QUESTIONS AND ANSWERS

Radio Record, Volume VII, Issue 5, 11 August 1933, Page 26

QUESTIONS AND ANSWERS

W HB. (Wellington) : Could you give ‘ *me details of broadcast coils for the "Sellens" short-wave set, using 0001 mfd. tuning condensers? A.: At least two sets of coils are re quired. For the first set, aerial 80 turns, secondary 140 turns, reaction 20 to 25 turns. Second set, secondaries 190 turns, details of other windings are the same. Place three or four more turns on the detector secondaries in each case. Note: The sample of wire you mention was not enclosed in your letter, but you will need to use a fairly fine gauge, say, 28 or 80 d.s.c., or the coils will be too bulky. "PHANKEFUL" (Oamaru): How many moving and fixed plates has a .0002 mfd, differential condenser? A.: Four moving and three fixed. 2. Could I use enamelled wire instead of ds.c. to wind the coils for the "Sellens" short-wave set? A.: Enamelled wire below 26 gauge is seldom employed for winding coils beeause the enamel chips off easily, with the resulting danger that turns may become short-circuited, V D.B. (Auckland): What value rei ¥ ® sistance should I employ in the cireuit I enclose? A.: R1, 1000 ohms, R3 100,000 ohms. Do not connect R2 in the position you have shown it, but between B-+- max. and the B-. terminal on the audio transformer. Also connect a 1 mfd. by-pass condenser from this terminal to earth. R2 should be about 50,000 ohms, and R4 from 1-2000 ohms. 2. Are the condenser values suitable? A.: Yes. i 3. What ratio transformer should I employ? A.: Any tapped pentode output transformer would be quite suitable. Note: From your circuit you will not be able to earth the moving vanes of the r.f. tuning condenser, and this will be rather awkward if you intend using a two-gang-ed condenser. A much better method is to earth the moving vanes and connect a imfd. fixed condenser between the boitom of the secondary and the moving vanes. ‘The bottom of the secondary should then be taken to the moving arm of the potentiometer. N B.G. (Auckland): In the article on ¢ earths published in the "Radio Reeord" dated June 30, reference is made to the aerial and earth as resembling the

two plates of a condenser. If such is the case, would not it be an advantage to bury a copper rod of approximately the same length as and directly underneath the aerial, as shown in my sketch? A.: Theoretically, yes-in fact, this is the principle adopted by most transmitting stations, where a good earth is all important. In receiving, however, you will probably find that a copper rod driven 5 or 6 feet down into damp earth directly underneath the aerial and as near the set as possible, is as good an earth as can be obtained. "PNTERESTED" (Wanganui): We eannot supply circuits of commercial sets, and suggest you write the manufacturers, The circuit was not published in the "Radio Record." GY W.N. (Port Chalmers): I have a 5- ° valve commercial set which is now 24 years old. When tuned to local stations a rattle is occasionally heard and tone becomes muffled, not so much on speaking or singing, but very much so on orchestral music, I am using a dynamic speaker which I built just two years ago. A.: If your valves are old, this fault is probably due’ to overloading and distortion. It is also possible that the cone of your spéaker has become warped and the ‘voice coil is rubbing against the pole-piece. , "4 VRO" (Waipukurau): My set, employs three 224’s which have been in use for 23 yeats. I replaced them with 224A’s, and had the set retrimmed, but volume decreased. replaced the old 224’s and there was approximately a 20 per cent, increase in volume. There is also a bad hiss when the 224A’s are in the set. Can you explain this? A.: Replacing your old 224’s with new 224A’s should have made an appreciable improvement to your reception, and we are inclined to suspect that one or niore of your new valves is faulty. 2. What are the specifications for the high frequency filter described in the "Radio Record," dated July 21? A.? Bach choke consists of 200 turns of 20 gauge d.e.c. in a single layer on a 24in. diameter former, Instal the whole device in a copper or aluminium box, which should be earthed.

"A.A. (Waikato): I have a commer- * cial 7 valve a.c. set, which, when tuned to a station operating on the lower half of the dial suddenly goes into a whistle. Is this oscillating, and how can I remedy it? A, Yes, The’ particular model you are using is rather difficult for an amateur to service, so your best plan would be to return it to your dealer for overhaul, "RTIG" (Gisborne): I have made the wave-trap described in the July "Radio Times" and find it makes a world of difference to my old set, giving it twice the power. Why is that? A.: The trap is acting as an extra tuned circuit, and is tuning the aerial. 9, With the trap connected, the stations between 30 and 50 can scarcely be heard, and to bring them in it is necessary to short the vanes of the waye-trap condenser with a screwdriver. A.: Evidently the trap is adjusted to cut out stations on that part of the band, If you wish to hear these stations, either alter the adjustment of the tuning condenser in the trap or fit a switch to cut the trap in and out at will. 8. What is the correct resistance of yolume control of my set? A.: 10,000 ohms. "KOT SURE" (Taihape) : Why is it necessary to have an earth for a set operated from the d.c. mains? A.: The set is already earthed through the mains, but some improvement may be effected by connecting an earth to the set through a 2mfd. high test fixed condenser. "IDHASATROL" (Paeroa): Communicate with the Registrar, Examination Board, Radio Servicemen’s Examination, P.O. Box 796, Wellington.