QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 16, 26 October 1934, Page 25

QUESTIONS AND ANSWERS

TW. (Paerou) : It would not be possible to adapt your commercial receiver for short-wave reception, though you could use a converter such as. the ‘"Lekmek A.C. Converter" (deseribed in the July "Radio Times’). in conjunction with your'set,- with good results,. ° You could obtain ‘a circuit diagram of your set from any modern service manual. "PLARTHQUAKD" (Seddonville) : t have built a 45 volt ."B" battery from flashlight batteries as described in the "Radio Record" of September 21. My set has two wires: connected to the 224 volt terminal. On which unit battery should I connect these wires to’ get 223 volts? A.: To the positive terminal of the fifth battery from the negative end.. "VyiEso" (Hangatiki) : . Your set is operating quite normally. The hissing you can hear when the yélume com trol is turned full on is due partly to valve hiss, but mainly to static and general "mush" picked up by the aerial The hissing is most pronounced when the volume control is turned full on, ag the set is then in its.most sensitive condition. "Me. X" (Christchurch) : Of late my set has developed rather a bad hum, which is much worse ‘sometime. than others. Also, when a very high note is reproduced thére is a suggestion of a rattle in the speaker, A.: This sounds as though the outpu: valve needs renewing; or that the voice coil of the speaker is a little "off centre." Re-centering it ig rather a delicate operation, and hence it is suggested that you return your set to the agents for overhaul, MSK" (Whakatane): What is the meaning of the tettn "diode bias"? I understand it means. the biasing of the triode section of a duplex -valve. A.: The resistor shown in your sketeh is the diode load. resistor, which in a diode detector serves the same purpose as a grid leak in a leaky grid detector, In diode detection, signal voltages appear-

ing across this load. resistor constitute the bias applied to the grid of the triode section of the valve. ‘Thus, the end of the load resistor connected to.the bottom of the last if, transformer secondary becomes more or less negative to earthie., to the cathode-depending on the strength of the received sigmal. The action of the diode detector ‘is yery fully explained in the July "Radio Times" in an article entitled "The-'Theory of the Diode." . B. AH. (Wellington) : The set you are planning to build would give only a very indifferent performance, as the circuit contdins a number of minor errors. Why not. build up the "Modern Midget Four" deserted in the September, 1933, "Radio Times,". or the "DX Beonomy Three," described in the last September issue?. ; D*97 (Wanganui): A’ book which covers the elements of short-wave reception very well is the "Amateur Trans: mittéi’s Handbook," 1934 eflition, This thay be obtained from ‘the Te Aro Book Depot, Courtenay Place, Wellington. 0.0. (Niho Niho): Is it possible to cut out reception of: second * harmonies? They are very annoying when one is dxing. ' A.: They could be eliminated by using ‘a wave-trap, but there would be no point in doing so, as the stations on adjacent frequencies would be trapped out as well. 2. Does 3ZE, Greyniouth, broadcast at night now? A.: On Saturday nights only, from 9.16 to 11 p.m. 3. Do European stations have any special times to announée their calls? A.: No, announcements from these stations are generally irregular. "CRYSTAL" (Inglewood) : What ig the. range of the "Selectra" crystal set ‘ deseribed in the.‘‘Radio Times?" A.: Ten to fifteen miles is ‘usually re: garded as the maximum distance over which a crystal: set will ‘provide good. headphone reception. However, there are many exceptions to this rule; as was proved by the series of letters published in the "Radio Red¢ord" several moiiths ago from readers, who claimed that: they. could pick up even Australian stations:on a crystal set without any additional ‘amplification .. whatsoever. « Apparently, locality plays an important influénce -on crystal set reception,. though ‘‘of . course an efficient. serial, and. garth ‘installation is always of: the first importance, . ~° 3. What would he: the cost of building this set?.. A.i About Bb, including headpliones. B.W, ‘(Auckland)’: "An ordinary. ent. * bon résistor of the brand, you men tion would, be: quite: satisfactory, "It should be sealed in a glass ‘tube, 28; -you,,, suggest, to shield it from exposure. to the_ weather, ‘ 2. What’ is the divection of Daventéy. from Auckland? **: nee Al: Roughly; B.N-H. Siete te " "TGNORAMUS": (Te Awwamiitu) = "y hive’ . a two-valvé: set which, gives. good * signals while it is oscillating, though re ~ production ia apt to be'distertéd,: How: 3 ever, as soon as I ‘turn, the reaction con:

trol potentiometer past the point of os-. cillation, volume drops cénsiderably. 8 this quite in order? Be A.: You should.-never listen. -t6 any station while your set ‘is oscillating, for three reasons. The first, and by far the most important, is that by so doing you are interfering with the reception of your neighbours for miles around, and this in itself should be sufficient reason for you to keep your .reaction control turned back. However, in addition to creating interference in this way, you are also spoiling your own ‘reception -by attempting. to listen while the set is oscillating, for. signals are not as.loud'as when the control is set just below the point of oscilla-_ tion, and also, as you admit, they are distorted.. A. reaction ¢ontrol of any type rarely, if ever, operates so that it is at. the optimum setting "when turned full on.* There’are a nuthber of factors. which determine the best. sétting for ‘a control of this type, the most important being the number of tutns.on the reaction winding ; type of, detector valve used ; the, amount of plate voltage applied to the’ detector; size of. grid leak and grid condenser, ete. .All these hayé an influence on the optimum setting of the reaction control. If it is operating. correctly, then’ when it is advanced slowly, a setting should be found where a hissing noise | comménces and the set sounds "live." When the control is rotated a‘ little fur-. ther, the noise should .inerease in inten sity and then usually a soft "plop" is’ heard. 'This indicates that the set ig occillating, and so the reaction’ contiol should never bé advanced.as far as this. With the control. set in- its. correct’ posi-° tion, you should not he able .to ‘hear a whistle.as the station: carrier, is passed over-just:a "swish-swish." If- possible, : you should try "and add’ a sereen-grid-r.f. stage to your set. This will not only give you much more powerful signals, but also. it will eliminate the "possibility of your set interfering with other people’s recép- _ tion, . .. A G. (Auckland): .We-cannot uider- * take to check: over wiring diagriins. In any case, yours could*nét: be checked thoroughly as the coil ‘conhéétions "are (Continued. dn: page 27).

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‘ Questions and Answers

(Continued from page 25.) not indicated. If you have a circuit diagram, send-it along- and. repeat your questions, XER". (Waitaka): I have recently had trouble with my battery-oper-ated. seven-valve superhet, which was installed by a serviceman about five months ago. Though it gave yery fair results, both yolume and tone are not up to expectations, Recently I gave the set a thorough overhaul, and found that the bias battery had been connected up wrongly. When the connections were made correctly, the set performed as it never had before. Has ‘the set in any way been ‘harmed by this mistake, and if so, what parts are most likely to be damaged? A,: The‘reversal of the two leads you mention meant that.the bias supplied to the driver valve (type L2) was + 8 volts instead of --3 volts.- This would have the effect of increasing the plate current of this valve far beyond its rated limit, and hence: by now its emission is probably ruined. ‘This also would account for the poor. tone and volume you have been experiencing, ‘Che only other component which could be affected by the mistake. would be "B" class transformer ; the high plate current passing through its primary might have burnt it out, but evidently this has not happened. Of course, your ""B" batteries will be a little flatter now than if ‘the ‘connections had been correct in the first’ place, owing to the greatly increased current taken by the L2, 2. The set whistles loudly on three or four stations between:245 and 253 metres, What is the cause of this? A.: It is improbable that this will be due to oscillation. It is more likely to be -heterodyne interference, which is very pronounced on this portion of the waveband. 3. Is the set harmed in any way by using it. with a pick-up? If not, how could one be connected to my set? A.: The set would not be harmed in any way. The simplest way of using a pick-up to your set. would be to connect it’ across the 1 megohm grid leak, I "M.I, (Auckland) : Could you supply *: details for a wave-trap to cut out 1ZM, ‘Manurewa? A.: Connect 'a-coil of 50 turns or so of. 28 ‘d.s.c.,. wound on a 2in. length of 1din, diameter ‘former,. across a .0005 aid.-variable condenser, Then take the

aerial to one side. of the coil-condenser combination, and take a short lead from the other side to the set. Then tune in 1ZM, and adjust the condenser until the transmission disappears, or greatly diminishes in volume. 2. Would an A409 be suitable for. the enclosed circuit? » A.: Yes, quite suitable. The A409 isa general purpose ‘battery valve taking 4 yolts on the filament, _ 8. What resistance would be suitable for a pair of phones? A.: The resistance is not very import-ant-any pair of phones with a resistance of from 1000 to 4000 ohms per earpiece’ would be quite suitable. "ENGINEER" (Lake Coleridge) : I propose to build the "Times" oscillator into the cabinet which already contains the "Times" analyser. I find it impossible to get all the components into the space I have available, unless the type 30 valves are mounted horizontally. Could this be done without. danger to the elements? A.: Yes. _ 2. Would the operation of the oscillator be affected by placing the components close together? . A.: Within limits, no. Try and keep the coil at least an inch away from any metal sereens or partitions. The space you have available should be sufficient. Note: You will find that you will obtain the best coverage of the broadcast and if. bands with plug-in type coila-by reducing the turns on the broadcast coil to 95 and on the i.f. coil to 400 turns, centretapped. ADIOGRAM" (Auckland): ‘The fault in your combination receiver is rather a puzzling.one, and providing the new 224A is not defective it is difficult to suggest a solution. The 224A is identical in type with the 224, but is of improved construction, and hence it is quite in order to use it for replacing the earlier type valve. No doubt an examination of the circuit of your set would give some clue to the source of the trouble, but unfortunately you have not quoted the model of your set.