Questions and Answers

Radio Record, Volume III, Issue 7, 30 August 1929, Page 28

Questions and Answers

A Variable Grid Leak, "GRD LHAK" (Blenheim) asks whether resistance is low ‘or high when the pellets of a variable grid leak are compressed. A.: he ieast resistance is obtained with the pellets compressed, in other words, with the compressor screwed right in, Defective Cone Speaker. AM using a horn speaker, but have a cone speaker. When the horn ‘speaket is tuned to the utmost volume, and then téplaced with. cone type, it is hardly audible-why?-‘"Cone," Auckland. A,: If horn is good, we fear a defect in the speaker. Have it examined by an expert. \ Speaker Connections. HAVE an A.C, six-valve set. Does it matter which way the speaker cords are connected to the set terminals?

There is no indication on the set, although one of the speaker, asks Query (Te Awamutu). A.: If there is an output>~filter-no. To make certain, screw the diaphragm to rattling pitch, and reverse connections. Tf music becomes smooth, replace connections as they were previously. On the other hand, if reception becomes ‘worse or disappears, leave terminals as they. are now, and readjust. If there is no ditfference it is immaterial, as this indicates an output filter. Ground Wire. wat is the best ground wire, writes A.E.B. (Auckland). I have my earth wire on to a water girs which runs under the house, about 40ft. before reaching the ground. A.: The heavy single-stranded wire, 12 or 14-gauge single copper, or. 7/029

stranded. are suitable. You are ‘wise in sinking a new earth, 40ft. is too far for the earth to run before entering the ground. Life of Dry Batteries. OW long should a 4-volt 30 amp. battery last with three valves, the filament consumption. of each being .06_ amps 7-"A.B.B." (Nelson). A. Does the correspondent mean a 4volt accumulator? If so, it should be recharged after about 60 hours’ use. dry batteries, they should last approximately 100-150 hours, depending on the time used without an interval. The correspondent encloses a sketch of a series parallel arrangement which is correct, but the order of the batteries should be changed round to allow of an even drain. ' Set will not Oscillate. "s H.R." (Wellington Soyth) cannot ‘Ve get his three-valve screen grid set to oscillate. A.: Adda few turns to the reaction coil, and if this will not bring about the desired result; increase the detector voltage. The set should oscillate quite readily for it is the adaptation of a well-known kit set. ; Transformer Error. T" constructing the power transformer described in the "Radio Listeners’ Guide," "R.H.H." (Lower Hutt) brought out the taps of the side of the bobbin which will eventually be covered by the laminations. He asks if he might alter . the width of these to 14 inches and turn them round. A.: This alteration is quite permissible. Selection of a "B" Eliminator. WwHat do you advise as a battery eliminator for 4 crystal set with a: 2-valve amplifier? Do you think this would be preferable to a 8-valve all-electric receiver? I am quite satisfied with the local station, asks "W-.G.S." (Miramar). A.: The correspondent has this to keep in mind: Although he may be satisfied with the local station, the chances are he will want to reach out to other stations before very long. A three-valve all-electric receiver, while being an excellent proposition for the local station, is, except in certain cases, not well adapted to bring in outside stations. If an eliminator of a good make is purchased the correspondent will be able, at a later date, to increase the number of valves in his set without going to any further expense as far as. the "B" supply is concerned. If, at a future date, he wishes to electrify his set, hé may do so by either utilising the A.C filament tappings on his present eliminator, or by the provisfon of an extra low yoltage transformer. An eliminator for this purpose should have tappings to at least 180.volts with grid bias. The construction of an eliminator is not a difficult problem, especially as the transformers with A.C. windings may be readily procured for a very small sum. ‘This, to us, seems the best plan for the correspondent to follow. Loudspeaker Connection. I FIND that by connecting my loudspeaker the wrong way, that is, the negative terminal to the positive of the speaker, I get a distinctly louder and clearer tone, Am I doing harm? —

A.: It is possible thatthe connections have been reversed within the speaker. Tighten the diaphragh if it is adjustable until at the rattling point. Now reverse the connections. If the. signals Jecppme further distorted or further diszppear this indicates that the connections are now correct, and the diaphragm should be loosened until the music becomes clear again. If, on the other hand, the signals become clearer the connections ate now wrong and should be reversed, the diaphragm being loosened. This will indicate whether the. markings on the terminals are correct or not. If the speaker is non-adjustable it- will be safe to assume that they have been wrongly labelled; that is, leavegthem connected to the set so as to get the best results. Short-wave Adaptor. " H.LP." (Hastifigs) submits a dia4k gram of an adaptor which he cannot get to work. It incorporates a one m.f.d. condenser between the platé of the valve and "A plus." This is the source of the trouble} as it is by-passing the signals. Remove it, and everything should be O.K, A Question of Resistance. "RAvi0" (Hastings) states that the reading across his accumulator reads the same as that across his valve irresective of: the position of the rheostat. e asks if this is correct. A.: A theostat will ‘break down volt- | age only when a certain amount of current is being taken from the source. This means that a load of definite value has to be placed in the circuit. _ Has the correspondent measured the voltage with the valve withdrawn? To measure the voltage on the valve a voltameter should be put across the terminals of the valve base, while the valve is in operation. 2. If my aerial is increased should I make any alterations in the coupling coil? A.: Alterations are unnecessary, unless it is to tighten the coupling slightly. Trouble with the 2-R.F. BrowningDrake. "my H.W." (Hauraki Plains) cannot-peu- * tralise the 2-R.F. BrowningArtike. He is consequently getting shrill whistles and finds difficulty in tuning stations in. The tickler has no effect on the volume. — A.: Incorporate Imfd. by-pass condensers between "B"’+R.F. and earth, This will no doubt solve the trouble. "A.C. Reflex." "L B." (Auckland) wishes to construct * a reflex to work from the A.C. Mains. a, ; A.: Reflex sets are unsatisfactory. "Phe correspondent should utilise his half-wave rectifier and A.C. filament windings, to make a straight-out 2-valve amplifier, using the 227 valve as first audio, but sée our special A.C, number,.September 20,-) Battery Charger Problem. \ « A G.0." (Te Aroha) has made the ‘Ae battery charger deserjbed in the "TLigsteners’ Guide," but finds his rectifying valve lights a brilliant green instead of the usual blue glow... é A.: Disconnect, the filament, and it will be found that charging will still take \

place, if the Gonnections other than the filament are correct. Now reduce the number of turns by, say, three,.and note the result. . . Double Grid Valves. wit a double grid valve be suitable for the erystal and valve with 3valve performance, writes "BH.T.D." (Petone). If so, -will it be equal to.an ordinary H.F. valve with 90 volts on the plate A.: Use "A441" and results should be found to equal that of the ordinary valve of 90 volts on the plate for a one-valye amplifier, ° 2. The set refuses to oscillate. ; as Put ‘more turns on the reaction coil. _3. I am now using a short aerial. What difference will it make to the coils?None. Q Interference from Generator. "Ge" (Takaka) complains that’ his * jgenerator,. driven by a_ Pelton "wheel, causes interference with his wireBREET RERESCSEEE Eee sea rMe ee ee Seas eTeeEeeeeeeeeeeeEe

i receiver. He asks how this may be ercome. A.: Connect in series with the leads to the lighting system H.F. chokes comprising three or four hundred turns of 22 gauge wire.: Try the effect of a little caustic soda on the commutator. Short-Wave Adaptor. OAN an A.C. short-wave adaptor be applied to a 7-valve all-electric receiver, writes "A.A" (Avondale), . A.: At this stage of radio evolution A:C. short-wave adaptors have not come into general being. For the best results it will be necessary to use D.C. Such a D.C. adapter could quite well be used with van A.C. set. . Shortwave’ Problems, " ‘W.Y." (Herekino), asks the follow-' *ing questions: 1. Is it normal for both rheostats to ‘work only when just open? Past this point.a strong howl sets up. A.: No, there is too much reaction. Remove a little wire from the tickler coil. 2. -Why is it necessary, to keep the set oscil¥,sng, to have sometimes to reduce the coupling between the aerial and secondary coils ; o A.: This js done to reduce the damping of the aerial, ,which varjes frequently. 8. I connected a fixed condenser to a choke and incorporated this in the set, but found it would not oscillate. A.: This was probably in parallel with the existing condenser, so that a very large capacity was introduced rendering regeneration impossible. . 4, With both variable condensers al] in I can turn the reaction coil full out and the only sound heard is the steady rushing sound.? Is this normal? A.: It appears’as if in this position, the fixed vanes are touching the moving vanes, also try another grid leak. Your other problems are being dealt with by the technician who designed the Supply of.a "B" Eliminator. « WeovLtp a Phillips "B" and "CO" eliminator supply enough current to run a séreen grid two: 201’s and two 17TA’s

in push pull, or would I get better results by using semi-power valves in the, last stage. A.:- This eliminator will supply 30 milliamps. at 150 volts. This would not be sufficient for two 171A’s in push pull. Try the new "B405’s" in push pull. 2. Do two power valves in p.p. take. exactly twice as much plate current as on Sa A.: Yes, unless they are double biased, in which case they take the amount of one. ‘ . S.G. Browning-Drake. °- -- "QELECTIVIT * (Kelburn) has constructed a 5-valve screen grid Browning-Drake, but cannot neutralise it correctly. He states that the set is not shielded, and complains that selectivity is poor. He adds that a good number of stations were logged while the valves were new, but now he has noticed a background of noise, and cannot get the same number-of stations. A.: There are a number of factors to be considered. A 5-valve set using the

sereen grid and reaction should be shielded, otherwise it is a hopeless task trying to make -the set selective or neutralise it. Introduce 1 mfd. by-pass condenser hetween B positive R.F. and earth. The shield grid valve is notoriously unselective, but a wave-trap should rectify this. The valves appear to have lost their emission; have them tested. This would account for the noise. The correspondent usks if the parallel feed B.D. would be more selective than the screen gird. It would if properly constructed. The correspondent has appended a diagram with the following questions: 1: Are the coils standing at rightangles for the B.D.? A.: Yes, though the diagram shows them arranged the neutrodyne style. 2. Are the valves in the right relative positions? A.: No, it appears that the lay-out is altogether wrong. Follow the lay-out for the 2 R.F. Browning-Drake published in the ‘Listeners’ Guide." — 8.: Is the tickler controlled by a cona correct? So far as we can seees. Booster for the Browning-Drake. I HAVE tried loose coupling for the aerial coil of the Browning-Drake without success, writes "J.S.K." (Ngaio). Signal strength is reduced too much. Would the parallel feed boster be bettct? — Yes. : Charging an "A" Battery. CAN a 6-volt 80 to 100 amp-hours battery be charged from a 32 volt storage battery? asks "B.S." (Market Cross). A.: A very heavy resistance would be necessary, involving a very big waste of current. Approxjmately 26 volts at 5 amps,, if this were the charging rate, or 130 watts would have to be dissipated by the resistance. A resistance-of these dimensions would be very difficult to obtain. In any case, the arrangement would be unsatisfactory unless the capacity of the 32 volt storage battery were greater than 100 amp-hours. Jf it were being charged while the "A" battery were being charged a workable compromise might be effected.

2, Could an eliminator be made ta provide 180 volts at 40 to 50 milliamps. from the 82 volt battery? A.: No, for this reason. Voltage can be stepped up only by means of a step up transformer. The principle of this is that if two coils are connected by their rn ’

magnetic fields only and an alternating or pulsating current fed into one of them, current will be induced uy in the other at a voltage in direct proportion to the windings. This step-up ean take place only with pulsating current. The induced current momentarily opposes te

the primary current, 80 that unless the primary current. fluctuates rapidly the magnetic field in the secondary ciront will. not operate after the first impulse. ; Condensers for Short-wave. How many plates are there. in J, .00015:.condenser? asks L.R.S. son A.: Four fixed and three moving, but they vary with the different sizes of the plate. Q.: Is it necessary to have the latest spaced ?-No. Round the World Two. AN a .0005 condenser with another condenser used in series be applied to the Round the World Two? asks "KK." (Auckland). . A.: The arrangement is not satisfactory, but a .0003 mfd. condenser connected in series would reduce the capacity from .0005 to .0002. The better. plan would be to remove plates, making a total of 11, Motor-Boating. I HAV®E recently changed over a different make of transformer, writes "N.D.B." (Patea), and find as a consequence motor-boating very pronounced. I have tried extra condensers but to no avail. A.: Have you tried the simple expedient of reversing the leads to the primary? It seems as though the two makers have wound the transformers in the opposite directions, as is often the case. If this has been tried to no avail, place a variable resistance of a few hundred ohms in the B supply to the primary, and between this and the primary connect a by-pass condenser to earth, a Defective Transformer. "PNQUIRER" (Wairoa) states that (4 his machine will work well for a while and then signals will fade away. A.: Try the transformers, as this is the source Jot a great deal of trouble with this receiver. A condenser may have broken down or a grid leak defective. 2.: Would any alteration be necessary to use 201A type of valves. instead of 199’s? A.: The resistances would have to be altered. The 199 requires .06 of an amp., whereas the 201A requires jamp., hence the resistances would be burned out by the change over. In addition, the set would want reneutralising.

D.C. Generator. S there a small generator available to give, say, 350 volts at 100 milliamps, to feed two 245 type valves in push-pull? writes: "W.D.S." (Urenui). A.: So far as we know there is not one of these available in New Zealand. The M.P. generator is an BHnglish product that will do the job, but is not.on our market. Try Johns Limited, Auckland, as they at different times hdve apparatus of this description on hand. 2.: Can an adapted megger be obtained in New Zealand? A.3 Not to our knowledge. It would need very great amount of filtering. — Short-wavé Adaptor. ‘M. ©." (Eastbourne) wishes to convert « a .0005 condenser into a .0001 and another condenser and a .00025 by detaching plates and opening these out double space--This will be O.K. 2: On plugging the short-wave adaptor into the detector socket of a five-valve neutrodyne. is. it necessary to have the R.F. valves turned on?-No. 8. To eonnect phones or speaker, do I plug in to the ordinary broadcast receiver jack or from the adaptor? A.: Plug the speaker into the speaker ‘terminals on the broadcast set, and the phones into the plate "B plus" terminals of the first audio valve.