QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 34, 1 March 1935, Page 47

QUESTIONS AND ANSWERS

NXIOUS NO. 1" (Taitoa) : There is possibly a defeet in the wiring system of your house, but the most probable cause of your set going on and off is a defective valve or a }:oor connection somewhere which intermittently opens anc closes, probably due to the action of heat. A qualified serviceman would soon locate the trouble for you. "ys M.C." (Timaru): Connecting a 00005 mfd. midget variable condenser in parallel with your present tuning condenser would enable you to bandspread stations and would make tuning much easier. A pre-set condenser in geries with the aerial lead would improye selectivity and would help to eliminate dead-spots, if any are encountered, ‘To make the tuning easier, you could take’ out three of the moving plates in your tuning condenser. Your present coil -. would then cover the 40-metre band. You would need to wind two additional coils, For the 20-metre bana one, put 7 turns on the secondary and 4 on the reaction 3 for the 80-metre band, 26 turns would be required, with 12 turns reaction. 2: ¥." (Dunedin): I would like fo * :mprove the selectivity of my sevenvalve battery set, which, though old, is sti#l good. Is there any simple way I can { this? A.: You could uge an aerial tuner such as that described in the November, 1933. "Radio Times." Alternatively, you could try taking séveral turns off the primary windings of all the coils, say 5 or 6 turus off the aerial primary, and S or 9 off the rf. primaries. M B.F. (Dunedin): The valve types * used in your set are still as good as any. M. G.H. (Te Awamuiu): There are two serious errors in your circuit which explain why you are not getting resuils. The first is that the .J mfd. by-pass condenser in the plate circuit of the r.f. valve is on the wrong side of the r.f. choke--1t is. by-passing all signals straight © to earth. It should be on the "BL side.

Secondly, there is no need for the .0001 mfd. fixed condenser in series with your: reaction condenser. It is reducing the

Circuit of the "Selectra" crystal set. effective capacity of the Jatter to approximately .00008 mfd., and this explains why you cannot get effective reaction. If you do not wish to insulate the shaft of the reaction condenser, then put a .002 to .005 mfd. condenser in series with it. The additional condenser will only have a negligible effect on the capacity of the reaction condenser, but will isolate the latter from the detector plate voltage. Connect the 001 mfd. condenser that you take out from either side of the t.f. choke in the detector plate circuit to earth, J W. (Wellington): Where could I ob- * tain the circuit of the "Selectra Crystal Set’? A.: This set was described in the August, 1938, "Radio Times," which is now out of print. However. the circuit and a sketch of the coil is shown’ on this page. For winding the coil, a 8in. length of cardboard or cbonite former, 3in. in diameter, is vequired, together with a small quantity of 24 and 80 gauge double silk covered wire, The finer wire is used for the S1S2 winding. The P1P2

winding, which consists of 50 turns, is tapped at the 10th, 15th, 20th and 25th turn. Thirty turns should be put on tbe $182 winding. G A, (Christchurch): Your set is evi- * dently operating correctly and merely requires vealiguing with a ealibrated oscillator, Any qualified serviceman will do this for you. °

OO OO Ell EOE eee IT SET" (Auckland): For all-waye work you would find a correctly designed doublet type of aerial of great advantage. To give compiete details of one of these aerials would be outside the scope of these columus. The subject will be very fully covered in the 1935-"Radio Constructor’s Guide," which will be published about the end of March. In ail cases the flat top should be as high as pos. sible for best DX results, RIEL" (Otago Central): I am using a five-valve battery set which brings in the New Zealand "A" stations: quite well, both day and night, but in the evenings the "B" stations are very bad. There is a- continual seratching noise all the time, extending from 70 to 150. My set is only seven months old. A.: As the "B" stations are not nearly ag powerful as the "A," you must expect to get a certain amount of noise with them. In other words, if you have to turn the volume control up to tune in the "B" stations you refer to, you must expect to get a little more noise. You can soon tel] if the trouble is in the set or not by (Continued on page 49.)

Information Coupon (To lno use(i with aI1 requests for information.) Na me o f set Mlodel Na me Addres8 Nom de plume To be kept in shsequent inquiries Please Note ; (1) Be specitic and briet; tabu: lating if possible_ (2) Write legibly and on one side of the paper, 3) We do not design circuits. (4) [imit three questions unlesg 1/ is enclosed. (T) Postul queries (itnjI 3 ques- tions; Enelose stamped and addressedl envelope and '1/ . fee:

Questions and Answers — \: (Continued from page 47.) detaching the aerial Lf the noise ceases, the trouble is in the set or batteries If the former, the most likely causes are a poor connection or a defective transformer winding, while if the latter . are badly run down, they would cause the crackling noise of which you complain. 2. My. aerial is 185 feet long and 30 feet high. Would I get better reception if I reduced the height and kept the Same length? A.: No, certainly not. If. possible try and increase the height-do not decrease it, Your aerial-is rather long, but pro vided your set is. selective enough for you, it is quite satisfactory. It would pro-bably-be worth while to have the set rewligned for the aerial you are using. 7 H. (Dargaville): You could listen- " * in to. short-wave stations on your by using a short-wave adaptor or Converter, preferably the latter. 2. Last year I changed my three-volt valves for the new two-volt types. Now

there are dead-spots over the tuning band, What is the cause of this? A.: Did you have thé set re-neutralised when the new valves were put in?, if not, this should be done. Also, if you are using the same "A" supply as before. have you made provision for reduciag the voltage across the filament terminals of each valve socket in the set to 2? [ff not, then the emission of your preseut valves will not last long. \ e N ‘C.P. (Cambridge): My set frequent- * ly makes an extremely loud buzzing noise, on which the volume control has . no effect whatever. It can be stopped by tapping the tone control, or one of two valves, ° A.: There is a poor connection or an intermittent short-circuit somewhere, most probably in the tone control circuit. A careful inspection of the wiring would no doubt soon reveal the fault. 2, What is-the correct method of adjusting the trimmers on my set? A.: Tune in to a station coming in at ‘about 3850 metres; then adjust the trimmer on the detector section of the condenser gang for best results. Next proceed to the third section of the gang, then the second, and s0 on, obtaining a peak on each. If one of the trimmers screws either right in or right out without a peak in volume being obtained, then screw the other trimmers a turn or two in the opposite direction and commence again. S G. (Auckland): To cure jncomplete * oscillation in my "Tiny Tim," I have moved the reaction winding closer to the secondary winding for all coils, until now both windings are touching. Is this all right? . A.: Yes, as long as the insulation on ‘the winding holds. If there is q short from the reaction to the grid winding your "B" battery will be flattened in no time. A better solution would be to replace the .0001 mfd. fixed condenser you have in the aerial with a midget preset condenser of .00005 mfd. capacity. setting this for best results. If reaction is still not satisfactory, connect the .0001 mfd. condenser iy parallel with your reaction condenser. 2. What should be the range of each of the short-wave coils? A.: From approximately 16 to 380 metres, 30 to 50 metres, and 50 to 80 metres. If you add 2, 4 and 6 turns to the grid windings of the 20, 40 and 80metre band coils respectively, this should give you the’ coverage you want. 3. How could I employ automatic grié bias? A.: Disconnect the "B-" lead from the "A+" and connect it to one side of a 2000 ohm resistor. Take a lead from the junction of the "G.B." terminal on the audio transformer. The other end of the resistor should be connected to earth. P O, (Poverty Bay): I intend: moving * shortly to a district supplied with electricity. Would it be possible to con‘vert my superhet portable for a.c. operation? ' A.: The change-over, while possible. would be a very expeusive one, and we do not advise it. By far your best plan would be to use a "B" eliminator to take the place of the "B" batteries.