QUESTIONS AND ANSWERS

Radio Record, Volume VII, Issue 4, 4 August 1933, Page 27

QUESTIONS AND ANSWERS

"DICK-UP" (Laumarunui): Could 1 connect a pickup to my commercial set, and, if so, where? , A.: Yes, in the grid return of the second detector. 2. The set employs two. 227 type valves. Are there any more modern valves which could be inserted in these sockets without alteration to the set, and, if so, would there be any improvement? A.:-Yes, you could substitute with 56’s, but. the improvement would scarcely be noticed. 8. The aerial is 160ft. long, cluding lead-in, 85ft. high, and there are numerous trees about 28ft. high underneath it. Would a shorter and higher aerial improve long-distance reception? "A.: The trees underneath your aerial reduce its effective height considerably, and it would be an advantage if you could change the direction to avoid this. A flat top of 60ft. would be ample. "NORA" (Rotorua): I have a 4valve a.c. set of German make employing the following Telefunken valves: R134, REN1004, RGN1500. These valves. are now off the market. Could I replace them with another make? A.: The ‘corresponding equivalents in the Philip series are B405, ©4388, and 506 rectifier. W.R. (Waihou): The Australian * School 6f- Radio. Engineering, Wambley House, Railway Square, Sydney. P B.L. (Ngaruawahia):-I can get no * stations at all when the dial of my set is turned past 60-only a noise ‘like static. A.: Evidently the moving vanes of one or more sections of your condenser gang are touching the fixed vanes, or are pattially shorted in some way. "QTUDENT" (Auckland): How does a bar magnet amplifier work? AL: The reed of a highly developed earpiece is connected directly to a small microphone in series with a battery and loudspeaker. A 6-volt accumulator is needed. The device is delicate to adjust and is rarely satisfactory for any length of time. .Why not.build a one-valve baitery amplifier? 2. Is much amplification obtained by this method? Av: Quiet speaker volume from _the more powerful of your locals is all you eould expect. 3. How can I improve the selectivity of my crystal set? A.: Rebuild it, using the cireuit of. the "Selectra" crystal set, which is to be deseribed in the August "Radio Times," It employs two tuned cireuits and is capable of a very high degree of selectivity. " HISTLB" (Central Otago): Our electric power is obtained from a small water-driven Pelton whee] gener- ating 230 volts, a.c. Though it is situated a quarter of a mile away, interference from the plant is experienced over the lower wavelengths. Could I cure this by installing choke coils and condensers at the set, ‘as mentioned in your issue of July 217 . A.: ¥es, but install the. filter at the motor end. You can quite easily make the chokes yourself by winding on 250 turns of 20 gauge d.c.c. in a single layer on a 2in, diatheter former. If necessary, install the:whole device in a copper or aluminium box, which should be: earthed, 2 With an indoor aerial the volume ©

conivol has to be turned on fully to give good reception. Does this shorten the life of the valves? , A.: No-or, at least, the effect is negligible. . "RIOVICE" (Auckland): When I switch on a blue glow appears in the 280, which soon after commences to spark, After a few minutes this ceases. A.: Evidently a defective 280, which should be replaced. Note: A first-class wave-tra}) was described in the July "Radio Times." i 7

J. D.- (Hamilton): I have an 8S-valve superhet. which recently has lost its selectivity and has become noisy. When the clip on top of the 224 is removed the set operates just as well, ITs this in order, or does it denote a fault? A.: It is certainly not in order, and it appears that your set is due for an overhaul. There are three 224’s in your set, and we do not know the one to which you refer. The fact that the set oper- . ates just as well with the grid eap removed indicates either that the valve is defective or that there is some break in the cireuit. 2.: Where can I obtain a circuit of my set? A.: In the 1931 edition of Rider’s "Official Radio Service Manual." URIOUS" (Carterton) : When aerial and earth are disconnected the only station I can hear is 2YA. Should it not be possible to get at least the YA stations at good loudspeaker strength without the aerial? ; A.: No-in fact, with a set as well shielded as yours is, it is surprising that you can pick up even 2YA. 2. When‘ the aerial and earth is connected up the only American 1. ean pick up is KFI. Should IL be able to get more? A.: Not necessarily, Your locality might be unfavourable. We recently came across .an interesting case of two ‘keen dxers, both of whom operate powerful

other. Their logs are equally goo.§, and yet though one can hear KFI cét full speaker strength, the other can scarcely pick this station up at all. j "PEADER" (Hokitika): Shorf-wave superheterodyne converters nee very careful designing and building if they are to operate successfully, espec {ally if used in conjunction with a broadcgst superheterodyne receiver. We = skrong!ly advise you against attempting tp build one, and suggest you get in toucfh with the agents for your set, who can ‘supply you with a converter specially bilt for it. L H.W. (Auckland): Lately my set has * been fading even on local «stations. A.: Have you had your valyeg tested lately? Evidently one or more of them is worn out and needs replacinsg. 2. Would seven valves of mix¢kl makes spoil the performance of the set P A.: Possibly not, but it is al ways advisable to replace with the one make. M N.S. (Te Aroha): Recently; my set * developed a hum at frequemt inter vals, and the volume fades or :teproduction becomes distorted. This ‘ happens on all stations. I have had my sett over 12 months, and with the exceptioz) of one. which I had to replace about tw months ago, I am still using the originfil valves.. A.: Have your valves tested, atnd if they are all in good condition, there nothing for it but to call in a servicem bn. Iam using a 2-point plugg. Would it make any difference if I used; a 3-point plug instead. as there are thiree wires from the set? ; A.: No-no difference whatever, A A. (Auckland): I ean onily pick up * one or two Australian :itations on my 5-valve A.C. set. When volume is turned full the set becomes unstable. Should I not get better results ti? A.: We notice your set is a 1931 model. Have your results ever been setter? If not, then it is probably due tc» poor locality, poor aerial, incorrect gan: fing of eondensers, or to some little fahilt initially present in the receiver. Tf. fesults have heen better, then evidently yo tr set is due for an overhaul. ‘ sets within a quarter of a mile each J ©.B. (Dunedin): Is full+wave detec- *** tion an advantage ovete half-wave, and could it be incorporatedt in superhet by using a 55? .: A special intermediage frequency ‘OO RGGnedeenecacnncnnenthnn...... 2...

=... Information Cor ipon (To be used with all rec juests for information.) Name of set Model tescoeocerse Be cccvccees ‘Name Address seeeeeceseseoases Peccccececs SOCSCOHTASEESSe ESE SOEEOEE "FPP PSOH TOSSES OSSEOEOCOBEEE Peccccccese a tet ee ere rwesesecsesscenee: Qreccecooone Nom de plume ......0 pescccscces To be kept in subsequer ft Please Note:(1) Be specitic ana iigriet, tabulating, if possible, ! (2) Write legibly and ®p one side of the paper (8) We do not design. :etreuits. (4) Limit three quest fons unless 1/: -is enclosed .(5) Postal queries Ben 3 ques- — tions. Wnelose and addressed envelop: and 1/- fee. |

When a set howls or "motorboats" in spite of the fact that the audio frequency side is de-coupled from the "‘B’’ supply, it is often possible to remedy the trouble by increasing the capacity of the decoupling condenser. A _ second condenser of | or 2 mfd. should be joined in parallel with the existing one, thus adding. to the bypassing capacity already in circuit. This scheme is to be preferred to the alternative of increasing the value of the de-coup-ling resistance, a procedure which may reduce the effective anode voltage too much,

coit is needed when ‘a 55 is connected as a fhill-wave detector. The main advantag of this type of detection is that less dis tortion is present than when using the half-wave type, but as the usual output eircluit is not above reproach in this re spe tt, the advantage is largely lost. 2, Is the tone given by a single 2A5 in ctx output stage good, or would a pair of 4\i’s in class "A" give greater fidelity? Ai.: A. 2A5 operating under correct conditions and correctly matched to a good qaality speaker is capable of excellent toney but not quite un to the standard of 45’s ‘in class "A." 3. To what extent does audio power make up for r.f., ie, by providing an outrut of 10 watts max. in place of 8, would this inerease the distance-getting abilities of the set? A. Very little, because the noise level woulgl be very high. "The" (Claudelands): I am enclosing a sketch of an amplifier and a Hartley transmitter. Could I connect the fcrmer to the latter for telegraphy and telepttony work? A.:' Yes, but generally modulated oscillators are not satisfactory, — P 2. ‘Where should I connect the amplier? A.: Ditectly in the B-+- lead. KF D.-(Cambridge): Could I receive 1ZH, * \ihich is 14 miles from here, on the erystal, cireuit 1 enclose? A.: No, or at least we would be very surpristed if you could. "RRAt10" (Huntly): When my volume edntrol is turned about three-quar-ters of ‘the way on, a loud hum is heard, which vanishes when the control is turned on fly, Reception is good until the hum apipears, when a erackle and hiss becomes. yiery evident behind the tuning. A.: FRvidently your set requires a new volume ‘control. J.B. (Dunedin): My set cannot pick * up stations under 620 k.e, or above 1380 k.ck IL have tried adjusting the trimmers) on my set, which by the way is a commercial model. A.; Tltis is evidently the tronble. You have ups¢tt the alignment of the condenser gang, with the result you mention. Call in a serviceman and have the set realigned. "TyRYAJZEL" (Christehurch): Whieb is thie better set for DX work, the "Skyline }five"’ or the "Diode Five’? A,: Thet "Skyline Five." 2. Is a £447 as efficient as 245’s in pushpuil? A.: No, and you would find the quality of the latter combination is better, An excellent gitticle on the merits and the de merits of vy arious output valves, including the latest ttypes, will appear in the Ang: ust "Radio Times." "PUZZLE(p" (Mosgiel) : When my a.e. set hits been working several minutes a crackling noise is heard, and volume drops, Also, if the set is tapped near the dial, ths volume varies, A.: Evidgntly there is a partial or complete breakdown in a small resistance or condensei% or there is an intermittent short to the: chassis. "QPARRO\WHAWK" (Carterton): | have jiist finished building a "Sparrowhawk" scxt with one audio stage. Oscillation is very ploppy. I have tried a new r.f. choke and varying the number of turns on t.be reaction winding-in fact. on the 80-metire band I have tried from 1 to 12 turns, Lwit the set still goes into oscillation with, a sharp plop. A.:. Possiblly the grid leak is faulty. Try different ‘values, ranging from three to ten megobmms. Also try the addition of a by-pass dtondenser, of value of about .0005 mfd., connected from the side of the rf. choke going to the primary of the transformer trp earth. Turning down the rheostat a trifle will alsg assist to give smooth reacti pn,