QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 23, 14 December 1934, Page 51

QUESTIONS AND ANSWERS

E.U. (Hastings) : Sorry, but we caa"not undertake to supply specifications of superhet coils, particularly with tke odd size of condenser gang you are planning to use. We strongly advise you to give up the idea of making your own coils, as you would have very little hope oi matching them accurately without proper equipment. However, if you would still like to persevere, you witl fird full details of winding data for aa cscillator coil to suit the 2A7 in the RCA. Radio Tube Manual. ‘C * (Winton): I have built up the * Lekmek shortwave converter, and find it works well. In conjunction with a five-valve commercial set, it brings in the Empire stations at good volume. However, I find that the 20,000 ohm resistor becomes very hot. I have tested and changed the .5 mfd. condenser bypessing it to earth. The 58 also works at a high temperature, the shield being uncomfortably hot to touch. Is this in order? A.: From the symptoms it is apparent that the screen current flowing is excessive. Have you measured the bias of the valye to see if it is correct? It should be -3 volts. The 20,000 resistor is dissipating just under one watt, so it shoul not become very hot. Should you intend replacing it then do so with a two watt resistor, to give a greater safety margin. An alternative cure would. be to interchange the 20,000 ohm resistor with the 35,000 ohm resistor in series with it, This will lower the screen voltage, but not to the extent of making an appreciable difference to the gain of this valve. D222. (New Plymouth): I have built up the "Simplex Superhet" and as an experiment would like to add an r.f. stage, separately tuned. Could a eail and condenser from an old set be used? A.: Yes, this would be quite in order. 2. Would the primary of the present aerial coil take the plate current of the 34, or could some form of resistanve . capacity coupling be used? A,: Choke capacity coupling would be. better, ie., replace the resistor shown in your sketch with an rf. choke. There is no reason, however, why you should noi apply the plate voltage of the 84 through the primary of the present coil, though for greatest gain you would need to winl on a few more turns. ADIO" (Tokomaru Bay): I hayea four-valve battery set, which does not seem to have the "punch" it shouid. I have tried all ways of lining up the condensers, and have failed to get the sez to oscillate above 75 on the dial. A.: If the set was correctly lined un in the first place, then no amount of experimentgng with the trimmers will improve the oscillation, Other remedies must be found, 2. How should the set be trimmed? A.: Tune in to a station somewhere the centre of the broadcast band and adjust the trimmer on the aerial section of the ‘gang to give greatést response, Then do likewise with the remaining trimmers. If it is found that one of the trimmers

screws either right in or right out without a peak being obtained, then screw the other two a turn or two in the opposite direction and repeat the aligning process. 2. Iam using an H210 as detector and a P2 ponver valve. Could I improve on e‘ther? They are about thirteen months old. A.: Replacing the power valve with 2 small pentode such as the type PT2 would give you greater volume. If you

order this yalvye, be sure to specify au Iinglish base. Also, you would find that you would have no difficulty in obtaining oscillation at the top of the dial if you used an L210 as detector, It has an appreciably lower impedance than the H210. However, before making the change it would be a good plan to have your present detector valve tested. If it is faulty, then replacing it with a new one would probably correct the fault.