QUESTIONS AND ANSWERS

Radio Record, Volume VII, Issue 7, 25 August 1933, Page 27

QUESTIONS AND ANSWERS

a} A. (Christchurch) :.I have construct- * ed the "Skyline Five" described in June "Radio Times.": When I turn the volume control up there is a rattling noise in the speaker, and reception ceases. It is necessary to tap the chassis before the set will start operating again. By accident I noticed that when the shield cans touch the rf. valves they spark across to the metal coating on the valves. A.: This is evidently the trouble. When this occurs .the resistor in the cathode lead is shorted and the valve left without bias. Slip a length of bicycle tube over each valve. H+: (Auckland): I am contemplat- * ing building an "Outspan Four." Is the tone of this set good? A.: Yes, quite good, if the speaker is correctly matched to the pentode. 2, Would my present speaker match up correctly. with the output valve? A.:. Yes, if you use a tapped output choke or a multi-ratio output transformer. 3, I expect 1 shall not. require the present output filter used with my sei, as av output choke is already provided for in the "Outspan Four." A.: There is nothing to prevent you from’ coupling your speaker to’ the pentode through your present output filter.

H L.V. (Lower Hutt): Could you give * me constructional details of the line filter mentioned in the article "Curing Power Interference," published in the "Radio Record" dated July 21? A.: Bach choke could consist of 200 turns of 20 d.c.c, wound on a former 2in. in diameter. The condensers should be of .01 mfd, capacity each and should be of high test. Enclose the whole device in a metal box, which should be earthed, A D.20 (Banks Peninsula): Would a * set with 150 volts available for plate supply and using a 78. a 6B7, and a41 be as sensitive as a set using a 58: and a 57, resistance coupled to a 47, with 200 volts on the plates? A.: Theoretically there would be no difference, as the valves first mentioned are, roughly, the 6.3 volt equivalents of the last-named. , 2, What are the characteristics of the new 6D6 and the 6C6? A.: They are not yet available. 8, Which is the more sensitive, fullwave or half-wave detection? A.: There is very little difference. Fullwave detection has the advantage that a larger signal can be handled with better quality. "AGPIE" (Wellington): In a recent 2 "Radio Record" the shielded leadin was mentioned as a solution to the interference problem. In a later issue, A paragraph on the same problem states "this lead must not have a large aerial to ground capacity." Could you explain this more fully? A.: When shielded wire is used as & jJead-in, there is a certain capacity between the shield and the wire it surrounds, Hence if the shield-is connected to earth a portion of r.f, signals picked -up by the aerial leaks away to earth. To minimise this loss as far as possible it -is necessary to use shielding of wide dia-

meter, thus making the aerial to earth ' eapacity as small as possible. 2. If the ordinary type of shielded wire is unsatisfactory, what should be used? A.: A special shielded lead-in is manufactured for this purpose, 8. At present I am using a Beverage aerial 800 yards’ long, and interference +s terrific. I do not think the special aerial for reducing interference describ--ed recently in the "Radio Record" would be a success, as the power lines are too close, A.: Cutting down power interference ‘in such a bad locality as yours is largely

a matter of experiment, but we still think that your results would be improved if you installed the special 10-foot’ aerial. oJ G.H. (Dunedin):, An aerial « filter * was described in. the January. "Radio. Times," ‘and full constructional details of a line filter will be given shortly. , af I. (Napier) : I have a "Modern Five" * chassis and parts. Could I employ the "Skyline" circuit? A.: Yes, quite easily. The main dif ference between the two circuits is that the split field method of bias is used in the "Modern Five," whereas the "Skyline Five" employs back biasing.

F A.T. (Temuka): 1 live about a quar * ter of a mile from a factory, where an electric motor provides the driving power. When the motor is running | get a noise like raindrops on the voof. Reception from 3YA and 4YA is seareely affected, but 2YA’s transmissions are eompletely spoilt. A.: You will probably find that a mains filter would be a solution to your problem, but, failing this, 4 filter should be fitted at the motor. "pUzZLED" (Waihi): ‘My set "gives better results when operating without an earth, Is this harmful? | A,: No.° . . ANX1OUS" (Rotorua) : I have a com~ mercial 6-valve battery set which has four neutralising condensers. * Will you please. tell me how .to- balance -the set? ,

j A.: This is rather a delicate operation and an oscillator should be: employed to obtain the best.results. Your -best plan. would be to-call in a serviceman. _ ,2, Where could I obtain a- book of instructions for my set?) J)... .A.: Write the International, Radio Co., Box 1048, Auckland. © . S D. (Cambridge): Would my recép- ,° tion be improved if I installed another earth as shown by the-dotted line in my sketch? , A.: This depends upon a ‘number of: things-the relative length of the twoleads, whether the water pipe which-you -: are at present using as an earth runs for any distance before entering the ~ ground, ete. ‘Your, best plan would be to try it. | os "CYMRO" (Southland): What would be suitable values for the rheostat and potentiometer shown in my sketch? A.: 6 ohms and 400 ohms. S G. (Auckland): I have been inform: * ed that it is preferable to bring my lead-in through the window fanlight and ° tack it around the wall until it reaches the set. At present it comes in by the shortest route through a lead-in tube, and the set is placed quite close to. the point: where itt enters the room. "Which is the, better method? "3 ~ A.: That you are at present employ-:. ing. It is never advisable tq run a lead-. in round the walls unless it cannot be "Sawoided. -~ : h, "IF BEB. (Betone): Could you tell me: * the equivalent of the R21 HV? A.: This is a rectifier and the only equivalent of which we know is the ‘Phillip’s 1201, which we believe is now off the : market, : vg »QTABILITY has been given to Ametii+ ean amateur radio status by the ‘Welcome decision of: the Federal Radio: Commission to extend all existilig: -. Heences for 2 period of two years ant; to issue new licences for three-yeat™ ’ periods. Besides saving officials’ time ° the new measure is encouraging fmia=: teur transmitters to experiment more’ _.ambitiously. The Radio Leagues _ BS hopeful of an increase in new. -inven=: tions. ? ln ane aneeey ecenttae.

Information Coupon- (0 be used with all requeste for- information.) Name ot set Model, Name ddress Nom de pjume 10 ~be kent _ iW subseaueut; inaultries Please Nore (1) Be specitic 4n( OcIcE: f445; " lating, if possible: (2) Write legibly aud o0 one: Side of tbe paner WVe do not desigu crrcuits Limit tbree questions iinleer 1Y, is 'erelosed ; '(5) 'Postal queries: limit: 82' ques; tions. Enclose stamped and eddressed_ envelope anti 1/+ feel;