Questions and Answers

Radio Record, Volume II, Issue 46, 31 May 1929, Page 10

Questions and Answers

The Full Wave Crystal Set. "B P.S." (Devonport) has constructed "* the full-wave crystal set, and states that it is a big improvement on the ordinary crystal, inasmuch as_ it brings in 1ZB quite clearly, also 2YA and the Australian stations faintly. He observes that signal strength from 1YA is greatest when the condensers are closed right up, and that disconnecting either crystal does not lessen the signal strength in. the slightest. A.: It appears that the coils are too small; add the same number of turns on to each, so that 1YA will be brought in further up the dial. If the two dials are not perfectly matched, a few more turns should be added to the coil, the dial of which reads the greatest. (The full-wave crystal set was described in our issue, December 14, and also on page 28 of the "Radio Listeners’ Guide.’’) Plate Melted Off. "S B." (Palmerston North) states that * the plate in the rectifying valve in his charger has melted off, despite the fact that there were only three blocks of 20 volts each being charged. Particulars as to the: type of valve were not given, so that it is very difficult to say what the cause might have been. Certainly a short-circuit is suggested, and the correspondent should examine his circuit very carefully. for. this. At the same time, he should see if his valve is not being overloaded. What is the purpose of a resistance lamp, and which requires a bigger re-sistance-40 watts or 60 watts? — A.: A resistance lamp is a safety device to prevent the high-tension current from the mains damaging the «charger. A 60-watt lamp -has a less resistance than a 40-watt one. Pa

Varied Set Ailments. " H.C." (Wellington) asks the follow- * ing questions:- . 1. How to test a Siftron? A.: A siftron is an output filter comprising an iron-cored choke and a by-pass condenser. The test can be made by the phones and cell method. Connect these across the two input terminals. A click will indicate that the choke is OK. Connect one of the input with the output opposite. There should be either a click or a silence. Now test the other pair. Tf a click was heard on the first test of these two opposites there should be a silence or at the most an initial click followed by silence. One pair should give a click at every application of the testing apparatus. One at the most-one, and then silence. 2. How to add a further audio stage to "Round the World Two." A.: Disconnect the connection between the plate of the last valve and the jack. Introduce another audio transformer, placing it at right angles to the existing transformer. Connect "P" so disconnec~ tion to "P" of the transformer. Disconnect "B plus" from the jack, and connect this to "B plus" of the audio. transformer. Introduce another valve socket connecting "G" of thia to "G" of the transformer. "GB" of the transformer is connected to the negative of a grid bias battery of about 44 volts, depending on the valve to be used in the last stage. The postiive of this battery is connected with the "A minus." Connect the plate of this last valve socket to the jack as before, and the other leg of the jack to the highest "B" voltage -available. It remains now to connect -- AS eR,

up the filaments. Connect one of these pins to an amperite, then to the filament connection of the first audio transformer as shown in the original diagram. The other goes to "A plus." 8. Can I connect a dry battery ‘with a wet "B" battery to increase its voltage ?-Yes. 4. Since inserting three new English valves in my American set, I have found the set very unstable on the lower wavelengths. I,am now using an American detector with English radio and.audio. The use of a Continental detector makes things worse. Would the addition of Phasatrol stabilise the set. A.: This is to be expected when English or Continental valves are used to replace American valves in an American circuit. The whole neutralisation apparatus of these sets is adjusted for American valves, either 201A or 199. If English valves are to be used they should be as nearly as possible matched to the American valves, that is, PM6 would give better results than the PM5’s now used, but the correspondent would be advised to return to the American valves. A Phasatrol would certainly add greatly to the stability. 5. Where may I obtain particulars of the Australian broadeast programmes? A.: The difficulty here is that the Australian mail comes in just a day too late to enable the programmes to be published. Where possible, we publish extracts from future programmes. It is doubtful if any daily paper would undertake to publish the full Australian programmes, ‘

A Neutralising Device. "CN C.H." (North Auckland) states that * he has noticed an arrangement similar to the regulator of a:clock on the back of his American-made receiver. The letters "D’’ and "S" indicate the extremities. On altering the regulators, he notices that the tuning becomes sharp and howling commences. He séeks an explanation. ; A.: This is the neutralising condenser, the letters standing for "dry" and "storage" batteries according to whether 199 or 201A type of valve-is used. . 2. What grid bias would be needed I increased the voltage on the power valve B605 from 90 to 135? A.: About 12 volts. Introducing an Eliminator. " Z." (Dunedin) wishes to change * from dry batteries to an eliminator, and asks if these are a success, az if any change in the set is necessary. asks also if a power valve could be used with an eliminator? A.: Where the mains current. is reasonably steady, as it is in all but a very few districts, an eliminator can replace a battery with every: satisfaction. The great advantage of an eliminator is that a power valve can be used with. excellent results. Voltage that is economically unobt: ‘nable through batteries, is quite possible with an eliminator. Many of these instruments have grid bias tappings going up to about 50 volts negative, thus enabling a power valve to be used with 180 volts on the plate and 50 volts grid bias, e.g.. the 171A type. The correspondent asks the amount of grid bias he would have to use, but that would depend entirely on the valve in the last stage. See ‘Listeners’ Guide." Concerning a Super-Hetrodyne. WISHING to construct the last word in short-wave receivers, "W.J.W." SL ST TTI ~e

a (Brixton) asks. advice regarding a shortwave super-hetrodyne described in "Modern Wireless." This receiver departs somewhat from the usual, requiring certain transformers that are unobtainable in New Zealand. A compromise is suggested, but they will not do. A.: The correspondent is advised to try something less ambitious. Superhetrodynes are not featured in New Zealand, and not only are parts difficult to obtain but knowledge of these is very scant. The regulations prohibit the use of a super-hetrodyne on anything but a loop antenna. ‘Try the receiver described in the "Listeners’ Guide." An American Hi-Q Receiver. " P.," of Oamaru, sends for our con- * sideration’ details of an American Hi-Q receiver that is described in "Radie, News" for March, 1929. He asks seréral questions regarding the coils. A.: -_We should strongly advise our correspondent to try something less ambitious. The set is very. complicated, using the tuned plate tuned grid system, the coils for which, to our knowledge, are unobtainable, and would require a great deal of experimenting if home-made. No less than five tuning condensers are employed, ganged in two groups. ‘This would complicate tuning very considerably. ‘These sets are intended for American amateurs who -have had years of experience and who can obtain most of the parts without any difficulty. If such a receiver were constructed great care would have to be exercised to pre vent losses, as any loss would offset the advantages secured by the intricacies of the circuit. -If a complicated receiver is desired the correspondent would do well to try the Adapted Screen Grid Brown-ing-Drake. Stray Capacity. " P.S." (Christchurch) writes: "I have placed my short-wave receiver immediately below my broadcast set and find the signals vary in strength as my hand was moved between the two. On opening the front of my broadcast re ceiver and moving the tuning condenser I found that the signals had almost disappeared when this was at maximum and minimum capacity." A.: It appears that between the two receivers a capacity effect is caused, no doubt through something being common to both. .The further statement that the "A" battery supplies each _ set strengthens this view. ..An aeria] going to both sets would have a like effect. The way to overcome this is to shift the sets further apart and use one only at a time and have no connections between them. The R.F. Booster. A LITTLE while back a correspondent described, through "Suggestions from correspondents," a booster comprising an untuned stage of. radio frequency. This has proved very satisfactory, but a correspondent raises the following points, some of which are of vital importance :- 1. The article says "attach the lead from the ‘B’ battery to the ground post on the set." Would this not short-cir-cuit the batteries and burn out the valves? A.: Yes, if the aerial coil was in: any Way connected with the remainder of the set. The writer presumed that the first transformer has its primary distinct from its secondary, but in many receivers, e.g. Browning Drake, this is not so, and the filament end of the secondary or grid coil is connected with the earth. ‘Those constructing this booster must examine their sets to see that there is no connection between the aerial and secondary coil of this first transformer. 2. How can the valve in the booster be expected to function when there is no current applied to the plate? : A.: The current is applied to the plate through the first radio frequency transformer, "B plus 45" -is attached to the ground post, and, prvoided there is no connection between this and any other part of the set

the current will pass through the aerial coil to the aerial terminal of the set and so to the plate of the booster. 8. What is the object of placing a resistance between the rheostat and the variable condenser? A.: None. It was mentioned only because the correspondent had used it to advantage. A wire wound resistance placed between the grid of the first valve and the coil will act as a stabilising device, 4, What is the difference between a bypass condenser and an ordinary fixed one? A.: None, the terminology merely indieates the use of a fixed condenser. 5. As the booster is not neutralised, will it not cause interference ?-"Constantinople" (Matamata.) A.: Yes, if not carefully handled. But were it to oscillate, the signals would be so distorted that it would be impossible to listen to them, so that the constructor in his own interest, would not oscillate. Lower Wavelengths Mushy, "[NTERESTED™ (Central Otago) finds that on wavelengths below 38YA signals are mushy and distorted, and the tuning is very broad. He asks if a condenser or R.I. choke in the aerial lead would sharpen tuning. A.: It appears that the difficulty lies in the construction of the receiver, and it will be indeed hard to make any satisfactory alterations. A condenser in the aerial leads should be tried, and the detector voltage should be kept low, about Be The same correspondent asks if it will be necessary to vary the grid leak if he alters his battery voltages, but there is no necessity. Battery Voltages. NE of our shortwave correspondents, "W.G.L." (Southland) inquires regarding his set. The last audio stage will not’ take any more than 90 volts without screeching, although the audio valves are correctly biased. The makers’ recommendations are 125 yolts and 9 volts grid bias. A.: Try a grid leak, value } megohm, in series with the grid of the last or gecond last valve, and make quite certain that the battery voltages are OK. 2, "While trying to experiment with more grid voltage, I removed the last C minus connection, and the set worked quite well without it, but with a slightly reduced volume." Short-circuiting of the secondary of the transformer is suggested. A.: The function of the "C" battery is to improve quality rather than volume. By disconnecting the grid bias, the amplifier is working from the free grid system, which should give slightly louder signals, but more distorted. There is probably nothing wrong with the transformer. An Unshielded Booster. " J.K." (Palmerston North) states *that he has the shield grid booster which works perfectly from 400 metres upwards, but below this causes the set to oscillate. A.: The whole booster wants shielding and the shield connected to earth. Otherwise remove it a considerable distance from the set and connect the plate to the aerial coil by a very thin lead of wire. Circuit Diagram Wanted. " B.," of Denniston, who has consult- * ed an expert regarding his set, comes to the conclusion that there is a short circuit, resulting in loss of volume. The set is a "Claratone," and he requires a blue print of this in order to be able to track down the trouble. A.: Although we have asked several dealers, we cannot obtain any information regarding this ‘‘Claratone" set, but the expert friend should be able to tell the type of set-regenerative or neutro-dyne-and from that follow out the wiring, which will be almost exactly the same as any other set of its type. A diagram is not necessary to look for

faults. The wiring need only be checked over as it appears in the set. In _looking for a short circuit, try first all byass condensers, testing them by the phones and cell method, Then examine for touching wires, defective resistances, burnt-out transformers, faulty valve bases, ete.,

Troubled with Morse. i "7 LF." (Lower Hutt) is greatly * troubled with morse. He is using a crystal set and a two-valve amplifier." A.: This is one of the disadvantages of living beside a port and near a Government station. The best method is -_ oe --= —

to make the crystal set selective. Selectivity with a crystal set has been fully discussed in the "Guide." In addition, it might be added that the writer has found the English carborundums to be very selective. In fact, using a plain coil it has been found that Wellington wil] disappear in about 10 degrees, This means that morse could most probably be tuned out. Converting a Moving Coil Loudspeaker. " H." (Dannevirke) wishes to convert * a casting to make it suitable for "Pentode’s" dynamic cone. From the diagram he submits, the following points oceur: 1, There is not enough room for the wire of the moving coil. 2. The general shape leaves much to be desired. It tends to loss in the joins. 3. It would work, but not well enough to. warrant the work proposed. The number of turns on the moving coil for a 171 valve is about 800. For the pentode valve,-any attempt at matching would mean the winding on of several thousand turns, so that the best play would be ‘to wind the coil for an ordinary power valve, and work the pentode with this, "X" Coils Wanted. F W. ENGLEHART, of Tauraroa, * asks if ""X" coils are obtainable in New Zealand. He is wanting one with 250 turns and one of 60 turns for a wavechange set. A.: As far as can be ascertained they are unprocurable, being wnnecessary where the wavelengths are more or less low. In England, coils with such a large inductance are employed where stations on wavelengths of over 1000 metres, such as 5X°X, Daventry, are to be received. as well as stations on the lower wayelength. Provision has to be made for a wave-change arrangement in ‘order that either high or low inductance coils ean be used at will. In New Zealand, there is no need for the high inductance coil, seeing that the only stations available on the longer wavelength are an occasional commercial station and Perth; this latter owing to the distance cannot be wellreceived here. However, if the correspondent wishes to construct his own coils, he eould do so by winding 250 turns on to a 3-inch former, and tuning with a .0005 condenser,

The "Round the World Two." "Tl M." (Auckland), who is making his * debut as a constructor, asks the following points regard the "Round the World Two" :- 1. Oscillation: He states, "I understand that one has to search for stations just at the point of oscillation. This point is so sharply defined that I cannot hold the set in that state in order to search for signals." A.: Reduce the "B" battery voltage on the detector. This must be kept low, if smooth oscillation is required. Increase the value of the grid leak. 2. Hand capacity: I am __ greatly troubled from hand capacity, which applies.when I place the hand at the back of the set to adjust the neutralising condenser. The proximity of the hand to the coils causes the signals to disappear and the effect of the tuning of the neutralising condenser is lost. A.: It appears that the panel will have to be screened, that is, a metal plate placed at the back of the panel and connected to earth. 3. Coils: "I am a little doubtful about the coils. I have been advised to use a third coil on the swivel-(a_ tickler). Would a ready-made set of coils improve the circuit?" A.: Reaction is carried out in this receiver by an extra winding and controlled by a neutralising condenser. This has the same effect as the moving tickler. A set of ready-made coils could be used to save the trouble of making them. 4, Battery voltage: I have been using from 20 to 40 volts on the detector. and have varied "B-+-" between 60 and 90. A.: Reduce the "B-+-" detector voltages and add more, tickler turns if necessary. Shortwave tuning is very intricate and requires a great deal of practice before a good log can be amassed. Half-wave Rectification. EGARDING the article on Power Transformers in "Listeners’ Guide," "SW.B." (Oamaru) writes: I would like to know if I can use a centre-tapped transformer, say 250 volts, on each side with a half-wave rectifier, disregarding the second half? A.: Yes, by winding the other end well

— out of the way, one half can be used with a half-wave rectifier. To Log the Americans. " A.D" (Levin), who owns a fac- * tory-made set, wishes some information on loggifg the Americans and Australians. He is using a good aerial which runs from north-west to southeast. A.: Such a set and aerial should bring in the Australians. ‘They can be logged at usually good strength after about 9 o’clock, and sometimes some considerable time earlier. As for the Americans, an aerial running from north-east to southwest would be the best. These stations can be logged usually before 8 p.m. Would a magnetic earth tube make a more efficient earth than one soldered to the water mains? A.: Providing these enter the ground at a short distance from the set-No. Life of Valves. "JT ISTEN-BUT-DON’T-TOUCH" has been using the same valves for 24 years, and assures us that they are still going strong. However, from his log of stations we are inclined to think otherwise, particularly as his sets works best from 45 volts only, and as well without the grid leak as with it. The best way to convinee the correspondent that his set is not giving him maximum efficiency would be to obtain a set of new valves, and bring his voltage on the audio valves up to 90, that on the radio valves up to 673, leaving the detector at about 22} volts. The fact that the set works better without the "grid" (by which we take it the correspondent means __ grid leak) would indicate that either the grid condenser has broken down or that the radio frequency valves are detecting. The condenser should be tested by the ’phones and cell method. Transformer for A.C. Amplifier. "TT C.E." (Auckland) in passing favour- * able comment on the "Crystal and Valve with Three Valve Performance," states that he now contemplates constructing the crystal and amplifier to work off the mains, but he would like to know where he could get the transformer ready made. A.: The Dongan Power Unit (Abel, Smeeton), as reviewed in our columns, would be quite suitable. Other power units, however, should be equally suite. What thickness or quality of paper do you suggest for wrapping round the layers of the wire? A.: Ordinary good quality writing paper. Operating a Moving Coil. "CAN I operate a moving coil speaker from a charger delivering .65 and 13 amps?-"C.R.A." A.: Yes, select the rate that gives the best results. It will not overheat the coil of the speaker. Power Interference. ‘C J.W." (Reefton) is in a rather un- ¢ fortunate position as regards power lines. He is situated 300 yards from a power house, 25 yards from one set of power lines and about the same distance from another line at right angles. A picture theatre is 75 yards from him. Reception during the daytime is impossible. A.: This is probably one of the worst cases possible as far as the location of. the radio receiver is concerned. From a sketch he has indicated that his aerial is at right angles to the power house. The length is not stated. The following are suggestions that may help the correspondent :- 1. Fully shield the receiver. This would minimise the pick-up of interference on the coils. 2. Try a counterpoise earth. This probably would be effective, especially if used in conjunction with a shielded receiver. A counterpoise is merely a: second aerial slung a few feet from the ground immediately below the usual

aerial. It is fully insulated as is the usual aerial. This is connected by a lead-in to the ground post and all connection with the ground broken. 8. Reduce the size of the aerial and try shifting round in different directions. loop antenna may be used to replace the ordinary aerial if the set 7 powerful enough to operate from the oop. 5. See "Noises" and "Counterpoise" in ‘Listeners’ Guide." Shielded-5 Neutrodyne. WiSHIN G to construct the Shielded-5 Neutrodyne, "H.R.Q." (New Plymouth) purchased a set of factory-made coils, and discovered that the diagram © was slightly different from that described by "Pentode." The main difference is that the neutralising is done from the secondary instead of from the primary as in our Shielded-5. Other alterations are that separate bias is applied to the last valve, and a higher voltage to its plate. A.: The. circuit diagram supplied is equally efficient as the Shielded-5, and ""Pentode’s" instructions can be followed with the small compromise regarding the neutralisation. Other alterations with regard to voltages should be followed. An Audio Howl. "G ¥." (Putaruru) states that his set * has developed a howl which obviously arises from the audio end. He has tried various voltages on both anode and grid, but finds that when he puts his hand on the audio fre-transformer, the howl changes from a deep note to a high shrill howl. A.: Try + megohm grid leak in series. with the grid of the last valve, or a bypass condenser or a resistance across the secondary of the last transformer. Material for Condenser Plates. "DUZZLED" (Wellington) wishes to know of which materials the plate of variable condensers are made-brass or aluminium. A Loose Valve Socket. a A J." (Tauranga) complains that his * set has a tendency of going out of operation on receiving the least jolt. It can be made to work again by pressing a particular valve hard into its. socket. A.: It appears that the connection to the valve socket is loose, or one of the terminals of this is not making contact with the pin of the valve base. This should be carefully examined for loose contacts. The set should be overhauled so that the operator can make certain there are no other loose contacts. Please state exactly what type and name of yalves to be used in the Round the World Two. A.: It would be unfair to one maker to stipulate another’s valves. By referring to the classified index in the Guide, the constructor can select any make of valve for any position. , What voltage should be used? A.: Abont 224 volts on the detector and 90 at least on the audio valve. This will require two 45 volt blocks. ‘ How are the batteries connected to the set? A.: Join "B"-+-45 of one to negative of the other. Take a lead from the free negative to the negative of the set. From the 224 volt tapping on this same block take a lead to the detector from the free 45 terminal, take a lead to the phones. Dial Readings Change. " W.Y." (North Auckland) complains ° that the dial readings for several shortwave stations have been recently altered. The set is that described in the 1928 "Listeners’ Guide," the coils of which are variable in their ‘relationship ~ to one another by a slider. A.: It appears that either the wavelengths of the stations have changed, or that the dial has slipped in relation to the condenser, An alteration in relationship to the tickler could also have an effect on the tuning. .