QUESTIONS AND ANSWERS

Radio Record, Volume VII, Issue 52, 5 July 1935, Page 47

QUESTIONS AND ANSWERS

. oN L W. (Wanganui): I wish to make the "Trans-Tasman Three." ‘Could you give me. the details necessary to make my own coils? A.: The following details would be suitable for coils wound on 1fin fermers: Aerial primary 20 turns, secondary 110 turns (primary wound 1-Sin. below ‘bottom of secondary). Detector coil, primary 60 turns, secéndary 110 turns, reaction 35 turns. The primary winding. should be put on over the bottom of the secondary, the two windings separated by. a layer of Empire cloth or insulation. tape. The reaction winding should be spaced about jin, below the secondary. Use 30 or 82:d.s.c. wire for all windings. IDGET ONE" (Waikaune) : I would like to build the. ‘Midget One," but want to use two .0003 mfd. condensers I’ have on hand, and also 5-pin. plug-in coils, Could: you give me the broadcast band and. 25 to 31-métre coil details? A.: Broadcast coil, aerial primary 25 turns, secondary 115 turns, reaction 35 turns, The primary should be wound either above the top end of the secondary or over the bottom end. In: the latter case, the two:windings should be separated by a layer of Empire cloth or insulation tape. ‘The reaction winding of 35 turns should be put on about din below the secondary. Unless you have a gvod. yernier dial, you will find tuning op the short wayes very tricky, Owing to the comparatively large capacity of the tun- _ ing condenser you are using.. However, the following coil should give you tbs coverage you want-aerial, primary 3 turns, secondary 4 turns, reaction 3 'turus, Put the aerial primary on abdye the see~. ondary, and: the reaction winding below, with a distance of abour 1-8in. to fin. separating the:windings.~ Use’32 d.s.c. for the broadeast coil, and any wire heavier than 26 gauge for the short-wave windings. 2. Would I need a éhielding can? LAs: No, this will not be necessary, S. W.E.. (Christchureh)': 1 have built’a *Comet Superhet, ‘Five," but cannot ° get rid of a bad hum. 3YL comes in on

three places on the dial. Would a different I.E. help at all? ' A.: There is something radically wrong somewhere, as thé hum level should be negligible. Also, you’ should not.get repeat points, 175 k.c. L.I.’s are quite satisfactory, and we do uot advise yuu to change them. It appears-as though for a start the set is not correctly aligned, and your best scheme would be to either call in a serviceman Or take. the-set along to a dealer to have this done with an ovcillator, Wave you tried shorting the grids of the successive stages to earth to see when the hum disappears? In this way you will be able to localise the trouble. . R.S.. (Auckland): In the article de- *. -seribing the "Air King. de Luxe Wight" it is’ stated thut practically 110 in.a, flow ‘through the 1000 ohm field plus the 25 ohm resistor,’ thus making 1025 ohms. This. gives approximately 108 volts bias ov the +5's. Ts this not too. high? . "Ac: It would be, except for the. small voltage drop in the. 10,000 ohm. decoupling resistor, This‘is due to -the slight current drain taken by the $‘mfd. by-pass electrolytic. ,In this elass ‘of condenser, a leakage current ‘up to.2 mils. is permissible. Your best: scheme --is. to measure the bias actually developea, and if it is too high, increase. the decoupling resistor to, suy, 50,000 ohms. If the bias is still too high, connec; a2 100,000 ohm resistor from the negative side of this resistor to eatth, thus giving a voltage divider’ effect. .By using different yalues of resistors any desired bias can be obtuined. "PXQUIRER" (Methven): It. would scarcely be’ practicable to alter your set to: give complete all-wave coverage. Your best scheme would, be to build. one of the short-wave converters described in the May "Radio Times." " J 8S. (Thames) :-The trouble in your * receiver most probably lies in the power-pack or the audio . transformer. The.smoothing choke is most likely the offender, .but. also you. should check over the electrolytics and .the . by-pass condenser from ,one.side of the mains primary to earth Another possibility is that the primary winding of the audio transformer is defective." ‘Try momen: tarily shorting the plate of the driver and that of each +5 to earth.. If the tvouble lies in. either. the trangformer or. the smoothing Choke.the heavy current — will probably open up the break compietely. 7 HLS (Bay of Plenty): You should’ use the valve types already:in yout ser for replacements. You could not use the Philips Octode because. your set las not been cesigned for it,. This. valye is a mixer-oscillator, combining the duties of first detector arid oscillator in one valve: ‘In your set separate valves are: used. . "RRA" (Auckland): 1 have had no _previous experience in. set building, Could I build: an‘ 8-valve superhet, from a kit-set? yo mos A.: Tf you have a fair technica: know: ledge you might be able to, but.we cer tainly do not advise you ‘to try Your best, idea is to make a start om a much. simpler-‘set which uses parts you could » incorporate Jater ina larger receiyer,. Of

the two sets you name, we would prefer the "Air: King" for DX. purposes; either of them, however, compares very fayourably with commercial models. L. 8.D. (Auckland): Try connecting a’ 20,000 ohm resistor between the B+ side of the oscillator reaction winding and B+ max. Evidently the AK1 in your converter is oscillating too strongly, 2. I have a "Tiny Tim" but cannot get it to oscillate on the short-wave’ bands. How can I remedy this? A;: If it oscillates al) right on broadcast, then the strongest possibility is that you "are. using too large an aerial, and the ‘excessive damping is preventing oscillation. Try connecting a-semi-variable widget of. 00005 or .0001 mfd.-in series with the aerial lead; and adjusting. it for best ‘results. Alternatively, attach a Gin. or Sin, length of flex. to the aerial termina] and then twist the lead-in around this. You can,.vary the coupling by,, twisting or untwisting: the flex. — ): The 227° in your set, is L.N J. svldaatly the detector. The. valve types you. are at present using in -your set’ would be best for replacements, J M.: (Wanganui) : The capacity of each * section of the condenser gang .you have on hand is-either .00085 or .000385 mfd., most likely the latter; You have not given the size of the plates. A JS. (Petone) : We have a seven-valve * superhet and are troubled very much by fading.on the Australian stations. The set has been completely overhauled because of the above reasons, but is none the better. A.: If you experience this‘trouble only on the Australian stations, then it is very unlikely that. the trouble lies in your set at all, but is. due to natural fading, If the trouble lies with the set, however, it could be due .to one of dozens of causes, Your. best plan:would be to return the receiver to the agents for another ‘overhaul. 2, The tone control is out ‘of order, but the serviceman says that this does not (Continued on pagé 49.)

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Questions and Answers (Continued from page 47.) make any difference to reception Do you think it would? -, . A.: It would not, provided, of course, there is no. short circuit from the plate supply to earth, If there is, you will notice a faint biue glow around the rectifier filament. A K.J, (Paraparaumu): Your set evi- * dently needs a thorough’ overhaul. You sheuld not be able to pick up morse all over the dial the way you do. ° 2. The valves have worked approximately 4500 hours, Are they likely to be at fault? In replacing them, should the entire set be replaced at the same time? A.: You would find a complete vew set of valves would go a long way toward curing the troubles you are experiencing. It is always best to renew all the yalyes at onee, particularly in your case, where they have given you several thousand more hotrs of service than can . normally be expected. "WARICRO" (Auckland) ; I intend inserting a Switch in the high voltage supply of my commercial eight-valye receiver to allow the heaters to warm up before applying the plate voltage. Would this be best inserted in the lead from the ceutre-tap of the transformer to earth? Also, do you consider this would materially prolong the lives of the valves? ‘ A.: The switch could be located where you suggest. The effect on the valve life would be uegligible. A K.A, (Christehurch) : ‘The capacity * of-each section of the condenser gang you have on band is’ approximately 00085 mfd. It would be suitable for use in the Exelrad short-wave converter. 2. Could you give me the coil specifieations? A,: Sorry, but we cannot supply details of coils used in commercial kit-sets, "RUDDY" (Dunedin): By far your |" best plan. would-be to install‘one of the new noise-reducing aerials for allwave operation. General details of an aerial of this type will be given in the July issue of the "Radio Times." "R" S". (Invercargill): TI have a pair of beadphones connected across the plates in the 45's in my set, but Iam told that I would get Jess noise if I connect them on the r.f. side of the receiver? How could I do this? A,: You -would' undoubtedly get less

noise-in fact, none at all! The phones would not work if used ahead of.the detector. Your best plan is to inchide a 01 mfd. .condenser’ between each headphone lead and.each plate of the output 45's, This would reduce the noise appreciably.