue 33, 28 February 1930, Page 36

Questions and Answers

A Silk Diaphragm Speaker. I HAVE constructed a silk diaphragm speaker and until lately it has given excellent results. One night, however, when switching the set on, the music was very "tinny" and distorted. The silk diaphragms do not appear to have the same sound when tapped as they did at first. Would weather conditions be the cause of this? asks "Nikau"’ (Hawera). A.: Probably the silk wants re-stretching as it sounds as though it has become loose. Make quite certain there is no strain on the driving reed. If the preparation used for "doping" the silk is not quite as it should be, weather conditions will surely affect the speaker. , 2. Would the Kylectron speaker be suitable for home construction? If so, could you publish the details. _A.: The Kylectron speaker is not suitable for home construction, so for the present at least, we cannot see any possibilities of publishing the details. 8. Could a bell transformer be used in the construction of an "A" battery charger? A.: If the secondary winding will deliver sufficient amperage, it will. Set will not React. I HAVE constructed a tetrode crystal set using regeneration but neither the tuning control nor the reaction condenser make any difference upon the volume. The regeneration is of the

Reinartz type and the tickler coil is wound in the opposite direction from the secondary.-‘H.E.L." (Kilbirnie). A-: Your circuit appears to be quite in order, except that the tickler and the secondary coil should be wound in the same direction. Try reversing the connections, too. You might try also altering the capacity of the fixed condenser between the crystal and the grid of the valve, making it slightly larger. A Vibrator Charger. I AM using one of these chargers, but it has become noisy owing to the points wearing. Can I use any of it for a valve charger, or should I renew the points?-"Charger" (Heretaunga). A.: The transformer can be used for a valve charger, but if the points are readily obtainable we think it would be cheaper for you to renew them. If it is noisy in its operation shunt a 1 mfd, condenser across the points. Resistance or Grid Leak. I HAVE what I believe to be a Bretwood anode resistance or grid leak. The apparatus has a red control knob. Which is it?-‘W.H.Y." (Herekino). A.: The anode resistance. 2, What happens when the knob is screwed in? A.: The fluid is compressed and the resistance is lowered. 8. On unscrewing the knob a sticky black substance is noticed on the thread. What is it? A.: The fluid-an oil.

Volume Diminished. I HAVE a screen grid booster which worked satisfactorily until I made an alteration in the circuit. This was not an improvement, so I, reverted to my old circuit, but I could not get anything like the volume that I had formerly, while the tuning was very much broader. A.: We suspect the radio frequency choke. This is very critical in a booster of this description, and providing you have made no wrong or loose connections, we can suggest little else. We presume that you have tested the condensers for a possible break-down, ex-

amined the pins of the plug-in coils for dirty or loose contact. The circuit is really quite a good one. D.C. Eliminators. I REGRET that you gave such an unsatisfactory reply to my question regarding D.C. eliminators, complains "W.J.D." (Reefton). The points were :-- 1. What are the specifications for a choke for an "A" eliminator? 2. What lamps to use in series when lighting valves from a 230 D.C. mains? A.: Our reply was merely to show the impractieability of lighting the filaments of valves from D.C. mains, but we regret having failed to give you the specifications for a choke. This was an oversight. Here is the problem regarding D.C. "A" eliminators, All the valves if not in series must be equal in fila-

ment characteristics, that is, they must be of the same voltage and the same amperage. If, on this arrangement, one of the filaments blows out, or one of the valves is removed from the circuit. the whole collection will be "snuff ' out." The reason for this is that there will be less current drain and the voltage will immediately rise. The best arrangement is to put the valves in series, but we cannot advise you what lamps to use to break down the current, because we do not have -sufficient details. Supply us with the number of valves, their voltage, and the current drain from the largest, and we shall do the rest. The specifications for a choke depend very much upon the amount of ripple left in the cur. rent. A 10 henry choke should be sufficient, built up of 3000 turns of 22 gauge wire on a 2 x 2 core with a gap of .1 of an inch. Follow the general directions given in the Radio Listeners’ Guide, but specific directions will be supplied in the 1980/31 Radio Listeners’ . Guide and Call Book. \ I~ Short-Wave Problem. I WISH to build a_ short-wave set, using three valves, with provisions for an extra valve. Can you give me a suitable cirecuit?-"A.P," (Oburu). A.: See the all-wave "Round the World" Two. We shall give an excellent short-wave receiver in the new edition of the Guide. Four-valve shortwave sets are not a good investment for constructors. 2. Should the core of a home-made audio transformer be left straight or bent round the coil? A.: Unless you are referring to the hedgehog type of transformer, in which case the wire coil is bent round the coil, we cannot see the point of your enquiry. The stalloy is usually cut so that there is no need to bend it round, in fact, it cannot be done. 3. Can an H.F. transformer be used in short-wave sets? If so, how many turns would be needed for, say, 17 az 100 metres? \ A.: If you require a high-frequency transformer for use in connection with a high-frequency valve, we should not advise you to do this, as high frequency amplification with a triode is rarely successful. See the Radio Receivers’ Guide. Types of Reaction. WILL you answer the _ following points? asks "W.S." (Whangarei). 1. Which type of reaction makes for the most sensitive type of detector? A.: The Schnell or Reinartz method, although differential condenser control, is now being introduced into England |) with considerable success. The dif-" \ ferential condensers, however, are not readily obtainable in New Zealand. When they are, we will discuss them through our columns, 2. In fitting an anti-motor-boating device, is it necessary to apply higher

Questions and Answers FREADERS of the "Radio Reeord" who are in difficulties about reception or set construction are invited to write to our "Questions and Answers" department for help. We particularly wish to assist those who know little about radio, as very’ often there is some ver, slight trouble which spoils completely one’s enjoyment of the pro- . grammes. . Correspondents are asked to observe the following courtesies: 1. Write legibly. 2. Make your questions brief and to the point; do not make apologies for writing, and, where possible, tabulate. Do not ask for a reply by post unless a stamped and addressed envelope is _ enclosed. Even in these circumstances, we reserve the right to answer any question through our columns. 4. Do not ask us to design circuits or send detailed lay-out diagrams; but we can offer advice regarding circuits. 5. Address all technical correspondence: "The Technical Hditor, P.O. Box 1032, Wellington."

‘Ny ps voltage to compensate for the ss in the 25,00 ohm resistance used {in it? y A.: Yes, use a high-impedance valve, and increase the plate voltage. Transformer Trouble. HAVE built the power transformer from the ‘‘Radio Listeners’ Guide," -but the primary heats up’ to such an extent that the. cotton smokes, the humming is terrific, and the core is highly magnetised.-‘D.McW." (Westport). , A.: It seems very probable that the shellac between the laminations is either poorly applied or of an unsatisfactory quality. The only solution we can offer is to pull the whole transformer down and re-shellac the lamin,MMitions then built strictly according to Specifications. You have probably left off a large number of turns on the primary. Anode Bend Detector. I WISH to bias my detector, but do not know how this is done. Could you inform me? asks "W.H.B." (Matamata). A.: The detector, of course, must be biased only when the grid leak and condenser is not used. See an article on this subject in the "Radio Records" dated August 16 and 23, 1929. 2. I have just added a stage of R.C.C. amplifieation, and it is working very well. The resistances are 100,000 ohms and 1 megohm. Could I improve reception by substituting for other r valves? I am using DDL610. ~~ A.: You should use DEH610 for a ‘. stage of R.C.C. amplification, and O.C. coupling condenser. Short-wave Problem. HICH is the most satisfactory circuit for a 3-valve short-wave set? asks "Shorty" (Tuna). A.: Build the "Round-thé-World" Two redescribed in this issue, with another stage of audio frequency amplification. 2. Is the screen-grid an advantage in short-wave work? A.: Yes, we hope to give a suitable circuit in the new "Radio Listeners’ Guide and Call Book." 8. Where can I get the details of a Schnell circuit? A.: The Schnell circuit is fully deseribed in the "Radio Amateur’s HandBook," obtainable from Te Aro Book Depot, Wellington, for 5/6.

A Two-Valve Circuit. WHERE can I obtain the particulars of a two-valve broadcast receiver? asks "H.A.G." (Nelson). A.: Build up "Round-the-World" Two following the modifications mentioned for broadcast reception. A Wave Trap. HERE can I procure a reliable wave-trap for a factory-built receiver? asks "M. McG.’"’? (Nelson). A.: You can either build yourself one from the description in the "Radio Listeners’ Guide" or you can have one built by any radio dealer. . Eliminator Problems. WovuLp you answer the following points regarding an eliminator? asks "A.W." (Auckland). 1. I am using variable resistances for the detector and R.F. Would it be O.K. to use a wire-wound fixed resistor for bias? As the bias value is more critical than the anode voltage it is almost imperative to have the bias variable, so that we should advise the use of the potentiemeter used as a resistance. The value should be 0-3000 ohms. 2. Allowing for the voltage drop in the chokes, and 40 volts for grid bias, would I get about 180 volts from a transformer built to give 260 volts? -Yes. 8. Should B minus be connected to earth when taking grid bias from the eliminator? A.: B minus is earthed at all times, when there is no bias. When there is bias the centre tap is not earthed, for if this were done, the voltage drop could not be obtained. 4, Is there any advantage in using the larger Ratheon valve? A.: For ordinary purposes, no. 5. Is it necessary to have a threewire connector to the power point? A.: The case of the transformer must be earthed to comply with the regulations, and this is most easily done through the third connection of the power point. However, it is not essential to do it this way, and it may be earthed merely by running a connection from the case to the ground, 6. When are you describing the dynamic speaker you promised recently? A.: Owing to the holidays and the reorganisation of the paper, we have not been able to present this as soon 4s we should have liked, but we shall endeavour to get it under way as soon as possible. * Aerial Problem. ' ]D° you consider a vertical aerial of, say, 35ft. more efficient of one of the horizontal type and of 7Oft. in length? writes "Ace" (Wanganui). A.: 80ft. high and 7Oft. in length would be the far better aerial. See our article next week. 2. How do these two aerials compare for selectivity volume, and directional effects? A.: The horizontal aerial will deliver greater signal strength, and will be more directional than the vertical, which wiil have the advantage when selectivity is concerned. Set not Up to Expectations. HAVE just bought a new 6-valve receiver, and cannot get anything like the results I have heard with other sets. I have a good aerial, while my speaker and valves are good. The set will not oscillate.

A.: You are leaving us rather in the dark when you do not state the main type of receiver, for we have no standard to which we can compare it. If you are not satisfied, consult the agent who sold it to you. If it is a neutrodyne it should not oscillate. Aerial Efficiency. PRECISELY what effect do neighbouring earth objects, especially

trees, have upon an aerial?’-‘H." (Wellington), A,: They considerably weaken sige nal strength by diminishing the effective height, but see our article next week, Note: Please supply us with your name and address 4s soon as pose (Continued on page 39.)

uu Quéstions and Answers (Concluded from page 37.) sible. This is essential for office pur-: poses, . Undér-Baseboard I HAVE putchased a copy of the "Radio Listeners’ Guide," and would be obliged if you would supply ine with a diagram of the under-base-board wiring. "C.H." (Linwood). A.: This is a big order, especially as we are now. particularly busy, Follow the directions given and you will encounter no difficulty, An under-base-board wiring is not essential, Recording on Wire. (SoULD you give me full particulurs of ‘the system of recording music on wire appearing in a recent issue of the "Radio Record?" asks "H.A.S." (Wellington). A.: Seé reply to this query on an adjacent page. Selection of Valves. WovLp you advise me on thé selection ef yalyés for my six-yalve commercial set? I have been usin? 201A’s in all radio and uudio stages with PM5 as detector, and I have been advised to use PM5B in the first two stages, PM256 in the last, and PM6 in all others.-"R.G.B." (Wellington South). A.: We regret that your adviee is erroneous, We would suggest that you replace by 201A Radiotrons, wiih & medium power valve such as 256 in the last stage. Do not use PM6 in unything but the last stage of a small receiver. PMS and PMSX are the weneral purpose yalves made by Muilard. With PM256 as power yalve, you. will ‘require about 18 volts bias. Constructing the A.C, Browning-Drake I PROPOSE building the A.C. Brown-ing-Drake as deseribed in the "AU About the All-Electric," and wish you

to answer the following questions cunnected with it-‘A.J.C." (Devonpori'. 1. Is a certain A, B and C. powerpack the most suitable to use for all requirements of the set? A.: We haye found other powérpacks more suitable, including the Pilot and Dongan, 8 2. Would another transformer be required if I added another stage? A.: The only method of adding another audio stage is by resistance capacity coupling and push-pull. which would require an altogether diivcrent lay-out. We would advise ‘you te cum struct the receiver as described, although the alterations suggested "by your diagram are quite permissibie. 8. I intend to use Radiotron or Cuuningham yalyes. What is the lst combination ? a A.: Either of these makes. 227, in all stages except the last, where wa should advise the use of Radiotron 171, PM256, Cossor 6251. or better still, Osram P625, if 250 volts are available. Two-way Communication. T WISH to establish a two-way com-, munication with a friend wlio Hyes ‘ three miles away. How might this be doné?--"RMLA." (Te Kuiti). A.: You both obtain a license to transmit from the Post acd Telegraph Department, and build a radio transmitter and receiver. The license can bé obtained only by those .who have passed a set examination, requiring ‘both theory in radio, and practice in Morse telegraphy. Set dees not Oscillate. I HAVE constructed the threé-valve short-wave receiver described — in the "Radio Listeners’ Guide." but I have omitted the reaction condenser and control. oscillation by. a yarlable resistance. I cannot get the set to oscillate on short wayes.-Bain-boozled" (Dunedin). . A.: Put a fixed condenser of about 001 between the reaction coil atid A, An Amplifier for Browning-Drake. HAT kind of an amplifier should I use with a 2-R.F. BrowningDrake? A.: A separate amplifier must not be used, hut if you wish to improve the audio side, use one stuge of resistance capacity coupling to a transformer stage to a push-pull output stage. jonbees ees -~ eer 9 a . he ‘ * te od