QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 17, 6 November 1931, Page 15

QUESTIONS AND ANSWERS

FRANK

KEE

The Technical Editor will, through these columns, be pleased to help readers experiencing trouble with their sets. Queries are limited to three-for more than this a shilling fee is charged, and a similar fee is payable for queries answered by post. Supplying layouts. -eircuits and solutions of intricate theoretical problems is beyond the scope of this service. A coupon must accompany all requests for information. Nonappearance of the coupon in any issue cannot be regarded as a reason for its not being used. Address all queries, The Technical Editor, Box 1032, Wellington.

H.C. (Swanson) : We do not think * radio science will totter because of your. disproofs, Before you can disprove a you must know something about t. We would advise you to look up a few xt-books and then start all over again. OMSE (Waikanae) : If I connect my earth to the aerial terminal I can obtain good results, but on turning up the volume a loud surging noise is heard. Why? A.: By taking off.the earth you are working your set nearer the oscillation ‘point. By forcing the volume you cause it to break into‘ oscillation or go very near to it. Hence the surging noise’ You are in no way harming your set, although your neighbours. may bless you. 2. If I remove the two power valves will I get less static and better volume? A.: No, if you take out the power valve your set will stop unless it is specially fitted to take. earphones from the first audio transformer. 8. I am troubled with a crackling noise in the day time only. A.: This seems to be a nearby interfering electrical apparatus. Probably something that is operated during the day only, e.g., an electric motor. KENNY (Kopaki): Your dynamo will charge probably at 1.5 amps, The method of calculation is by determining the watts output and having the voltage you can ascertain the charging rate. two sets of figures, however, do not "agree. When charging at 6 volts it appears that it is capable of a 12 watts output, while at 80 volts it appears to be 20 watts. Taking intermediate result of 15 watts and charging at 10 volts you can®see that the rate would be 1.5 amps. 2. What may cause the primary of an audio transformer to burn out when connected into the detector plate: circuit?

A.: Excessive plate current taken by a power valve used as a detector or a poor transformer. 3. My "A" battery runs down very fast. When I connect the dynamic speaker to it there is a small flash. Does this indicate trouble in the speaker? A.: No; it merely indicates that the speaker field takes a fairly large. current and this causes a flash. 4, Does the speaker still draw current when the set is shut off and the speaker left connected to the "A" battery? A.: Certainly ; when you have finished cut off the field of the speaker. Perhaps this is why your battery is running down. rs Boe AND LOW (Frankton): Your propostion is not a practicable one. unless you know a fair amount about electrie sets. D C. (Gore): I have built broadcast * coils for my four-valye short-wave receiver, but it is very broad. How can I improve matters? A.: Tap the coil in the anode circuit of the s.g. valve and try taking the connection from the anode of that valve into one of the tappings instead of to the top as at present. Reduce the number of turns in the aerial coil to 20. By doing this you may lose a certain amount of sensitivity. Bias the radio valve by inserting a 14-volt grid bias battery between the end of the coil and its connection with ae Try a midget condenser in the nerial. OPTICS (Dunedin): The volume control on my set is far from steady. Instead of giving graduated volume it will suddenly jump from very quiet to a roar. Jf tuned to the local station further rotation of the volume control will cause the signals to become weaker, but on moving the tuning dial off resonance, the signals come back strongly again. A.: The volume control should not be jumpy. ‘This indicates a defect in the resistance compassing the volume control. The diminishing of the signals on the local station indicates that the set is being overloaded. When you, tune off resonance you weaken the applied signal, and consequently the overloading ceases and you again get the maximum amplification from the set. By swinging the small tone control from zero to maximum I cannot detect any difference in tone other than an increase in volume and noise. A.: Some tone controls are like that. Instead of accentuating the bass they merely by-pass the higher frequencies, which. by the way, are the ones causing the crackling noises. The fact that these disappear indicates that your tone control is operating. ‘The apparent reduction of volume i. caused by these high | frequencies being cut out. | 8, What parts of the valves are affect- ' ed or controlled by, (a) the volume con- ' trol, (b) the tone control? | A,: The volume control affects, usually, ' the voltage on the screening grid of the $g, valve. In the newest seis it operates

fs) on the eathode. A tone control is a resistance between the grid of one of the audio valves and a condenser connected with some point of low potential. Dx}w (Shannon): How many turns and what gauge and type of wire would be required to be used on _ the stampings of an old audio transformer in order to make an output choke?’ A.: You will require about 10,000 turns of No. 36 gauge enamelled wire. Take out tappings at every thousand turns. If the bobbin is not quite full when you -_

have finished the 10,000, continue on, until it is full. 2. Would the wire have to be wound in regular layers or jumble wound? A.: You need not worry about insula= tion, but wind on the wire jumble fashion. ‘Take out the taps by carrying the wire over to one side, There is ‘no neéd to be exact in the positioning of the taps. a few turns either way will not make any appreciable difference.

voreveterarce Information Coupon * (To be used with all requests for information.) Name of set... ...sceeee eoveceee Number of valves ..ccocececceccvces Name Prerrrrrrrrrrrrrr Trt irr tii te ACAreSS secccccecccvccccvceccesee: sss eeceeccvensccesccceenecaseeccoouss Nom de plume eeteceereeoeeeeeeeart To be kept in subsequent inquiries Date «cece eee eee eee See ewes Please Note:(1) Be specific and brief, tabulating, if possible. (2) wee legibly, aud on one side of the paper. (3) We .do. not design circuits, but accept suggestions for feature articles.

Economy (Hakataramea) : I wish to use a "B" eliminator for my set. Would an electrolytic one be satisfactory? A.: We think the better plan would be to use an eliminator employing valves or dry rectification. You will want one that will supply at least 20 mamps. at a maxi mum of 150 yolts., HORT-WAVE (Ruotoria): I wish to buy a_ short-wave set. but do not know anything about them. Would it be. possible to get a set like your SuperA.: Your best plan would be to buy a commercially made short-wave adapter, and suggest your writing to one of the better-known firms and asking their ad- . vice, as it is impossible for us to recommend any particular make. We have tested one only-the "Add-a-phone’-made by Mack's Radio, Wellington. AUSSIE (Wanganui): We think it ‘ would be inadvisable for you to shorten your earth. Shorten the lead, if you like, but do not take away any of the actual connections with the ground. INY (P.N.): We suggest your using \ A209 1st radio, AG15 detector, B605 audio. We'do not happen to know the agents for your eliminator, but suggest our writing Abel, Smeeton, Ltd., Auck- nd, whom we think may know something about it. Any good _ radiotrician should be able to put it right for you. _ 2. Regarding your last question, it is to diagnose this without further particulars. lt seems that the coils

are slightly out of balance or the condensers not quite of the same capacity. STATIC (Denniston): I find that by disconnecting the earth the volume becomes 50 per cent. stronger, Is it injurious to the set to work it in this manner? A.:By disconnecting the earth you are working your set nearer the oscillation point, and so long as it does not squeal it is quite satisfactory worked in this manner, 2. From 15 to 20 degrees on my set ihe stations become very congested.

Would shortening the aerial help to remedy this? A.: It is doubtful. That part of the ether is very congested. Try the effect of a midget condenser in your aerial. You can take this out if the loss in volume offsets any gain in selectivity. od B.H. (Auckland): How should the °* copper sulphate solution of a Daniells cells charger be made up? A.: Dake sufficient water to half fill the vessels in which the porous pots are to be placed. Add to this ordinary bluestone until no more can be absorbed by the water, ie., it will not dissolve. The solution is now saturated and ready for use, 2. How often should the sulphuric acid solution be renewed? A.: When the voltage drops off, but often the addition of a little fresh sulphurie acid to the solution will bring it back to normal strength. . GIMPLEX (Waikanae): I cannot get 2YA as strongly on three valves as 0 n two. A.: Although you have had the valves and transformers tested, we still suspect the transformer. Are yon quite certain you do not have it connected round the wrong way, or that there is a breakdown between the installation of the primary and secondary? You need not worry about matching the valves, 2. How many turns should I have on the tickler? A.: About 25 turns. [t should not be necessary to use the rheostat to cut down the whistle, which should be controllable by your condenser, Try reversing the connections to the tickler, 3. Does it make any difference which ends of the rheostat are connected to the battery and set respectively.-No. TIIC (Dunedin) : What would kill * the 245 valves in my commercial set. They go blue and squeal when starting up from the cold, and the set is generally poor until I replace them? A.: If this happens with one set of valves, we would say the valves themselves were faseous and at fault. Tf it

happens regularly with all valves you put in the circuit, we should imagine the 800 ohms bias resistance between the centre tap of the 245 filament and the ground ‘to be at fault. 2. Could I put an s.g. valve in the first stage? A.: It would be possible to do so the way you suggest, but you would take a big risk, Do not forget that the grid is" now On the top of the valve, and you would have to break the connections in your set. You might, too, throw your condensers out of alignment and so lose all you would gain and perhaps a little more. The same applies to the s.g. valve used as a detector. ' JC AREBANA (Kelburn): We suggest you adding the anode coil. Just offhand we cannot tell you the number of turns to use. but try half the number on the grid coil to start with. If the set oscillates too readily-you will detect that it is oscillating, because it will squeal-take off some of the turns: until the oscillation is controllable by the variable condenser across that particular coil. ASS. (Wanganui).-I have had uittle * suecess with the "Sparrow Hawk" one on shortwave, the only coil working being the 40-metres one. A.: You have not described your symptoms, so we cannot help you. ‘Try reversing the tickler, and adding more turng to this coil. 2. Results on the broadcast band were at. first unsatisfactory, 2YA taking up half the dial, so I connected a .0005 condenser in the aerial, and results were* very much improved. A.: Such a set as the "Sparrow Hawk" is very unselective, and it is not surprising that 2YA should come in over half the dial, Using a midget in the aerial will always have the effect of improving selectivity. 3. How would I use two 45-volts batteries with the "Sparrow Hawk" one so anne I could break the voltage down to A.: Use a 10,000 ohms variable resistance in the "B" lead which you wish to break down. Otherwise take a tapping off the battery. W J.C. (Wanganui): What circuits do °* you recommend to use in the shortwave adapter with my three-valve electric set? A.: It is difficult to make a shortwave adapter. As for a battery one we can recommend none better than the "Sparrow Hawk," described in the 1931 Guide. It will cost approximately £3/10/-, the batteries being extra, AERIAL (Thames): We think you will get. better results by using a commercial shortwave adapter. 2. Recently I extended my aerial to nearly twice its original length, this increasing the volume but making no difference to the selectivity. The length is now approx. 65 feet and 30 feet high. Do you think a further 30 feet to the length would make any difference? A.: You have brought your aerial nearer the ideal, consequently you would not expect that the increased length and height would make an appreciable difference to selectivity. It ig doubtful whether your proposed extension would be any improvement. We are writing "Digger" asking him to get in touch with you.

LL? (Auckland): How is the voltage divider resistance calculated? A.: By Ohms:law in the following man-ner:-A voltage dividing system should pass from 10 to 20 on constant drain, and, by knowing the drain at each point, the total resistance in the divider can be calculated by Ohms law. First set out exactly what must be supplied, say, 30 mamps are required at 260, 10 mamps at 150, 5 at 100, and 2 at 50. This makes a total of 47 mamps, and to this must be added the constant drain, say 15 mamps, ° making a total of 62, which is the output the system must be capable of delivering. If the power pack delivers voltage at the highest potential: desired (in this instance 260), it will not require a resistance. This is desirable because of the heavy current which. would have otherwise to be passed though such a resistance, The voltage divider will then be called upon to break down 260 to 150 passing 10 mamps, 150 to 100 passing 5 mamps, and 100 to 50 passing 2, but to each must be added the "bleeder" of 15 mamps and the. current to be supplied taps further down. Then through the first section will pass 32 mamps (10 + 5 + 2 +. 15). Through the second 22 (5 + 2 + 15), through the third 17, and through the final section, which will reduce the 50 volts to zero, 15. The tesistance values can now be computed by 0} formula: R= — for the first section this equals 110 — 32 x 1000 — 3450 ohms. For the second. 50 -- 22 x 1000 — 2280 ohms. For the third, 50 -~ 17 x 1000 = 8400. For the fourth 3340. moling a total of 12,470 ohms. (Note: The figures are approximate only.) 2. How can capacity be measured other than by using a capacity bridge? A.: There is a method by using a neon lamp. For full details see "Television," Vol. 4, Nos. 87 and 38. 3. What are the winding specifications for a microphone transformer? .: A. microphone transformer has a ratio of 100 — 1. It is not advisable to attempt to make one yourself as very fine wire must be used on the primary. (QQUILP (Te Aroha): I have a battery Set with an eliminator, and when ee set is turned on a noise like static is eard, A.: This sounds like condenser trouble. Probably one of the smoothing condensers in the power pack is at fault. Your best vlan would be to take your eliminator in to a dealer and as him to put it on a set and see if it is all right. Failing this, it would be necessary to get somebody who knows something about radio to go over your set, as it is possible there may be (Concluded on page 30.) Ps

Questions and Answers (Continued from page 15.) a defective condenser or a loose connection. M2? WHARE (Feilding) : In the folder which accompanied a PM6 valve it says that the maximum voltage is 100, I this, going to 185, but the valve becomes hot. Is this in order? : No; you should not exceed the voltage recommended by the manufac‘turers. In your ‘case you have one of the older type PM6’s, the anode voltage of which must not exceed 100. On _ the more recent valves 150 volts can be used. Often the voltage on valves can be exceeded, but in this case we think you would got no appreciable improvement by increasing the voltage. beyond 100. -P*914, (Pukeroro) : What valves a thould I use in a Radiola 20? ° 199 in all stages except the last, where use 120. 2. The address you require is L. M. Silver and Co., Taranaki St., Wellington. F.L. (Auckland): Will the "Kestre! * Three" bring in Australian "A" class stations on the speaker? A.% Fairly late at night you should ‘be able to get the main Australian stations "at speaker strength. We were able to pick them up’ quite easily when testing out the "Kestrel.’"