Questions and answers

Radio Record, Volume II, Issue 53, 19 July 1929, Page 32

Questions and answers

Full-wave Crystal Set. AN this cirenit be used without the primaries? ‘I am haying trouble. with the serond erystal, A.: No, the secondary would be shortcircuited. See answers to corgeapondents last. week. Various Points, DD H." (Qhbristchurch) | _ Writes Paes asking: = 1. Which is the best method of marking a panel? . A.: There are principally two methods which might be employed to adyantage: pasting a, piece of paper over the face of the panel and marking it out from that and scaring out on the back and rubbing with chalk to bring the marks up clearly. If lettering is to be done, it should be placed in the hands of a manufacturing firm who have the equipment necessary for this work. 2. Can a "B" battery trickle charger be used as an eliminator? A.: It is most unlikely that such an eliminator would be of the full-wave type, and eonsequently would not’ be able to deliver a very substantial current. By the.time this was filtered the voltage would be too low to be of service. However, if a small set is to be fed the addition of a choke in the positive lead and 4 mfd, condensers on either side connecting with the negative, would allow sufficient current to pass to work the set. If a filter ‘SYS

tem is not employed the hum would be too great, 8, Can an extra R.F. stage be added "to a three-valve reflex set as a separate unit? A,; It would be far better to rebuild using the parts for a four-yalve set. The R,F, booster described by a carrespondent wauld probably be effective in this case, but not a0 effective as a rebuilt set, ‘ 4. What .is adaptor harness, and could it. he used on this ci-cuit? © A.; An adaptor harness comprises a transformer and a’ series of adaptors: ‘connected so that on plugging these into the valye-holders A.C. valves can be fitted without any alteration to the wiring. Although this harness has been found to work successfully on a neutrodyne and on a regenerative set, it cannot. be said authoritatively that it would work on a reflex though we ean see no reason why it should not, 5. I have heard it said that a battery. charger can be employed ‘as an eliminator merely by floating a B battery across its terminals and connecting these to the set. The B hattery need not be new. Is this feasible? A.: Cases where an A battery can be fitted up in this manner are quite common, though we haye never heard of a B battery being used in this manner. It hes been suggested that the A battery accumulator shunted across the terminals to the chargers acts as a condenser, but the precise explanation is

not available, In the case of the B troubles caused through a high resistance battery would no doubt disturb the smooth operation of the set. If the charger Gould be borrowed, the experiment. would be well worth while, The Value of Inductances. ORY. 8." (Wellington) wishes to use a. ,000125 .condenser where his dare for a .0002 variable condenser, He asks the following queéstions ;- 1. With a 2} inch coil former, how many turns should be placed on the nerigl éoil to tune from 15 to 80 metres, and secondly, from 80 to 80 metres? How-many would the secondary require? .. A.: To tune from 15 to 30 metres three turns on the aerial coll and four on the ‘secondary coil. A .000125 condenser will not tune hetween 380 and 80. metres, Two coils will be ‘necessary. 2. How many turns will be necessary for the tuned anode stage?-Four on the secondary and five on the tickler if close wound. . B What size of former do you recom-?-Three inches, "4. I have on-hand a reaction condenser, .0002, and a quantity of 20 gauge D.S.C. and 20 gauge enamelled wire. What other sizes -of wire would be more efficient?-These will be O.K. 5. Will I. have to make any other changes in the circuit to change the con denser values ?-No, other than changes in. the coils. . 6 For the high-frequency’choke, and 80 gauge, enamelled wire on a 5-inch formes how many turns are required? 7. Will the "Radio Record". choke do for the detector stage ?-Yes. .8. I have a 625 screen grid valve. will this be as good as any other? ~ . A.: In this eirecuit it will be better™ probably than the vertical type of §8.G.° valve.

‘Browning-Drake Problems. AN I use a 22-volt battery for the detector stage only and run. the eliminator for the two stages ef audio in my four-yalve transformer coupled Browning-Drake? _A.: Yes, if it will oscillate, always use, the lowest voltage possible, ‘2: I get good results, but since under baseboard wiring has been put in I have been unable to put more than 3° "A" voltage on the filaments of the audio valyes, when tuning low wave- | lengths. A.: Reverse the primary connections of one of the transformers. If this dees not. step the trouble, introduce a ‘little damping by means of a low value grid leak in series with the grid ; of*the last valve.’ : 8. I.used to put a .0001 condenser in the aerial to -balance the dials, but when I do so now the set bowls, when-

ever the tickler is up or down, and I ean, cut it off anly with the B.F. rheostat turned down two-thirds, A.: The set appears to be not neutyalised. Try re-neutralising, or, if this will not stop the trouble, use a hgher impedanee valve in the radio stage--

Hammer

Chewer

(Bluff),

A Wave Meter, CAN you'tell me how I can make a wave meter?-"A.I.M." (Wellington). I do not know the wavelengths of my. coils, yAvi The description of a wave-meter be beyond the seope of "Questions and Answers," It has been fully described in Handy’s Handbook, the journal of the United States amateur transmitters, It can be obtained from several booksellers,. but probably an amateur transmitter would be able to lend you one. Its construction is anything but simple. The easiest way to calibrate coils approximately is to make a graph for each, plotting the wavelength against the dial reading and a curve will thus -be obtained which will show quite clearly the Jilit of the coil in particular. Accumulator Runs Down. "DEAD Battery" (Eketahuna) complains that when his set is turned on every night for more than 23 hours the battery runs down, and the trickle charger has to be on'for 24 hours te make un the -deficiency. A,: Hither in the battery itself or in the set there is a short circuit, Charge: the battery as. full as_ possible and leave it stand, to note whether it is-discharging. If this is the case, the battery should ‘be taken to a service station for repair. It is possible that some of the.plates have become dislodged or that a sediment has fori-

ed at the bottom. In this case, the eorrespondent may be able to. effect agremedy himself, The battery should bé, emptied of all liquid and washed out with distilled water. -Pure rain water collected in an earthenware vessel is a good substitute. Once the battery is quite clean inside sulphuric acid and water, the density of which is 1.25, should be added. (This can be readily obtained from a chemist.) The liquid should just cover the plates. If the battery is found to hold its charge, then it is evident that there is a ‘short. cireuit within the set. To conclusively prove this, withdraw the valves, remove the "A" battery, and connect "A" plus to a "B" plus of about 224 volts, Connect "A" minus to one’ terminal of the speaker or a pair of ’phones, and the other to the "B" minus of the battery. A distinct click; should be heard, and this will indidate that there is a short circuit. The set should then be overhauled by the owner if he is competent to do this, or by a radio dealer. Great care must be exercised that the "B" battery is not connected to the "A" battery terminals while the valves are in.: If no click is heard, and a dummy valve is available, try. inserting it in each of the sockets moving it about and listening. for a click. This will point to.an error in the sockets. If a battery eliminator is used, substitute a low tapping for the "B" battery. ‘ Vaive Consumption. "PUZZLED" (Nelson) writes: (1) . * Will you explain why it is necessary to use six dry "A" batteries in series parallel with valves, using a total filament current of more than .2 amiperes? . , A.;- When filament consumption exceeds .2 or .25 a heavy drain is placed upon the dry cells. These are adapted Leclanche cells, and not built to stand a heavy drain. When this is placed upon them they rapidly polarize, the internal resistanee. rises, and the H.M.F. drops-in other words, they soon become useless. If, on the other hand, they are. used but slowly, polarisation does not take place readily and ‘greater service is extracted from the eell. By using six cells,. in series parallel instead, of three in series,’ the E.M.F. remains the same, but the resistance of the battery is halved, and current is taken out of each cell at just half the rate it would be taken out of each of the three cells .used in series.

This means that the "A" battery will last probably three times as long as the "A" battery, comprising three cells only. 2. Does one get: as good results from a-.06 valve as from a .1? A.: It all depends on what is meant by "as good results." The performance of both valves is very much, alike, and opinion is divided as to quality, ete. The statement is sometimes made that the .06 valves are harsh, but. there is no theoretical justification for. this, and our observations have shown us that ‘except in a very few cases, .06 valves are by no means harsh-and these exceptions are very dubious. 4, As a .06 valve uses much more "B" battery current than a .1 valve, is. there much difference in the annual running ‘cost? A.: If two sets of batteries are used with the "A" battery readily rechargeable, there is little difference in the running cost. 5. Is there a standard size copper wire used for connecting dry "A" batteries? A,: No, but use the thickest copper ‘wire obtainable, 6. My receiver is a factory-built fourvalve regenerative. I am using Bnglish. valves with a consumption of .1 ampere each. ‘Hither these or lower filament consumption valves were advised by the maker last year for use with dry "A": batteries which I am obliged to use. Which would be the more economical? A.: It is difficult to say. Probably the .1 valves, unless liberal grid bias were applied to each valve of the set, will be the most economical, 7. When I have to change my ‘valves what type shall I replace by? A.: The wisest plan is to use the same make of valves now employed. Set Will Noé Oscillate. VERYTHING is in order in my set, wrties "C.D. McG." (Wellington), but the set will not oscillate. A.: Try a few more turns on the tickler, and if the set will not oscillate under these conditions try reversing the connections to the detector valve. If another valve is handy, it should be tried, for the one now. used as detector may-have lost its emission. Parallel Feed Browning-Drake, — "J. S.C." (Nelson) asks concerning this receiver :-

1. What difference, if any, would it make by fitting a 1 mfd. condenser in the plate leads instead of the .5 condensers as. specified ?-None. 2° Which is the most efficient method of controlling volume without ‘Spoiling tone? A.: Place a high value variable condenser between the aerial. and the earth. This resistance should be at least 10,000 ohms. 3, Would it be of ‘any advantage to replace 201A valves in the R.F. stages with valves of higher impedance? If so, with what impedance? "A.: Providing the set will neutralise without difficulty, the 201A type of valve gives the -best results in the radio frequency stages of the Browning Drake. 4. Would it be advisable to replace the detector which has a resistance of about 9000 ohms by one of higher impedance? A.: The tendency now is to manufacture special detectors with impedances of slightly under 10,000 ohms, eg., PM4D and A415. Unusual Troubles. P.B." has experienced an unsual * trouble, in that he has to turn his rheostat in different positions on different occasions to get the best results. He notices that when he has a wavetrap the rheostat when turned to a certain point will allow of signals from the local station being received. A.: If a battery eliminator is used this may be accounted: for in surge’ in the main current, otherwise it is rather difficult to explain, providing everything in the set, including the valve, is in perfect order. The fact that the local station is received when the set is brought to a certain degree of sensitivity indicates that the wavetrap is inefficient, or if this is beyond doubt, that the receiver is situated in such a position that it is impossible to get past the local. station when the set is fully sensitive.

2.. Sometimes I go to the set and I can hardly touch it without a growl and. a howl. Another night I can handle it without any trouble. A.: It appears from this that the set is not properly neutralised, and that the signals being received have an unstabling effect and cause the variations noticed. 3. Some nights the wavetrap will cut out the local station, but on odd oc» casion, irrespective of the wavetrap, it will come in all over the dial, A.: It appears that the wavetrap: is inefficient under certain conditions. One would suspect that this is a matter of chance rather than being the result of the conditions existing from night to night. Wxamine the wavetrap carefully and see that it is not possible for a short cireuit to take place. The condenser vanes, for example, may have worked a little’ loose and be touching one another. Uncontrollable Oscillation. MY trouble is fierce oscillation of the detector, writes "A.J.C." (Te Aroha). I have reduced the "B" battery and tried reversing the tickler connections; also, unless the set is right up to the oscillation point there is no volume and consequently poor tone. The R.F. stages seem to be wrong. There is now howling from this. no matter how the neutralising condenser is altered. . When it is turned low it sets the detector howling. The audio filaments on being turned higa cause a howl also. I thought of reducing the number of turns on the tickler, but being a factory-made set I thought this would be correct. A.: The major trouble appears to be in the layout, and results from interaction between the rails or grid and plate wires. Next week we shall publish a few hints on layout. The best results could accrue only when the primary had been increased by two or three turns.