Questions and Answers

Radio Record, Volume V, Issue 7, 29 August 1930, Page 21

Questions and Answers

'O. M.R. (Lower Hutt): I find the de‘tector stage of my 5-valve set oscillates fiercely. I am using 135 volts B on the plate, and propose to shift the /reaction condenser to place it between the reaction winding and A--. Will this care the trouble. A.:Phe potential applied to the plate is excessive. Reduce to 45 or 674 volts, otherwise place a .1 megohm. resistance in series with the h.f, choke and p. of the transformer. The new position for the reaction condenser will be quite satisfactory. GPARKS (Auckland): I am constructing a coil for a receiver which uses 40 turns of 24 d.c.c. wire on a 3in. former. If a 24in. former is used how many turns will be required for secondary tickler and | primary ¢ ? A.: Generally speaking when the number of turns on a 3in. former is used and you wish to find the number of turns for a 24in, former multiply the number by 1.3 If you are using the coils to tune over the broadcast bands the following will be required: Secondary, 105; tickler, 35; aerial, 15.

2. What value fixed ‘condenser will be used in series with a .00025 to reduce the capacity to .0001? A.: .0015. 8. What value condenser in series bios é, .0005 will reduce its capacity to A.: .001. 4, Where can I obtain the best quality ruby mica and tin foil for making fixed condensers? A.: Try Johns, Ltd., Auckland, or George Wilton and Son, Willis Street, Wellington. We do not know the price. 5. On a. piece of #in. ebonite tubing with a number of one-eighth discs I wish to construct an r.f, choke for the broadeast band. Please state the number and widths of slots. You will require about five slots and wind 200 turns in each. 6. With the .0001 tuning condenser and .00085 reaction condenser please state the number of turns a 1%in. diameter former to suit the following wave-bands: and,

t. signifies turns. 7. What gauges are the enclosed pieces of wire? A.: There was no wire in your letter when it reached us. 8. In my set B minus connects with A minus and © plus. In the adaptor B minus connects with A plus and A minus with C plus. A.: Disregard.B minus when using the adaptor, as B minus is already connected to the set, otherwise separate A minus from the combination on the set and substitute it by A plus. §, Will the 100 turns 30 DSC on a Tin. former be satisfactory for a 10 to 100metre choke.-Yes. F BH. (Dunedin).-I propose to add a ° line voltage regulator to my set. Is this valve used in series with the a.c. leads to the transformer? A.: Yes, you can use either power clarostats or regulator valve ballast 876. These are designed for the 115 volt lines. 2. Would it be an advantage to install this?-No. 3. What tapping should I use-210, 230, or 250? A.: If the voltage is steady use 230, otherwise 210. 4, How would I be sure if the circuit was correct when the wall-plug fits the socket in either way? A.: If a slight hum igs heard reverse the plug. ‘There is no polarity on the a. line, but one way is better than the other. TRICKLE CHARGER (Khandallah). -I have a trickle charger and on connecting a volt meter across the output the needle flashes over to about 11 volts and vibrates vigorously. Is this charger functioning correctly? : Yes; a charger supplies a greater voltage than that required by the accumulator to overcome back emf, The fact that the needle vibrates indicates only that the current is not perfectly smooth and while it is sufficiently smooth for a charger, would not do for an eliminator, The purpose of chokes is

to smooth out this residual ripple. Note. Your request for information concerning the design of inductance coils has been forwarded to "Cathode," who will devote & special article to it. S W. Adaptor (Wanganui)-I am con- * templating making a push-pull bat-tery-operated amplifier for my set (the Could you tell me:1. How to change the diagrams in the ‘Listeners’ Guide" to d.c.? A.: A d.e. push-puli amplifier was described in the "Radio Record" of November 2, 1928. If you wish to work from the a.c. model described in this year’s "Guide"? you will disregard the cathode connection and connect A to B- and +. 2. Would this work without too much noise after the two audio stages in my set? A.: You could use the push-pull part of the amplifier with the last valve of your set as the first stage of the pp. amplifier. You could not use the complete amplifier after the last valve. 8. I bought some new wet "B" batteries. After the initial charge one appeared to have sulphated and does not now hold any current. What is the reason of this, and what the remedy? A.: It appears that there has been an internal short circuit which has ruined your battery. Treatment was outlined in the 1929 "Guide." You will probably have to take the battery to a battery house for expert attention. UNION (Pukekohe): Concerning a sereen-grid all-wave battery set, I should like to know why an h.f. choke is included in the plate lead of the screen grid valve when mine works perfectly without one.

A.: A choke in either or both of the plate and screening grid leads minimises radio frequencies feed back to the battery or eliminator. In some cases due to the characteristics of the circuit, the batteries or the valve, feed back does not take place, and this seems to be happening in your case. 2. Is my set large enough to install a heavy power valve such as DFA 6 or LS 6A.? A.: No. To obtain satisfactory results from these valves you would be required to load them more than your set is capable. The best valve would be a 256 or P265A. Probably P265, with 250 volts would prove a good valve. 8. Would an a.c. power valve taking up to 6 volts work satisfactory from an accumulator? A.: Yes, providing you keep the accumulator regularly charged. Are two smoothing chokes of 20 henries each sufficient for a B battery eliminator to be used on short-wave? You may be successful with these two chokes, but the chances are against you. The smoothing inductance has, usual~ ly to be higher than this, and plenty of capacity is also required to make a successful job of eliminating the batteries on short-wave. OFTIN-WHITE (Canterbury) — I have endeavoured to add a stage of r.f. to the L.W. amplifier, but this has not been successful. A strong hum, which practically disappears when the potentiometer is adjusted, appears. On adjusting the grid-leak regular clicks are heard in the loudspeaker. A.: You are experiencing one of the difficulties with the Loftin White circuit, that is the addition of radio stages. Our

own experiments in this direction, coupled with the remarks made in overseas jour nals, indicate that this is not an easy task. However, we suggest your removing the .02 condenser-and the earth connection. » By ‘taking the screen volt age from the amplifier you are asking for trouble. This must be taken from a separate resistance connected with the power pack. By taking another drain through the resistances provided you are altering their value for the other current flowing through them. Continue your experiments and let,us know your results. It would be worth trying with the gridleak and the condenser removed. ERMINAL (Dunedin)-I found that 4ZL tuned in quite distinctly on a frequency near TZ. ‘Would this be an harmonic, for I understand harmonies ean be only multiples of the fundamenta' frequency? A.: It cannot be, as you suggest, an harmonic, so we suspect that it is a re flected frequency, that is, the waves from the station have been striking a nearby object and re-radiating the natural, frequency of that object. S R.A. (Nelson): Is it possible to buy * short-wave coils for my set, or could 1 make them. A.: It would be bette. for you to buy them, because we do not happen to know the numbers of turns for all the coils. One coil the details were published in the "Radio Record’ a few months back. .. Would the set take another audio stuge? It has already a stage such as described in the "Record." A.: No. Two stages of transformer doubled audio are the maximum, unless push-pull is employed. 3. Would a dynamic speaker run from the mains be satisfactory for this set? A.: Yes, dynamic speakers are now fairly sensitive and are satisfactory on small sets. PUZZLED (Petone): I have’ con- ' structed the "B" eliminator from the 1929 "Guide,’ but it has burnt out

the filament and -the plate on one side of the rectifier. I have tested the transformer and can find no signs of a short circuit, A.: To test for a short circuit obtain a voltmeter and a "CO" battery. Arrange so that the battery and meter are in series and that when the free end of the voltmeter is brought into connection with the free end of the battery a fullscale deflection results. Attach either the free end of the voltmeter or the battery to the centre tap winding and > then bring the other side of the meter into eontact with the laminations and the frame. If there is any reading, then there is a short circuit. There will be a reading slightly less than directl across the battery when the free ter minal is brought into contact with the end of the secondary. By making care ful observations, the drops on each side of the secondary can be compared, and if the centre tap is in the proper position this will be about the same. there is-a difference the centre tap is not correct, and it will be necessary to rewind the secondary. 2. Is it necessary to have sufficient resistance to drop the 280 volts on no load ?-No. 8. Is it necessary that the filament wire of the rectifying valve should be centre- tapped? : Yes, either the mechanical centre through rewinding or the electrical cen: tre by using a 60 ohms centre-tap rheostat and using a centre tap of this. The value (60 is not critical. B.C. (Timaru): I am building the ¢ L.W. amplifier and wish to know if the following coils are right for the radio valves :- Aerial. coil: 80 turns 22 D.S.C. Space 3-16in. Secondary: 80 turns same gauge, 2}in. former, .000375 condenser. Second coil: Secondary 80 turns, primary 40, 36 enamelled close wound and placed inside the secondary coil. Third coil: Same as No, 2,

A.: The primary is possibly a little high. If you remember an article in the "RR." a week or so back, the question of primary coils was discussed. However, you can start with the large number and reduce to a smaller one. Probably the primary on the same former as the secondary would make a better transformer. ‘OVICE (Kilbirnie)-My set has started to give me more static than music. r replaced the valves, but it is just as ad. A.: Have your eliminator tested for it sounds something like a broken-down filter

me ae ED MO ED We El OE ED condenser, ‘Try replacing the grid-leak and examine carefully for loose connections. 2. I used to be able to get the Austraae stations, but now 1 can get only A.: This sounds like a broken-down condenser or defective grid-leak and coudenser. CREEN-GRID (Wellington).-Which is the better pick-up for use with the Loftin White (and _ here three are enumerated) ? A.: We do not know the characteristics of these pick -ups, as we have not tested them. All are first-grade. 2. Which would be the better speakera silk diaphragm or a cone, with an 18in. baifle? A.: The latter, particularly if the baffle was increased in size to 20in. or more. 3. An American magazine says it is not practical to use the screen grid detector if other stages are placed in front. It advocates using transformer coupling following the detector. If this is so, which is the best combination of valves. A.: 224’s should be the best combination, but adding radio stages to the L.W. is a difficult task. 4, Would one stage of s.g.r.f. followed by a 227 detector be as good as a 5-valve Browning Drake? A.: It should be, if everything works right, but you would have some fun in getting it to work. 5. Is it a difficult job to build the L.W. and to keep it adjusted? A.: No, our laboratory model was entirely home-built, including the transformer and choke. It goes perfectly with very little adjustment. Sometimes adjustment in the hum balancer has to be made when changing from radio _ to gramophone, but this would not be so if it were properly shielded. 6. Would strong tin or sheet iron be suitable for shielding?

A.: The latter is to be preferred. 7. If you are publishing a tuner, when will it appear? — A.: As soon as we can get it to work properly. . (CURIOUS (Masterton). The valves have been changed in my | factorybuilt receiver. Is the original plan superior to the adapted one? A.: The designers’ specifications would probably be slightly less sensitive than those now used, but the "B" consumption would be increased under the new combination, 2. Are the valves now used in proper sequence? A.: Providing you do not get feed bac ind audio howling due to over-amplifica -ion on. the audio side of the combination hey would be quite satisfactory. The * Dias on the last valve is decidedly inadejuate ; 12 volts at least should be applied ‘9 B406. |; 8. Are the connections right? > A.: The connection going to fhe first ‘udio valve should be better from the 90volt tapping, that is, the connection between the 45-volt battery and the 223. 4, When removing the speaker plug T frequently get a sharp shock if I touch the transformer at the same time. A.: Your set apparently does not use an output transformer, and although you might get a nasty shock, no harm cau come of it.

4, L.D.X. (Palmerston North). How * many turns are required for valve base coils to cover the 11U to 200 metres band by using a .00015 variable condlenser ? A.: 20 turns of 24 swg. dse. Primary and tickler 45 turns of 20 swg. dse. secondary. 2, How many mils do A415 and three 2OLA’s take? , A.: This depends upon the bias and voltage. Find out these and look up the tables in the 1930 "Radio Guide." . 3. Can a "Formo’’-condenser .Q0085 be made into a .00015? A.: Yes; reduce the number of plates to nine. 4. How many watts does an electric induction gramophone motor and_ the Loftin-White consume? A.: About 25 watts. 5. You recently published a diagram of an adapted Daniell cell-charger. How

ISTENERS must attach this coupon to all queries sent to the Technical Editor (Box 1032, Wellington). Questions arriving without it are likely to go astray or be delayed. Name of Set ..cccceceeceeveccvecers Number Of Valves ..seceeescecncece Name reer errr errr eee eee ees Address see eccee ves eceeeeeseseeees iieeeeeeceserteeeeeeeeesaseeaasteg seers: Nom de plume ...cccevscccvccscesess To be kept in subsequent inquiries. Date ...... te eeeee ce ccecceccccccces Please Note:(1) Be specific and brief, tabulating, if possible. (2) Write legibly, and on one side of the paper. (3) We do not design circuits, but accept suggestions for feature articles, F | Solving trouble, as different from advice, is difficult by correspondence and while letters are given every consideration, answers are not necessarily correct-they are only ; our opinion based on the matter supplied, which may be quite inadequate. Intricate and involved specifications cannot be supplied without a specialist’s fee.

is the wuestone prevented from mixing with wu. distilled water? A.:.The bluestone consists of solid erystals. They are not made into a solution before being put into the cell. » How many turns are required for a 25. henry choke with a core 1#in. wide, 24 in. long, and din. high? Window #in. A.: About 7000 with a .05in gap. W R.B. (Dunedin) : Can I expect good ‘¥¥ ¢ results from a.3-valve screen grid receiver by. using short-wave coils? . A.: Fairly good, but better by using an adaptor. .. 2. About what is the best time for short-wave reception? ; A.: See the short-wave notes pby. Mr. Sellens. 8. Can I bring in Dunedin on the loudspeaker with my rheostat turned right off. Am I overloading any valves? . A.: No, you.are quite safe in doing » 3. I am intending to add another valve to my receiver. How much would it cost me? Will I be able to bring in Japan? Would this valve make a great difference to’ the set. Will it be harder to eliminate Dunedin with a wave-trap? A.: About £2. Yes. Yes. No. 4, What is the best make of "B" battery to use? A.: There is little to choose between the makes. You are using a first-grade one, but it is advisable to use the heaviest batteries you can afford. 5. How am I to know where to find the short-wave stations on my two dials? A.: Only by experiment. Locate the station by its whistle and then tune it to maximum strength, then note the setting. 6. When I tune out 4YA I partly lose 2YA. Why is this? A.: Because they are close together, though with careful manipulation you should be able to. tune in 2YA quite well. % I can seldom get 1YA when Dunedin is on. A.: You should be able to if you are ‘using an efficient wave-trap. MADER (Auckland): How long should I be able to use my 30 amp.. hour accumulator before charging, using it three hours a day if my "A" current is about # of an amp.? A.: About three weeks. 2. Where can I get particulars of a suitable charger? ; A.: From the "Radio Listeners’ Guide," 1929. Use a resistance of about 10, ohms to break down the charging rate. J UARE (Matata) : Which valve bases are used in the construction of the Pf and detector coils? A.: The make is of no importance. All standard valves with the exception of the 199 type have the same diameter. 2. Which valves would give the best ‘results in the detector and audio stages? A.: Detector, 112A, 4DX, 415; Audio, 412A, PM8, 409, 1.410, H.: Is my earth sufficient? I en- * close a sketch. A.: If you punctured the copper with holes to allow percolation of the water it would be better. Te Sounr (Lower Hutt): What is the capacity in mfds. of my condenser which is marked 500 mmf. A.: .0005 mfds. 2. Can this be used in place of a .00035 im the set? Yes, if you use in series with it a .001. fixed condenser. 8. Would a .00025 condenser be satisfactory to substitute for a .0002. A.: Yes, if you use a .001 condenser in series with it. 4. Is a midget variable condenser a iqeutralising condenser ?-Yes. ‘ 5. Will a transformer with a ratio of 8:1 be satisfactory when 34:1 is specified ?=y-V es. 6. a short-wave h.f. choke. satisfacgory for RW3?-Yes. A M.T. (Otago): What is the value of * the current flowing in the 100,000ohm, resistance in the "L.W." amplifier? A.’ See the diagram in this week’s issue.

2. What is the current flowing through the .5 meg. resistance?-See diagram. Would there be any advantage. in building up the 5475 ohm. resistance to include a potentiometer, so as to allow for some variation in sereen potential. A.: Yes, if you use a 6000 ohms resistance along which clips can be placed to take off any voltage you could get the total 5475 without any difficulty, and then adjust another clip so as to obtain the optimum screen voltage. . 4, Will idin. stalloy built up to lin. thickness be suitable for the choke?Yes ANODE (Waipukurau) wishes to know if differential reaction is superior to the ordinary. . A.: It is claimed for reaction that onte the oscillating point is found it does not vary. with the frequency. le ae 2, Could you give-me a diagram of the circuit? A.: We shail describe such a_ set shortly: 8. When are the P.M.G. examinations held, and where could I obtain entrance papers? A.: You should write the "Radio Inspector" of your district (Wellington) for further particularg. 4, If I pass the examination is there any reasonable chance of getting employment? A.: We believe there is quite an opening. for. qualified radio-operators and radio technicians. For. further information on this point write Johnson’s Wireless School, Wellington. RAYL!I0 (Manawatu).-Can PN 16 be used for UX 222 in the 4-valve s.g., sw. receiver in this year’s "Guide"? A.: Yes, but note that the connection at the top of the valve is the plate whereas that on the top of 222 is the grid. 2. I suffer from considerable interference from telegraph lines. What can I re) e A.: Your only plan is to contact the District Radio Inspector. 8. Can an re. unit be used with s.g., s.w. set ?-Yes. ; S D. (Hikurangi).-My set howls when © the audio rheostat is turned on full, and no matter where I shift the speaker the set howls. . A.: It sounds like a microphonie valve. Try another in its place, or reverse connestion to primary of first audio transformer. 2. fhe speaker will sometimes crackle. ‘A.: It seems as though there is a loose connection in the speaker. 3. By connecting two leads in the primary of the first audio transformer will I lose any volume if the pick-up is not connected? A.: You ean leave the low potential end of the pick-up connected with one side of the transformer. The other side, however, must be disconnected when the apparatus is to be used as a radio receiver. OTSHOT (Wellington): I have an 8-valve a.c. set, and one of the valves is continually burning out. What ean I do? A.: Consult the agent who soid it to you, for there is little that an amateur ean do with a set such as this. JK ZEN (Wellington): I am about 800 yards from 2YA, and I ean bring in 8YA and 1YA only very weakly. I have been told that an electric local is the only thing for me, as I am shadowed by _ A,: This seems to be the case, although in the 2YA station a set is used which ean bring in outside stations quite easily when the local is operating. _ A.: Try shifting your aerial round, and increase its height. 2. Would I get results on short waves? You would stand more chance than on . ere cou get a cireuit of "Round the World Two’? A.: R.R.-February 28. 19380. 4, What would be the approximate eost of the above set, and what would be the range? A.: The cost would depend on the class of components used. You should get

a good set for £5. It would bring in the European stations on the ’phones. ICROAMP. (Gisborne): I have a 3valve all-wave Reinartz set. Can I add a stage of screen grid untuned radio frequency ? A.: Yes. But it would be better tuned, even if you put the tuning condenser on the same spindle as the detector tuning condenser. 2. I have tried several times to add the r.f. stage, but can get nothing. A.: Are you sure you are connecting the radio valve the right way? If you are using an Hnglish or a Continental valve the terminal on the top of the valve is the plate. With the 222 it is the grid, The terminal made free in each case by the transfer of the electrode terminal to the top of the valve is the screen grid. 8. In both cases I used New Zealandmade coils. Should I expect results with these? A.: They should be quite satisfactory, providing they are designed for the condenser you are using. an I adapt either of the circuits enclosed to use broadcast and short-wave coils? A.: The circuits can be adapted, but designing a set of coils would be rather a long job. You will find our screen-grid short-wave receiver described in this year’s "Guide," together with the broadcast coils described in the "Record" quite as good as anything of the kind:

Metres. Primary. Secondary. Tickler. 10-20 8t. 18 DSC earns. ry 80 DSC 18-30 5c. 20 enam, t. 30 DSC 28-38 8t, 24 DCC 134. "D4 ‘pce 30 DSC 84-60 10t.26 DSC 22t.26DSC 15t. 30 DSC 56-100 12t.30 DSC 45t. 80 DSC 24t. 80 DSC