ANSWERS TO CORRESPONDENTS

Radio Record, Volume I, Issue 11, 30 September 1927, Page 14

ANSWERS TO CORRESPONDENTS

"‘Megolim’’ las had a uumber of appreciative letters from readers who have obtained improved reception by carrying out hints given in answer to inquiries. It is always interesting to hear the result of suggestions that have been made, and those readers who have taken the trouble to write in and report are thanked accordingly. Chemical Rectifier Tips. An Auckland reader writes an appreciation of the test-tube type of B battery, having had a 72-volt in use for a Joug time, charging with a chemical rectifier. He recommends the coveting of the upper half of the aluminium electrode with rubber tape, tightly bound on to exclude all chemical ace tion, and also serving the purpose of reducing the area of plate surface. This is an idea that is frequently used in chemical rectifiers, and is easily tried. Its chief object is to prevent corrosion of the aluminium near the surface of the liquid. When charging a test-tube battery in series the current required is very small, so that it is always safe to keep the size of electrodes down, and also to keep them as far apart as possible. One inch has been stated as the minimum distance apart.

Loop Aerials. W.D. (ielensville).-Vresumably you require a loop for experiment. It will only give about one-sixth the volume of your good outside aerial. A loop for broadcast reception should contain about 70 feet of wire, preferably not finer than 18’s, spaced a little over one-eighth inch apart. ‘Iwo feet for each side would be suitable, and could have ten turns of wire. Make a cross with three arms equalandone longer to goinastand. Drill ahole near end of each arm and put short dowels through. Notch dowels for wires. Slicllac the whol2 frame before putting the wire on. Estimating Wave-length on Dials. C.F.B. (Alexandra).-The whistles you hear around 29 and 380 degrees on your wave-length dial are probably the carrier waves of 2UE, 297 metres, 250 watts, Sydney; or 2KY, 280 metres, 1500 watts, Sydney. You should be able to get these stations easily on favourable nights with your five-valye Browning-Drake, and both wet batteries. Probably it is only experience in tuning-in that you lack, and this will soon be acquired with practice. It iS difficult to give the range of the set without some idea as -to its efficiency or the name of maker. It should be able to reach nearly 10,000 miles on odd favourable occasions if it is in effi-

JUIN TTT TTT TTT COTTE cient order, The four-valve BrowningDrake just described in this column for constructors. has reached WBBM (Chicago), 7000 miles; KFKX (Hast ings, Nebraska), 6500 miles; three Japanese stations, 5000 miles, and a good number of American Pacific coast stations. Familiarity with positions of known stations on the dials enables one to estimate approximately the wavelength of & newly-found station from its relative position to the nearest ‘known station on either side, But there is a method by which a given wave-length may be found on the dial to a degree. ‘Take a piece of graph paper and along the bottom mark off 200 at the. left, 800 two inches to the right, 400 and 500 cach two inehes apart to represent metres. Every tenth of an inch will equal five metres. Now up the side we want the degrees, usually up to 100, commencing at 0 at the bottom. At every half-inch mark 10, 20, 30, up to the total number on dial. Now tune in all the stations you can, one by one, and with a list of wave-lengths, place a dot on the graph to represent each station, the position being found by reckoning the degrees on dial upwards and number of metres wave-length reckoned across. Where these two readings intersect ‘a dot is placed. When a dozen or so stations have been marked, well spaced across the graph, the dots are joined by a line running through ¢ach, which will probably give a slightly curved line running diagonally upwards to the. right. The ling can be extended in the same direction at either end for a short distance to the edge of graph. Now, suppose you wish to find a Sstation you know to be transmitting on 330 metres, look for 850 on the graph, and following this Jine upwards until the curved line is met, the point of contact gives the number of degrees at which that particular wave-length may be found. Of course a little searching a degree or two on either side may have to be done, and the other dial must be brought into resonance iu the usual way. ‘The degree readings may be taken from either dial, but are best taken from the resonance or secondary tuner, because that one is the least likely to be altered. Any alteration in the arrangement of the circuit may upset the accuracy of the graph and neeessitate making another. The graph will also show the approximate wavelength of a station when the dial reading has been obtained. An interesting letter has been recciyed from an enthusiast in the Veilding district, but is rather lengthy to deal with at present

Crystal Improvement. "Tuquirer’? (Stratford) wishes to know how to increase volume on an inexpensive crystal set he has constructed, and which receives 2YA, 1YA, and 3YA, the two last-named faintly. Your diagram shows a slider tuning arrangement, which is quite a good method. Wire on coil should be 20’s or 22’s gauge, space wound. Look to earth connection, which is important. Possibly a galena crystal would give better results than the hertzite now in use. (2) ‘The single pole double throw switch in the aerial circuit does not take the place of an arrester, which must be provided in addition. Crystal Amplification, C.ILW. (Wellington) asks for particulars of resistance-coupled amplifier to add to crystal set. Resistance coupling of the usual type cannot be used immediately after a crystal detector, as the principle depends upon the anode potential of a preceding valve. Transformer coupling is the most satisfactory for crystal amplification, and if good components are used, very fine results are obtained with one or two stages. ‘Tlicre is a circuit known as the Sharman circuit that utilises capacity coupling between crystal and valye on an unusual principle, and is said to give good results on long-distance work. B Accumulator, "Radio" (Auckland).-The holes in battery plates should be kept small, about one-sixteenth incl or a shade larger only, so that the paste will not tend to come through. (2) About forty holes in each should be sufficient. (38) If new paste is used, lead peroxide is

for the positive and litharge or spongy lead for the negative. These are separately made into a stiff paste with sulplintic acid, diluted to usual battery strength. ‘The paste is slow in drying, plates must not be put into battery for use until thoroughly dry. MIleavy pressure is used in placing the paste in factory-made accumulators, Forming these will take some time. Crystal Amplification, LW. (Parihoulou).-No. 6 of the "Record" shows how to couple an amplifier tc ervstal. If dry cells are to be used for filament heating, then two--yolt yalves- are the most economical. With an accumulator, four or six volt valves may be adopted. ‘Iwo ordinary dry cells will run a two-volt valye, The B battery may be a dry one of 45, 60, or 90 volts. (2) When reading reports of long-distance crystal reception in America it must be remembered that the stations mentioned may haye a power of 15,000 or 30,000 watts. Some splendid results are being achieved in New Zealand on DX crystal reception. |

Neutrodyne Troubles. Ftarth Wire? (Balclutha).-If your five-valve neutrodyne squeals as you state, it is probably not properly neu‘tralised. Carry out instructions givet on neutralisation in last instalment of Browning-Drake article in No. .10 of the "Record." ‘The usual midget variable condenser should be quite suitable for a neutraliser in your set. If after neu‘tralising you are still troubled with ‘squealing, try reducing plate voltage on R.F, valves, and if this does not effect a cure, try a slight reduction on ‘these filaments. | Crackling Noises in Set, N.H.M. (North Otago).-You cat test audio transformer windings for continuity with ’phones and a ‘‘dud’’ dry cell giving about a volt. If these are O.K., then the trouble is elsewhere, Replace the suspected condenser, and see there are no loose connections. Make sure’the crackle is not from B battery. If it is more than one of these matters, the best course would be, under the circumstances, to send it to the distributors for overhaul, as they suggest. a