Questions and Answers

Radio Record, Volume III, Issue 10, 20 September 1929, Page 44

Questions and Answers

High Resistance Connections. HI volume rheostat of ‘License _ Number 27807" has to be turned full on to get passable reception, when suddenly the volume rushes up very strongly and the rheostat has to be turned down again. A.: It appears as. though either in the speaker cord or in the receiver itself there is a high resistance connection, which at all times allows a certain amount of current to pass. When. the _ resistance breaks down ‘or is brokensdown and everything comes up normal. We suggest that the correspondent takes his speaker cord, and while the set is in operation, pulls it about to see if anything happens. If it does replace the cord. Take a piece of stick and move different wires about, likewise looking for its effect. Bxamine the valve springs to see that tight connections are’ being’ made. If any resistances are in the circuit, examine.them as carefully as possible, ‘and test them by the,’phones and cell method. Likewise test transformers and condensers. Crystal and Amplifier. WovLp it be possible to add another amplifying valve to Pentode’s crystal and amplifier to work from the A.C. mains? Asks "L.B." (Christchurch). A.: Yes, but the circuit would need several fundamental alterations. It will require a separate article, and this will appear in a future issue.-

2. Would it be possible to add a detector valve using an accumulator for the filament and getting a plate current from the A.C. set? A.: Yes, but we will need to describe this apparatus more fully. Aerial Trouble. A S. (Wellington) has a crystal set * which works a loud speaker, and thinking to still further better his reception, erected a high aerial. Contrary to expectations, this aerial weakened reception. . . A.: The correspondent has omitted the vital point from his question.. He has not stated the direction of either his old or his new aerial. On-the sur,face it would appear as though the old aerial were directional to the station, whereas the new one is not. This would weaken reception considerably. Coil Specifications. G H. (Otahuhu) has.made the 2RF * Browning-Drake, but has not had success... He is intending to make a few alterations including throttle control.. He a&ks also .for more coil particulars. A.: See the specification f.: the AllElectric Browning-Drake published, in this issue for coil specifications... 2. Am T right ia joining the tickler coil to the top of the tuning coil, so that the plate lead of the tickler. coil is joined to the moving plates of the grid condenser? ~ 7 A.: In condenser control reaction,

i where both coils are continuous, the. common lead goes to the filament negative or cathode and B negative. ‘ The énd of the tickler coil goes.to the moying plates of the reaction’ condenser, the fixed plates of which go to the plate lead, which goes to the transformer. B plus connects as usual. R.F.. Booster. S, W. (Northcote) has constructed an R.F. screen grid’ booster, which he cannot get _to function, properly, as ‘the circuit. ‘is a straightforward’ one, it would appear that it would be almost impossible to track the trouble without putting instruments on it: We can suspect only the R.F. choke, which, if defective, would by-pass aill signals to earth. A broken-down condenser would likewise cause trouble.. ‘The coil. which is small te limit the field and thus preyent interaction, appears rather unsatisfactory. A binocular or toroid, would be more effective. Various Points. A F. (Ohura) asks: 1. Should the * Lead-in wire be insulated? A:: If uninsulated there is a great possibility of. short-circuiting © with such earth objects as a wet wall, guy-~ ropes, roof, etc.’ It is far preferable to keep these leads insulated. ‘2. Could you give me information about the Stewart Warner Noi 315? A.:, Communicate with the Stewart Warner factory representatives, Hope Gibbous Building, Dixon Street, Wellington. 3. What is the charging rate of radio batteries? A.: They vary: according to the amp. hour capacity of the batteries. A small single cell may be charged at half an amp, whereas a 100-amp. hour battery could be charged tip to 5 anips.. 4, Is the "Northern Primary Bat- _ tery" ‘made by Seaman’s suitable for eharging accumulators? A.: We doubt it, but for further information about this battery, eom‘muniecate with Cory-Wright and Salmon, Box 1650, Auckland. Transformer for Small Set. HAVE .heard that a_ bell-ringing transformer can be used for-a small A battery eliminator, writes "EH." (Wellington). Is this so; if not, could you. describe a transformer for this purpose? A.: It would be ‘suitable if made to handle a. great amount of current. The voltage tappings would not correspond with any well-known make of valve. Small transformers can be purchased which should have a secondary that will deliver four amps..or. more. It is doubtful that the beljl-ringing transformer will suit the purpose. 2.Why are small capacity condensers used in short-wave. sets?. o

A.: Small capacity condensers are used in order to separate the stations ° on high-frequency by a reasonable space. Where a degree of the dial means the movement of a very little capacity, stations can be put farther apart. than. when, as-in the broadedst band, movement means a great deal of capacity. Furthermore, the efficiency of a circuit is governed by the relation L/CR, where L is induction, Cis capacity, and R.is resistance; so that it is advisable. to keep capacity as. low as possible, and inductance (coils) as high as possible. The larger the condenser, the smaller the coil.’ ° . A.C. Amplifier. . T.H. (Christchurch) asks if the * transformer described by Pentode for his erystal and valve set to work from A.C. mains would supply enough current for a .06 valve in the first socket, and a 15-power valve .in, the last. ‘A,: A .06 D.C. valve i in the first audio ‘stage could not be fed with A:C, cur-")" rent. A separate amplifier would have’ to be constructed, and this will.be described in a forthcoming issue. Your further queries will also be, covéred by this article. ,

Static on a Crystal. QNCE or twice a week, writes G.E. B., Lower Hutt, I get a noise like static coming through my loudspeaker, which I am working directly from 2 erystal set. It is only momentary. A.: It may be a power noise, and, ‘ although this is unlikely, it is not impossible: You may have a loose connertion, or something may be loose within the speaker. The speaker cord may be defective, and again it may be interference through another set. 2. Can you suggest a better horn speaker ‘than the one I am using? I " can hear this one all over the room, ana even through the house. A.: You are evidently getting very good results. It would be difficult\.to suggest a better speaker, although (a very sensitive cone may give louder signals. 3. With regard to your third query, the fees vary, the minimum being about half a guinea an item. 2RF B-D. I AM intending to construct the 2RF B-D, writes "3.W." (Timaru), and I want one or two points cleated up. 1. The shields "Radio Record" standard size are to be placed on a copper. sheet, and the wires soldered to it. — Can I remove the aluminium bottoms and mount my shields on this copper? The wires can then be soldered. through it. Aus Yes,. drill the copper. base: so 1s to take the screws in the uprights of the shield-supports. They ean then be ‘mounted quite sguventently ‘in the manner you suggest,: . F

2. As I am building a separate amplifier, would I put the condenser controlling reaction on the right-haifd side of the detector shield? A.: Yes. Variation is possible in lay-out, as.long as grid and plate wires are kept short and free from one another and coil interaction prevented. | 8. What was the difference between the lay-out and theoretical of the 2RF B-D in‘the "Guide"?. I cannot seé any. A.: The.difference did not occur in all thé issues. The bias on the second radio valve is; positive, that is, it is connected "to the "A"-+- instead of "A" -. The correct connection is shown in the lay-out diagram. 4, Will the transformers used in my four-valve B-D be all right in the fivevalve B-D?° Yes. 5. I cannot get the linen diaphragm Cs work properly. The tone is very etallic. A.: Is. there a tension of the reed of the unit? If this is so, it will have to be lengthened by the edition of a tap and coupling. Straining the whit will pull the reed over. to one side, vand cause the metallic tone you speak .of. We regret that we cannot undertake testing, but a local déaler will do it for you. _Impedaformers Versus Transformers. HAVE an -impedance coupled receiver. Would these:be more suitable than ‘transformers? A.: If your set is two radio, one detector; and two audios, then you could change over to transformers with advantage, but if there are three. audio r’ stages, the addition of trarisformers ' would cause audio howls, and other troubles. The only way three audio stages can be operated successfully is with push-pull in the last stage. Carrier Whistle. How is ‘it; asks J.0.0. (Fiji), that I can get strong carrier whistles, bat cannot tune in the stations? A.: The edirier of any station is always very much stronger than its modulations, and although this can be heterodyned with-a set causing some--times a very strong whistle, it is impossible t6 get the modulations owing to the weakness. ‘This explains why Morse transmission carries further than telephony. Speaker Terminals. HY have Philips loudspeakers three terminals? A.: By changing a connection, the impedance of the. last valve can be more nearly matched than with: one terminal. Move the connections to the speaker from one: to the other, until the best result is obtained. This will probably be with one of the red terminals, . : Converting a Set to All Wave. How could I convert my 6-valve fac-tory-made neutrodyne into an allwave set? A.: Don’t attempt it. Obtain a shortwave, adapter and plug into the detector socket. Your aerial is a good one, 1 Transformer Designs.. GAN you design for me a transformer to give the following characteristics, writes Q.R.A. (Rotorua)?. Input 110 volts A.C., output 5, and.8 volts A.C., 14 amps, 200 volts and 100 mili- | amps. A.: Sée "Listeriers’ Guide," latest édi-

tion, page 55." The following are the characteristics :-Core,: 1jin. . x 1}in.; primary, 28 gauge, 5356 turns; sécondary, 26 turns of\18 gauge for a 5-volt wiring, 44 turns of 18 gauge for the 8 volt turning, 1100 turns, 32 SWG, to give the 200 volts winding. If the rectifier has a high resistance, a few more turns. will have to be added. For Q valve rectifier the number of turns would be about 1042, A Battery Charger. CAN get only a slight hum from my A’ battery charger, writes A.J. (Tauranga). I am using 280 valve and rectifier. A.: The 280 rectifier will deliver only 1 of an amp. Use Philips one " Use of an "adapter. " CAN a short wave adapter he used with a small all-electric receiver? asks J.M. (Kelburn).° ~ ‘A.: Not satisfactorily. This receiver is designed purely for local reception, and has‘ not a strong enough audjo side for your requirements. ° 2. Can a wave trap be used to gbt other stations? |. A.: Not with any satisfaction,