Questions and answers

Radio Record, Volume V, Issue 1, 17 July 1931, Page 14

Questions and answers

B $.D. (Wanganui): I cannot procure * 5-Sin. former for the r.f. chokes. I can get either 4in, or Zin. former. Can either of these be used? If so, will st be necessary to change the gauge of wire and the number of turns? A.: You can make the change to suit Sin, former without altering any of the specifications. You will find that it will do quite well. ; 2. What type of speaker do you recommend for use with the Advance a.c. shortwave receiver? A.: Any good quality speaker will do, preferably for tone, a dynamic or an inductor dynamic.’ "

op UST ONT (Otira): Do I have to disconnect the speaker from the set to use phones?-No. 2. How long would I have to use my set on one unit of power?-About 100 hours. o 7 8. Which would .be the best short-wave adapter in the 1981 "Guide" to make and fix to my set? A.: We suggest the super-heterodyne converter, although, if you are not accustomed to construction, you. had better get some competent. radio dealer to build it. oO

H E.R. (Hokianga).--I have a 4valvs * set, using the American §.G. valve, butLit does not light. However, the set operates just as effectively as before, A.: This is rather strangé, unless the glass of the valve has became obscured. If this is.so, all is well, but, if the valve is really done you will find that a new one of the same type will give you better results: than you are getting now. It is not surprising that a set will work witb a dud valve in the socket. The h.f. current is getting through between the grid and the plate, but of course the valve will not amplify. SoLDbo (H.B.).-I am _ building the Radiogram all-electric receiver, and would like to’ know. what size transformer to build. , A.: We suggest a 100-watts transfor. mer, for although one of the smaller ones would do, yet by making this size you will have ample to spare in case you ever wish to increase the size of your set. 2. I do nat understand how the high voltage. secondary. turns are computed for fullswave rectification. A.: The figures giyen in the "Guide" refer to half-wave rectification. Where full-wave rectification is to be used, double the number of turns on the secondary, and take the centre tap. BoONzO (Masterton).I have a slight aiteration to a Majestic set, for I have found that by winding two coils.and using these between the aerial terminals, I can get better results from’ the local switch. However, results with the distant switch ave distinctly inferior. . A.: We cannot give a full of this phenomenon unless we knew the model of your set, because the antenna systems of the models'vary slightly. It appears, however, that when your gadgets are used on the local switch you are really putting in extra inductance and making for the losses brought about by the local switch. When, however, you go over to distanee and the set is now as it was really designed, you are putting in another inductance that was not: intended to be there and, of course, throwing the set out of alignment. . 7, L.D.X. (Palmerston North): Where * ¢an I obtain midget condensers of a .00005 straight-line frequency type referred to in this week’s "‘Radio Record’? A.: Try Ballingers, Wellington, or Johns Ltd., Auckland. 2. What is the principle and how does an electric induction motor work? A.: The electric induction motor is perhaps most easily understood when considered as a dynamo worked backwards. If a coil of wire is rotated between the poles of a magnet, the flux through it 1s constantly changing, and an electro-mo-tive force (T.M.F.) is induced in it. If the ends of the coil are joined, a current, which a galvanometer will show to he alternating, will flow. The coil ends are attached to "slip rings’ which in turn are attached to the terminals by two brushes. If direct current is wanted, a "commutator" is substituted for the rings, The effect of this is to keep the ‘brush’ voltage always in the same direction, but it still alternates between zero and its maximum valye every half revolution. If the dynamo has.a large number of coils following one another round, éach will contribute a fluctuating BLM.F,, but

the maxima will come at different times, so that the total effect is that of an almost constant yoltage. As remarked previously the electric motor may be considered as a dynamo worked backwards, i.e, a current is passed through a coil which, because it then has poles, like a magnet, rez volves in the magnetic field. When -ite N face of the coil faces the S pole of the magnet, the commutator reverses the current, and so the coil keeps turning. -The above explanation is very simple, but to go into the matter in greater detail will take much more space than, as a subject outside the realms of radio, could be allowed. . 8. Are the aerial turns correct? Aerial 10, grid 30; 25, 22; 30, 32. A.: No, for selectivity the aerial coils are far too large. Try these for best all-round results: Aerial, 9, 17, 25. pre WAVE (Whangarei) : I have constructed a four-valve receiver using a three-coil tuner and wish to convert the set for short-wave. How can I do so with a minimum of expense? A.: A three-coil tuner set is not the easiest thing in the world to convert over to a sw. set. You must make coils that ean be pulled out and put in at will, like valve base coils, but with a set such as the three-coil tuner you ‘would find this difficult. Your best plan is to' read wp the instructions to be given for the Kestnal Three and see how you get on. Rather than convert a broadcast set to all-wave, dismantle the whole thing and use your parts for a special all-wave set. D V.W. (Wellington): I have a five- * valve neutrodyne set and am unable to tune out 2YA A.: Try taking out one of the r.f. waves and you will probably find that the station will still come in quite well, but it will be more sharply tuned than _before. Otherwise you will have to use a wavertap. Shielding would probably be effective, although it should not be necessary. pisAko (Hauraki Plains): You are referred to "Diagnosis of Radio" for the answer to your query. GUBSCRIBER (Pahiatua): Owing to out of phase reception here, the: Wellington stations are almost useless in the evening, and we have to rely on the morsé distant ones, which, of course, do not come up ta requirements as far as signal strength is concerned. I wish to erect 2 better aerial to bring these stations in more strongly. Would the addition of a new arm to my "L" aerial be of any benefit? The proposed new addition will make the aerial into a V shape. | A.: We think not. A V-shaped aerial with the lead-in in the centre would be more or less the same as a "T" aerial, and this would be no more satisfactory than the one you already have. Possibly a little more height would be of some value; but the improvement would not be very noticeable. However, try say an extra 10 or 15 feet in height. You could lengthen it out to about 100 feet if you could get the space to extend it in the direction it already runs, ~ EyNVox (Timaru): What is the capacity of a Ford spark coil condenser? -A.: About 2 méfds, 2. What is the duty arid the approximate freight charges on- American receivers? \ ‘ Y }

f A.: The duty is 40 per, cenit. on built4up receivers, and the freight from New York 60 cents a cubie foot, 3. The current passed by 226, 201A in $-wave rectifying circuit? A.: The exact amount. of current will, of course, depend upon the amount of voltage supplied, but. assuming you are ‘applying to the plate grid about 180, you 4 f may reckon that those valves will pass about 20 or 25 milliamps. TPRANNY (Waitara): I have built the "Radiogram Five," and the power pack as described:.in the "Radio Guide and Call Book.’ I am very pleased with the results, but the power transformer has given a lot of trouble by burning out, It has done this twice, and although I have re-wound it very carefully. I have not noticed any defects in the wiring. It became hot and useless. A.: Without any further information, it would be almost impossible to say what the trouble was. Undoubtedly a short-circuit is taking place, probably in th primary eoil. To ‘xvoid this, fix fuses in™both the primary and secondary leads, and your transformer will not then burn out. The electrostatic shield may be shorting against the wire, and in leaving it out you are not doing anything to cause harm, : N 2 (Auckland): Would the inser- * tion of a fixed condenser between the plate of the. detector valve and the earth lessen a.c. hum? A.: It is questionable. It all depends upon the source of the a.c. hum. Some time ago an article was published in the "Radio Record" dealing with the tracing of hum in a.c. receivers. We advise you to read this up again and carry out the tests. You will then have some idea of where the hum is originating. The connection of a, say, 2 mfd. between the plate of the detector valve and earth will certainly have the effect. of by-passing some of the hum frequency, but whether or not it will eut out the hum about which you are complaining, we cannot say. You ‘are using a three-valve commercial receiver of a rather intricate circuit. and it is just a little bit risky to experiment to try to find out from where the hum is coming. However, read the article. BRED. (Herekino) : I built the Round the World Two, but am not geting the best results. I am using a .00014 tuning condenser and a .0005 hr. reaction condenser with two plates removed. How many plates must I take off to reduce this capacity to .00025? A.: You will require 11 plates, 5 moving and 6 fixed. 2. I also have another .0005 condenser, with only 5 moving plates remaining. How many should I leave in to get to .0002? A.: You will need 4 moving and 5 fixed. ° KAttoA (Nuhaka): I am using A615 and B605. valves in my five-valve American set. Do you consider that 221’s .or 201A would be an improvement? *: A.s We do not recommend the use of A615 throughout. Very often A609 used in all stages, except the detector and the last, will give- quite good results in an American receiver. However, generally for safety, use the 221’s in all stages exeept the last. You can, of course, use B605 in the last stage. It is an excellent valve for a medium power output. A615 ean usually be used as a detector. It is in the radio frequency stages where care must be taken with the selection of the valve and in this stage we strongly recommend the 221. ecrom (Wellington) : I have a sixvolt dynamic speaker. Can I use a 1.3 amp. charger to energise the field? A.: Yes;.you may -be troubled with slight hum, and-if this is the case shunt an electrolytic condenser across the field winding and -you will have no further trouble. 2. Is it possible to rewind a 6-volt dynamic speaker field to make a d.c. 8000 ohms field to build up into a Loftin White?

A.: It is possible, but if it is a com-mercially-made receiver we do not advise you to change it. You could quite easily make it all-electric by using the battery charger, If you wish to make a d.c. field you will require-3lb. of 88 enamelled wire. It will pass. 80 mfds. and have a resistance of 6500 ohms, 2. Have you published a circuit using a 245 valve in the output of the Loftin Four? ., A.: We have not published a circuit of the four valve complete. However, you can use the ordinary Loftin White amplifier for the audio side and use the two radio valves as shown for the Loftin Four. C W.B. (Martinborough): Is a erystal * set, a circuit of which I enclose herewith, suitable for reception at Martinborough? A.: No. The set is a rather fantastic one and uses many devices that the average crystal set has no need of. The utility of such extra components is always to be questioned with a. erystal set. Very little can better the straightout set-a coil, condenser, crystal and phones. Nothing else is needed, _ 2. Tf the set is not suitable, would take much extra to make it into a small valve set? -; No, you_could use the parts for the "Ranger Two," a set which was described in the "Radio Record" a short time back. You can use the .0005 variable condenser for tuning and a .0001 for regeneration, It might be as well to wind another ordinary solenoid coil, although probably you could use the basket coil and get good results. All your components, with the exception of the erystal, could be used in this set. A M.T. (North Otago).-I have con"structed an output transformer with the idea of matching a pair of 245’s in push-pull to a moving coil loudspeaker. In actual working the M.C. speaker is brilliant and pleasing, but it is doubtful whether the very loud notes frequencies are produced on quite the same level as the middle and upper. I am using a 20-1 output transformer. A.: The rule for a transference of energy is that the speaker impedance should be double that of the valve. If it is not this, then you are likely to suffer from a lack of base in the production. This appears to be your case. Without knowing the impedance of your speaker, you ean safely reckon it to be 30 or 40 ohms. A pair of 245’s in push-pull has an impedance of 3800 ohms. The formula for computing the value for the output transformer is :- R=(/2Ri- Rs: 1 Where R is the ratio of the output transformer, Ri impedance of the valve and Rs, the impedance of. the speaker. The equation in your case comes down to this :- R= = :1=10:1 As you are using a 20-1 transformer you are not getting the full base. Try adjusting the secondary of your output transformer until the ratio ‘is correct. You will then find that the base will come up much better than formerly. N. (Gisborne): I have a five-valve * sg. set. How far should I get with an aerial 60ft. high? A.: You should be able to get the main New Zealand and Australian stations at satisfactory strength. You may possibly get an occasional Jap or an American, but the reception of these is very much dependent upon locality. R L.F. (Lower Hutt): Could you tell * me if a .0005 variable condenser would be suitable for the tuning condenser of the "Sparrow Hawk One’? A.: Yes, it would be suitable. You would, however use 15 turns less on the broadcast coil. The ‘short-wave coils would remain as specified for the .00085 condenser, a ,

2. Would a choke consisting of 100 turns of 32 gauge d.s.c. wire spaced to she thickness ‘of the wire do for the same set A.: Yes, it would make an admirable short-wave choke. HD-UP (Otahuhu) : My set breaks into a hissing noise after it has been going for half to one hour. It is not noticeable at night, and if another set is connected between the aerial and the earth the noise persists. A.: The fact that the noise persists when another set is used shows quite

clearly that the noise comes from power interference. Your best plan would be to check up on the neighbours and see if they get the same noise, and if they do, communicate with the radio inspec tor of your district. If théy do not, then look to_your aerial and earth installation. _ Possibly it is in some way at fault and causing the trouble, but we think that it is power noise interference. Ww. H. (Gisborne) : We refer you to the article to be published next week concerning the "Kestral Three." ee

Questions and Answers «Continued from page 15.)

V C.B. (Taranaki).I cannot get loud * music without a rattle, I am using an electro-dynamo speaker with PM 6 in the last stage with a Balkite eliminator A.: The rattle may: be due to overloading the PM 6. -'This valve cannot be used satisfactorily with the M.C. speaker. It has a high impedance, and consequently needs a special matching transformer. Apart from this, this yalve used in conjunction with an M.C. speaker, does not always: give satisfaction. It is a medium valve, really for a cone speaker. However, as you suggest, some of your trouble may be coming from the eliminator. 2. I have ‘poured out all the contents of the small jars in the eliminator. What do I replace it with? I understand it» is a ‘special mixture. : It is a solution of sulphuric acid and water. However, if you write either

Ballingers, Wellington, or Jolins, Auckland, you. will be able to find out a good deal about your Balkite eliminator, as they are the agents. 3. What valves suit the set. best? A.: Three or four 221 valves, or three 221’s and one 615, with an output valve such as PM256. 8. I am not using a "low" terminal. Is at harmful to the set or the eliminator ?- O. ae Kr (Frankton) : Is it possible for me to use earphones with my commercial set? A.: Yes. An article will appear in the "RR." shortly on that subject. It will not be a very great advantage to use phones, for, if you can hear the station on the speaker, you can rest assured that the noise will be too great to hear it on the phones. . , A™MBROSE (Auckland): I own .a fivevalve portable receiver, but :cannot eliminate a strong Morse station: on 600 metres, I have been using a wavetrap, which, although it will separate the broadcast stations, will not cut out this Morse. on A.: We can suggest but little. The wavetrap will not cut out the station because it cannot tune to it. If- it were possible to tune in the station on his proper frequency and then adjust the wayetrap, you would probably be able to cut it out. However, by juggling the wavetrap about on the lower frequencies you may possibly be able at least. to diminish his strength. Other than that we can only suggest altering the direction of the aerial and loosening the coupling between the aerial and grid coil. If you had a more sharply tuned set the trouble.would not be so apparent, although these. Morse stations can sometimes make grave nuisances of themselves, and they ate ifficult to eliminate. Lodge a complaint, with the radio inspector, who may possibly do some good. ‘

Motnoure (Hastings) : Is it possible to hook phones on to an ‘electric set? A.: Yes; an article fully dealing with the of phones to an a.c. set will be described in the "R.R." shortly, 2. Could you give me some informatie about the Pierce earth system? Aw: Look up the "R.R." of March 6." Novice (iiltham) : I have been oparating g my set for nine months using a 6-volt "A" battery and am using: the original set of "B" batteries, which are now getting very low. The set emits a continuous whistling noise when switched on. Last time I charged the "A" battery it lasted 20 hours. Are these-faults due to the "B" batteries being about done? A.: Without doubt the whistling is due to the "B" batteries being ° exhausted, but this will not explain the "A" battery

running out. This could possibly be due to a broken-down by-pass condenser, When next you ¢connect your "A" battery, connect oné of the terminals and hold the other connecting wire in your hand. Now flick it against the free tex minal, which normally you would connect it to. If the set is turned off there should be no sign of a spark. If, however, there is a short cireuit in the set, there will be a slight or a big spark, dependiug upon the nature of the breakdown. If you have a torch lamp you could hold it between the connection and the terminal on the battery; If there is anything wrong in the-set the lamp will light. TTATTO (P.N.): I have an audio transformer which I use in a push-pull circuit. Can you tell me the value of the resistances to shunt ‘across the secondary? A.: Almost any value of grid-leak up to 1 or 2 megs. will do. Say two 250,000 ohms. each. A J.S. (Oamaru).-We hope we got * your questions right. O.M., but it would have been clearer had you tabulated them. You have written q rather long Ietter, and it is just difficult to get at exactly what you want. We hope our summary is correct. _ © ; 1. On looking up the "Radio Guide" to see what valve should be used with 160 volts, I find that PM256 requires 250 volts, yet it works very well with 160. A.: This, unfortunately, is incorrect. The PM256 requires on the plate 180 volts, It will. however, work excellently with 160, 150,'or even 100 volts. It should not have a very detrimental effect wpon your battery. 2. Could I use an "A" battery and make a 50-watts transformer, using 40-gauge wire for the "B" supply? A.: We do not advise you to use 40pauge wire. It is far tou fine. Use the 86 wire specified. 8. Is stalloy a silicon steel or soft iron? A.: A silicon steel. ; ' 4, Must TY wire for 250 volts on my

transformer, and tap 150 volts for 6 volts and 224 volts, or can I wire for ess on my power valve? A.: We advise you to wire for 180 or 200 volts, and if necessary take the tapping out at 150, but it is better to break down your voltage with resistance vutside rather than tapped secondaries. We cannot see the need for the 6 voits and 224 volts. A-6-volt accumulator would be needed to supply the current to the filament. .The 223 volts bias must likewise be supplied by batteries’ or. by using yoltage drop resistances in the set. ‘There is no point in tapping the secondary of your transformer to také off "A" and "B" current; They would need different rectifiers, and in the case ;of the "A" eyrrent, very heavy wire. 5. Can 201A be used for. a rectifier? A.: Yes, but it would not supply suflicient eurrent for long enough for: your set. * U* (Te Kuiti)-Can I use qa Clough system audio transformer to follow as. valve if I changed the 10,000 ohms plate resister to.a .2 meg. leak? © : A.: Yes; in all probability it will work quite well. In any case, it is worth €xperimenting with. me 2. Cau you give me any information about the grid-glow relay valve invented by the Westinghouse Laboratory? — A.: This question is rather beyond the scope of questions and answers, However, we shall do our best to look out some information for you. It will be published elsewhere, if we manage to locate it. 8. Is there any chance of an article on a set-tester appearing or, at least, a cir cuit of one? . A.: The demand for this would be fairly small in comparison with the amount 6 work entailed in preparing a special article. We refer you to the "Radio News Service Manual." A copy of this could be obtained from the -Te Aro Bookshop, Courtenay Place, Wellington. . ae