Questions and Answers

Radio Record, Volume III, Issue 30, 7 February 1930, Page 35

Questions and Answers

Killing Parasites by Radio. "T.D.C.K." (Whangarei) asks if it would be possible to kill bacteria dn/the soil without damage to the plants th high-frequency electro-magnetic Waves, and then asks several questions concerning this méthod. A.: We cannot suggest any economical means of coping with such a problem. There was an article under the title "Q.8.0., Rats, Mice and Bacteria," in a recent issue of "Q.S.7.," which the correspondent may read to his advantage. If he cannot obtain a copy we can lend him one. A Double Grid Ampiifier, I HAVE constructed the crystal and valve set described in "Pictorial News," and would like some information, writes "H.P.L." (Kilbirnie). The correspondent then asks several questions concerning the circuit. A.: We can hardly be expected to look through "Pictorial News" to find this circuit, without which we cannot suggest anything. If the correspondent will ‘send this along to us and repeat his , question we shall do our best to assist him. . Set stops Suddenly. I the middle of an item my factorybuilt all-electric receiver stopped suddenly, and although the valves light and there is the usual hum in the speaker I cannot receive any stations. What is the order of eliminating the various faults that are likely to occur? asks "Query" (Te Awamutu). .A.: The correspondent should disconnect his aerial and earth and rig up a temporary aerial consisting of some covered wire or plain wire insulated from the ground and tune in to his strongest station. If there are no signals, he ean take it for granted that there is trouble within the set. It would still be possible for the valves to light up and the speaker to hum while yet there is a defect in another section. See that dust has not collected between the vanes of the condensers and that all valves make positive contact with their sockets. To test for this, tune in to the strongest sMi@tion or, rather, where the strongest s@™mion should be, and move each valve tturn in its socket. If the reception is intermittent a valve grip should be tightened. Take out the screws underneath the set to lay bare the wiring and see if any component has dropped out of place, and if the previous test has shown a faulty connection tighten the offending prong with pliers. Take out the valves and have them tested. If these tests reveal nothing unusual, thén the set must be taken to a dealer. Distortion. FIND that, after 8 p.m, my new seven-valve receiver distorts from 2YA. Before this time it appears to be all right-"F.Y." (Wanganui). . A.: Apparently your receiver cannot take the volume delivered by this station after dark. It is probable that some adjustment to the grid bias or "B" supnly is needed. See what type of valve jis in the last stage or stages. If anything but a 171 type (see "All about Allelactric," page 65) is used, then distribu-

tion will result. If the set can handle the same amount of volume, either from a pick-up or from another station, it is apparent that the distortion either is emanating from the station or is the result of locality effect. It is unlikely that station distortion would take place regularly at a certain hour. It appears that distortion is present in the earlier sessions, but that it is not apparent until the greater volume at 8 p.m. has to be handled. Mushy Reception. M* batteries have just been renewed, writes "H.J.S." (Takaka), but when Tt am working 2YA the station fades and the reception is mushy. A.: Providing the set functions well when the station is not fading it is probable that the mushiness is the result of

the fade; this frequently happens. Otherwise you will have to examine your set, noting particularly the amount of grid bias applied to each valve and the condition of the resistances and condensers. The trouble is resulting probably through the valves. being a little old or not worked with their correct quota of power. Had you told us the types of valves we eould have advised you better. When I added the new "B" batteries the set howled. . A.: It seems as though there was a defect in the neutralisation system in the R.F. stages. A broken-down by-pass condenser would cause this. Place a 1 mfd. condenser across the "B" batteries nd the howling should disappear. 3. 1 am vsing four Philips 609 valves and one P.M.6. Could I get a hetter combination? 7 .

A.: Yes, you should have a power valve of the U.X.171 type in the last stage, say, P.M.256 or Philips B605. ‘These would have to be cgrrectly biased, le., about 12 volts with 130 volts on the plate. The "250" Power Pack, OULD you answer the following questions concerning the "250" power pack described by ‘""Megohm" in a recent issue? asks R.A.M. (Auckland). 1. Is there enough space and wire for a 500-volt and a grid bias winding? A.: Probably, but do not cut the laminations until the transformer is wound. The window can then be widened if there is too much wire to be accommodated in that provided. 2. Could the same condensers be used? A.: To be on the safe side, use those tested at 1000 volts. The others might last for a time, but a replacement would be inevitable.

in assembling the choke core are the smaller pieces of stalloy made into thre¢ piles pushed close and clamped? A.: Back the full lengths tightly in the centre of the winding, then assembl¢ the short length’s ends and the remaining long side alternately-thus short long short, short long short. 3. Could two or more filament wind: ings be put on the one layer separated by a small space?-Yes. 4, What is the thickness and materia of the spool ends of the transformer? A.: They are usually of 1-8 ine} fibre. but see the "Radio Listeners Guide" for full information concerninj the general principles of transforme) building. 5. Where can I get a revolution coun ter? A.: George Henry and Co., engineers Christchurch, 12/6. & Do the layers of brown puper com after or before the layer of tape?

Questions and Answers READERS of the "Radio Reeord’" who are in difficulties about reception or set construction are invited to write to our "Questions and Answers" department for help. We particularly wish to assist those who know little about radio, as very often there is some ver. slight trouble which spoils completely one’s enjoyment of the programmes, Correspondents are asked to observe the following courtesies: 1. Write legibly. 2. Make your questions brief and to the point; do not make apologies for writing, and, where possible, tabulate. 8. Do not ask for a reply by post unless a stamped and addressed envelope is enclosed. Hiven in these circumstances, we reserve the right to answer any question through our columns. 4, Do not ask us to design circuits or send detailed lay-out diagrams; but we can offer advice regarding circuits. 5. Address all technical correspondence: "The Technical Editor, P.O. Box 1032, Wellington."

P A.: After, to give a level working surace. 7. Are the chokes wound regularly or in heaps? A.: They are left to themselves. A layer is wound on at great speed, and no attention is paid to regularly. Trouble with the H.R.4. OULD you inform me of the date 1 when "Round the World Two" was published?-"R.B." (Petone). A.: April 26, 1928. Our supplies of this number are running very short and would advise you and prospective constructors of this very successful receiver to secure a copy of the issue before it is too late. Loss of Volume. ‘ALTHOUGH I nave renewed my batteries and valves I find the volume from my factory-made receiver is. unsatisfactory, writes "G.H." (Point Chevalier). The carrier wave is no longer clear, but there is a shrill whistling when I try to tune in the stations, I have had to reduce the voltage on the detector to 67’4 volts. The tone is not at all clear. A.: It appears that the set is not neutralised properly. HWspecially where valves other than those recommended by the manufacturers are used there is trouble with the neutralising, and this would cause the howl referred to in the tuning. Other possible causes of trouble might be defective aerial and earth systems, open grid circuits or defective joints, lack of resonance in the tuned circuits if single-dial control. (See that all the tuning condensers are in step. Turn the dial to 100 and get all the moving vanes within the fixed), Burn-out resistances in the grid circuit (if the set is neutralised by this method), poor contacts between the valve prongs and the socket, shorted tuning condenser or one that is not moving when the dial is rotated, defective radio transformer, and broken-down by-pass condenser. Many of these likely defects can be remedied by the continuity test, using ’phones and cell or voltmeter and cell methods. These have been outlined in the "Radio Record" and in the "Listeners’ Guide." Weakness on a Crystal Set. of et following troubles have been spoil- ' ing my reception of 2YA on a crystal set.-"‘RJ.S.," Wellington. 1. Reception is weak on a set comprisjng a four-inch coil tuned with a 32-

plate condenser. The aerial is long, but low. The phones are rusty, and ‘one ear is silent. A.: Probably your phones are not in the hest order. The fact that one will not function shows that there is trouble here. Oan you not borrow a pair to try out the set. If you are using a permanent erystal, tap it, and then try. Dirty connections are one of the most frequent of trouble causes. 2. Another aerial hangs from a tree, and the swaying in the wind breaks the insulators. Can I leave about two feet slack? A.: Yes, but we should imagine that this would introduce a greater swaying. Note-We are preparing an article on aerials and earths, which should be helpful. . W S.R. (Martinborough) has incorpor- * ated in his factory-made receiver the push-pull amplifier described by "Pentode," but is troubled with overloading, probably, he suggests, the detector. The following are his queries :- 1. Can I work two pentodes in pushA.: This is a difficult task owing to the very high impedance of the valve. None of the devices used in the output stage are designed for this valve, and when it is worked in push-pull the impedance is doubled, and so are the attendant difficulties. 2. Assuming that I ean put out two watts of undistorted power with the 256 valves at present used, what would be the output if I used pentodes? A.: Two 256’s in push-pull deliver about 1.8 watts, and two pentodes, if correctly matched in their circuits, 2.25 watts. 8. How might I prevent overloading? but as components are now designed, the ehances are against the pentodes. A.: Do not attempt to work 2YA with all the radio stages in the circuit. Bring the aerial into the grid of detector or last radio valve. Is the speaker capable of handling the volume that can be supplied by the set? Minally pay great attention to anode and negative grid voltages. Hum in an A.C. Receiver. I have recently converted my 2 R.F. Browning-Drake receiver to all-elec-tric, but I am troubled with a continuous humming. All the valves are Philips and of the cathode type. The mains is of

60-cycle frequency. Would this make any difference? A.: There are more than one type of cathode valves made by Philips. Some. are better than others, but for your circuit, presuming that you have the better for your purpose, 415, we would suggest that you have the powerpack tested, for a great deal of trouble can arise through a broken-down filter condenser. The mains frequency is normal. 2. When I switch on the set there is a surge of power than necessitates detuning, and then the set signals down to normal again, A.: Probably a defective unit. Band Pass Filters. WouLD you give me constructional details of a suitable band-pass filter? asks "Band Pass" (Auckland). A.: This is a topic that is receiving a great deal of attention in the American technical Press of late. We would advise you to obtain "Radio News," late issues for full particulars. We shall devote an article to this subject as soon as possible. Crystal Set and Amplifier. (THERD are several points that are troubling me in the article on this subject in "All About the All-BHlectric," writes "S.W.S." (Silverstream). 1. Which is wound on first, the filament or the rectifier winding? A.: It does not matter. If there is sufficient room, both may be wound on side by side. 2. Which of the primary and which of the secondary windings are connected to the mains? A.: The primary input and output are the only wires connected to the A.C. mains. 8. Does each layer in the primary and secondary have to be insulated if I use 86 d.c.c, wire?--Yes. 4, How is the value of "I" arrived at in the equation, R=B/I, and what is the bias resistance for 201A? A.: The value of "I" must be obtained from the makers’ figures or curves. Consult a chart either supplied or possessed of by dealers, and obtain the anode current consumed when the valve is pronerly biased. Thus, for 201A the bias with 185 volts is 9, requiring an anode cur rent of 8 milliamperes. ‘Thus the bias yesistance is 3-9 x 1000-350 ohms approximately; a 0/400 ohm _potentiometer used as a rheostat would do exeellently. 5. Could I use the A.C. equipment in thig circuit as an eliminator for another set: if so, what are the adjustments? A.: The "B" part of the outfit can be used successfully, but for the filament supply the changes shown in the article must be made. For the first stage it is absolutely necessary to use an A.C. valve of the 226, or better still, of the 227 type, and wire it according to the diagram on page 31, or the text on page 36 respectively. Text Books. HAT are the names of the text books suitable for study for a sea operator, and are there any text books on jrelevision worth while?-B.O. (Marton). A.: Communicate with Johnston’s Wireless School, Wellington, for information concerning an operator’s certificate. There are no good text books on television available as yet in New Zealand, but the "Television" magazine (Te Aro Book Depot, Wellington) is an excellent monthly publication. Improving Reception. DURING the year there are only three . Australian stations that I can work, "Subscriber" (Barrytown). Even the New Zealand stations are not very good. I have a good aerial, and I think I should get better results. : You have omitted the critical statement. How long have you had your set, and have the valves been renewed? This is one of the greatest factors in deciding whether a factory-built set will be good or poor. Very many owners:

note that their sets are becoming weaker, but forget for how long they haye had the valves. If the aerial and the, earth are in good condition, and theér are no high resistance joints, then the correspondent should consider a new set of valves (if the others are over 12 months old), or should have his set overhauled by the lacal agents. Trouble With the H.R.4, I BUILT the Hammarlund-Roberts 4 valve receiver, but cannot get satisfactory results. I have not heard of anyone building this set, so conclude that there are better ones for an amateur to build, writes "Gridleak" (Blenheim). A.: The H.RA4 generally gives good results, and they are made commercially by Johns, Limited, Auckland, with whom you might correspondent to advantage. , Our experience has been that the Brown-ing-Drake is an easier receiver to miake,, and gives equally good results. It has been described in our last "Listeners’ Guide." 2. Could you tell me the price of a complete transmitter, such as ZL 2 AX? A.: You should communicate with the secretary of the New Zealand Amateur Transmitters’ Association c/o Superadio, Queen Street, Auckland. 8. I am hoping to sit for an amateur transmitter’s license in the near future. Could you tell me the name of a textbook containing all that ig necessary for this examination? A.: We would advise you to secure the "Radio Amateurs’ Hand-book" (Handy’s Hand-book), obtainable from the Te Aro Book Stores, Courtenay Place, Welington. Price about 3/6. Again we would advise you to communicate with the secretary of the N.Z.A.R.T.A. Hand Capacity. I AM troubled with hand capacity on short-wave, writes J.S. (Dunedin). ,. A.: Not very long ago we published a | very complete article on overcoming hand ) capacity on short-wave, and we would advise you to look up this article. You should use a high-value gridleak, about 8 megohms, and it may be necessary to shield your panel; reversing the connections to the reaction condenser is very often effective. Local and Distant Switch. CONCERNING this switch on factorybuilt received, T.T. (Dalefield) asks the following questions :-~ 1: Is it advisable to use the local instead of the distant switch? A.: The local and distant switch is only a tapping on one of the coils of your set, and plugging in either to the tapping or to the whole coil controls the volume. It does not matter which one you use. 2: Does it consume more electricity on local or on distant? A.: There is no connection between the coil controlled by the switch and the consumption, 8: Which is the hardest on rence i A.: Again, there is no difference. 4: Will any alteration to the a have any effect on static? Yes, a short aerial brings in less noise than a long one, although the signal strength is thereby reduced. With a powerful set such as yours, it is generally advisable to use a short aerial, i.e., one no greater than 80 feet in all.