QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 20, 30 November 1934, Page 43

QUESTIONS AND ANSWERS

""RIEGATRON’ (Wellington): Tvilowing are details of suitable coils for your short-wave receiver. 15 to 28 metres; sec. 4, reaction 4: 28 to 43 metres; sec. 8, reaction 5: 40 to TO metres, sec, 12, reaction 7: 70 to 120 metres, sec, 22, reaction 10. Use 22 or 24 d.s.c. for the secondary windings, and 28 or 30 gauge d.s.c, for the reaction. A.T. (Auckland): My set, a seven- * valve all-wave 1984 model, has a short-wave band of 49 metres. I was wondering how I could increase the — wavelength in this case, A.: Sorry, but we do not understand what you want, particularly as your set fovers the main short-wave bands. "TILECTRA" (Takapuna): 11° fixed and 12 moving plates would be suitable. "C(yUIZZY" (Niho Niho): Would you please give me the dimensions for an aerial tuner fora 300ft. aerial? A.: A .0005 mfd, tuning condenser in series with a coil consisting of about 70 turns of 26 or 28 d.s.c. wound on a. 2in, former would be suitable. Alternatively, you could build up the tuner described in the 1934 "Radio Guide." Note: A wave-trap should not be necessary for your set. Also, in a good loca- tion you should be able to pick up quite a number of overseas stations at good volume. R M.P. (Wellington): What is the * cause and remedy of a loud shrick which occurs for @ few seconds just as the set-is warming up? A,: This fault is usually due to a defect in the assembly of one of the valves. 2.: What is the cause and remedy of a crackling noise when the dial is turned? A,: The moving plates of one or more sections of the gang may be fouling the fixed. Other alternatives are that the plates may be dusty or that there is a pigtail in the gang making a poor connection, "NEVA" (Napier): Recently the 80 rectifier in my: set burnt out, and in place of it I inserted a 281 (halfwave). Would this be the cause of both the electrolytic condensers swelling at the top, and of the discharge of electrolyte from the top holes? A.: No, using a_ half-wave rectifier would not overload the electrolytics in this way. 2.: The ‘circuit has since been changed, d both condensers are now working satisfactorily. Is it safe to continue using them? A.: Quite safe. E G, (Raetihi): You .should not be * experiencing morse interference from ships in your locality. Jf you get in touch with your local -radio inspector: he will investigate the matter for you. "NEVA" (Napier) : If you have not an a,c. voltmeter 6n hand, then your best plan would be to take your power transformer along to a dealer, who would identify the terminals for you. ‘ PD*10LA (Helensville): The value of condenser you are using in conjunction with a potentiometer to provide tone control is far too high. A. condenser of

.02 mfd. capacity would be ample. This should be connected in. the plate of the grid circuit of the 2A5, as shown in the "Comet Superhet Five’ described in the November "Radio Times." "@>4MSK" (Whakatane): The by-pass condenser used across the self-bias resistor serves two purpose. Firstly it minimises grid bias variations caused by current surges in the resistor; and sccondly, as the bias resistor has a very ap-

preciable impedance to all frequencies, the by-pass condenser is necessary to provide an easily negotiated path from the grid veturn to cathode. Hence, within limits, the higher the value of the condenser the better. For example, with the 2A5, the by-pass condenser should be at least 10mfd., while as high as 25mfd, is often used. If the jntermediate frequency of a receiver is not known, then it can be quite (Continued on page 45.)

Questions and Answers (Continued from page 43).

easily obtained by the use of a calibrated oscillator, A.C. (Auckland): I would like to * build up an a.c, receiver for myself, preferably a superhet, but my knowledge of radio is rather small and I am not sure whether I could make a success of it or not, Do you advise me to make the attempt? A. Yes, by all means, for building a set like the "Comet Superhet, Five’ described in the November "Radio Times" is simply a matter of common sense. The construction of the set is described in detail, and in addition, a free blue-print is given with each copy showing the circuit and under-chassis wiring in full detail. . 239 (Raetihi) : I have two "B" class speakers which I would like to connect to the one amplifier. How could I do this, One speaker is 80ft. away from. the amplifier? A.: Connect the primary leads from the input transformer of the second speaker through two .5 mfd. fixed condensers to the plates of the type 19 valve. The primary centre tap on the second speaker is, of course, disregarded. ~

Information Coupon (To be used with all requests for information:) Name of set Model Name Address Nom de plume To be kept in subsequent inquirieg: Please Note (1) Be specific and briet, tabu: lating if pogsible (2) Write legibly and ow one side of the paper: We do not design circuits. Limit three questions unless 1/- i8 enclosed. (5) Postal queries limit 3 ques- tiong: Enclose stamped and addressed envelope and 1/- fee: