QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 29, 25 January 1935, Page 43

QUESTIONS AND ANSWERS

IDGET Il" (Humilton): I am thinking of taking my "Midget . IT" away on my nolidays, to. a place about 150 miles from 1YA. At present it is using two 20LA’s which are just about done. To ensure daylight reception, I propose to use three 45-volt batteries instead of two,. with a 6-volt accumulator. I would like to know whether two 201A’s would be the best valve replacements? A. You would probably find that two AG09’s would give better results. Witn a small set such as this, 90 volts "B" would be all that would be required. A M. (Newtown): .Most. of the parts you. have on hand couid be used to build un. the. "Duodyne One," a one-valve all-wave receiver de-' scribed in the July, 1934 "Radio Times." A coil consisting of 95 turns of 26 d.s.c. wound on a 1fin. diameter former three inches long, and tuned by a .0005 mfd. variable condenser, would make a suituble wave-trap for your locality. Remember that the lead from the trap to the set should be kept as short as possible as’ of course the trap cannot eliminate signals picked up on this lead from the unwanted station. It would not improve matters to alter «the number of turns on the coil of the ‘aerial tuner.you are using. Evidently your set is not ~ery selective, and a wave-trap would be the simplest and cheapest way of effecting a cure for this. "Glvad MOT" (Wellington): I desire to attach a tuning. meter to my set. Where do I connect it? A. In the plate return of the i.f..amplifier would be the best place. 2. My aerial shows decided signs of corrosion. Would it improve reception if I replaced it? A. It is difficult to say. Theoretically, it would, particularly as your set. is also used for short-wave work. On the whole we would recommend you to replace it. 3. I intend to earth the far end of my aerial. How should I do it? : A. Hither direct, or through a resistor of 1000 to 5000 ohms. Your -best plan SSMU MUSH BCBS R ERB CU ERAS

would bé to purchase a 5000 ohm potentiumeter and adjust jt for best results. You may be interested in the following details of noise-reducing aerial developed by an American experimenter, and which he claims gives extraordinary DX reception. ull details of the aerial are shown in the accompanying sketch. Shielded lead-in wire is employed for the aerial. This wire has 25 feet of its shielding removed, beginning at a distance of 35 feet from one end. This leaves a wire which is shielded for a distance of 20 feet at-one end-and 85 feet at the other, leaving an unshielded space of about 385, feet between. The 20 feet section is the lead-in, and under no circumstances should the shield over the lead-in

be earthed. At the other end of the aerial the opposite state obtains, and the end of the shield-not the core wire-is earthed to as good an earth as can be. made through a resistor. The earth wire from the set is shielded, and the shield is not earthed either to the set or to earth. Two wire-wound non-inductive resistors of 2000 ohms each are employed, mounted in cardboard tubes filled with paraftin wax. One is connected across the aerial and earth terminals on the chassis, with leads not exceedirig 3in., while the other is connected in series at the end of the wire shield at the far end of the aerial, and as close to the earth as possible. This resistor should preferably be mounted on .- the earth rod itself. The whole insulation should be very carefully made and all joinis should be well. soldered. The designer accounts for the very fine results he has obtained with this aerial by the fact that it is directional, greatly favouring signals coming from either end, but relatively insensitive to signals or noise coming from’ either side. G S. (Pio Pio): I am enclosing a sam-. Xe ple of lead-covered cable, comprising a pair ‘of insulated wires covered with a lead sheathing. Could this he used as a shielded lead-in, and jf so how? a A. Yes, it could-be used as a low impedance transmission line to replace the lead-in, by attaching a step-down trans former where the lead-in joins the aerial and a similar step-un transformer ‘at the serial, terminal of the .set. With vegard, to the transformers. you could’ experiment. with the number of turns to find ‘thé best ratio. However, as a basis. wind 80 turns of, say, 26 d.s.c. on a Qin. diameter former, One end of this winding goes ta the aerial and the other to the earth. At the bottom of this winding put on an other eight turns in the same direction, The two ends of this.shonld he. connected to the wires within . the lend-enyered erable. This transformer. together = with the leads from it. should be enclosed in a water-proof container. A similar trans-

former: should be wound and attached at the set end, the eight turn winding ‘being connected to the transmission line and the 80 turns to the aerial terminal and earth. When the installation is complete, try the effect of earthing the metal shielding at the aerial end; and then at the centre and set end as well. C.A. (Christchurch): Would four-pin coils works equally well with the "Tiny Tim" as the originals? I have a set on hand. : oe A. Yes, four-pin coils could be used quite well, by omitting the primary winding. In this case a semi-variable "postage stamp" type of condenser should be connected. between the aerial terminal and the top of the grid winding. .0001 mfd. max. would be a suitable capacity. Also you should check up on the connections and number of turns on the coils you have on hand, as these may differ appreciably from the originals. 2. Would an audio stage improve reception of short-wave stations? A. Yes, you would get much greater volume. ‘ "rpiecy" (Carterton) : The manufacturers of your set also make a special short-wave converter to work with jt. The iocal agents would be able to supply you with full details. ot E, (Greytown): tlow can I attach °a cone type-speaker to my set without interfering with the wiring in. any way? . A.: Procure a five-pin wafer adaptor and place it beneath the output valve in your eet, i., the 47. Attach.an insulated wire to the plate soldering lug: of the adaptor and connect it to one terminal of a .1 mfd. fixed condenser. The other side of the condenser goes to one cord tip of the cone speaker; the other tip should be earthed to the chassis frame, A switch can be ingerted in one of the voice coil leads of the d4snamic speaker so as to cut out the latter when. the auxiliary speaker is being operéted. (Continued on page 45.)

POEL CEE LE et ME Gn Gt TT CEE GE Information Coupon (To be used with all} requests for information.) .Name Of SCE cocecscveccccccccceseces MOde] — sasccaccceccncccccccccvcepecse NAMC = veccvvccccccsceccrecsncccevess AGATESS cecccccccccccccccsccccccccss SCORER EES EHEEEHHERSHE TEESE EE EEE HEEEE ’ Nom de plume ecesen eeeeee To be kept in: Please Note:(1) Be specific and brief, tabulating if possible, te (2) Write legibly and on one side of the paper. (8) We do not design circuits. (4) Limit three questions unless 1/- is. enclosed. (5) Postal queries limit 8 ques: tions. BHnelose stamped and ‘addressed envelope and 1/- fee, PT ee SMM SMB SS

Questions and Answers (Continued from page. 48.) A J.C, (New Plymouth): ‘Che volume * control on my set is no longer as effective as it used to be. The variation in volume with the conto] turned full oa and right off is very slight. A.: It seems obvious that the earthed end of the volume control potentiometer us open. If the potentiometer. is wirewound the strongest possibility is that there ig a break in the winding... A replacement would no doubt cure — the trouble. HLF. (Dunedin): The best place to " ° connect a pick-up in your set would be from the grid of the 55 second detector | to earth. If you find that volume is too © iow, connect an ordinary audio transformer between the pick-up leads and the two points mentioned. You will find that volume on the pick-up is lower than on radio because the output from even tlie best of pick-ups is very low-it seldom exceeds half a yolt. Hence, a little extra boosting is needed when the set is being used for reproducing records. "BoLECTRO" (Napier): We do not ad- , vise you to replace the 24A’s in. your superhet with 58’s ‘The slight improvement in results would not justify the expense, and besides you would quite probably introduce instability. The 58 is a high gain valve, and you would prob-ably-encounter difficulty with oscillation in your three r.f. stage receiver, " B.C.". (Queenstown): My battery "* charger incorporates a wet electrolytic rectifier, using lead and aluminium plates as electrodes. How could I renew the electrolyte? A.: Prepare a saturated solution! ot borax and distilled water. and nearly fill {he container with this solution. "J AXGEE" (Otangiwai): I have a 12- ' volt windmill electric generating plant and wish to rum my battery set ‘rom it. However, as I have only 12 volts available is there any way jn- which I ean substitute the "B" batteries with some form of eliminator?

A.: Presumably your 12-volt generator charges a 12-volt storage battery or two six-volt batteries connected in series. One six-volt battery will be needed to light the filament of che six-volt valves in your set. The same battery can be used to operate an automobile "B" battery eliminator, a device developed in recent years to provide "BRB" battery voitage for auto-radio sets from the storage battery, system of the ear. The "B" eliminator operates on six volts and steps it up to a higher potential like any a.c, transformer, except that an interrupter or vibrator is used to break the steady d.e, battery current into a 60cycle pulsating current. The output is then rectified: and smoothed and. finally

is passed on to the set. There are sever- ‘ al excellent makes of these eliminatoze on the market in New Zealand. J). (Heatherston): I wish to replace the magnetic speaker used with my a.c. set with a dynamie speaker. How could this be done?A.: You could either use a speaker of the permanent magnet type, or one with a field to replace the second filter choke in your set. An output transformer >t the correct ratio to match the output valve to the speaker should be used in either case,