Questions and Answers

Radio Record, Volume II, Issue 51, 5 July 1929, Page 26

Questions and Answers

Answers in Brief. " J.B." (Onehunga): Your circuit is enot of the best. Try the two R.F. Browning-Drake with your own amplifier. The two R.F. was described in the "Radio Record" and the "Listeners‘ Guide," where full instructions are given. The Cockaday Receiver. A REQUBST has come from "R.S.," Nelson, for the ‘circuit of the "Cockaday’" receiver. A.: We shall publish this with a few notes on its constriction next week. Selling a Receiver. CQouLD you supply me with any information regarding the private sale of a wireless set? writes a correspondent from Hawke’s Bay. A.: A wireless set may be disposed of by private sale without any restriction. As soon as there is any suggestion of making or buying wireless sets to sell or resell, a’ dealer’s license of some description is necessary. The Crystal and Valve. I SHOULD like to make the crystal and valye with 3-valve performance with condenser control reaction. Could you supply the circuit?-‘H.A.§." (Morningside.) . A.: This set, which has proved very popular, can be made equally well witb reaction controlled in either way. As g00n as possible wé shall publish full constPudtionnl details. The R.F. Booster. "Ty WAS interested in the booster deI scribed in the ‘Radio Record,’"

writes "Inquirer," Ngaio, "and built one to specifications, but find on reading subsequent article that it could not be used with my Browning-Drake. I would appreciate advice as to whether the circuit of the booster could be modified so as to make it suitable for the Browning-Drake; A.: The simplest method is. to rewind the aerial coil of the Browning-Drake. On examining this first coil in the set it will be found that there are three terminals on it. One connects directly with the aerial, one with the fixed plates of the tuning condenser and the grid of the first valve, and the other to earth and filament negative. This coil has to be removed, and a Joosely-coupled coil constructed to take its place. On a 24-inch or 3-jnch former wind 75 turns or 55 turns of 20guage (according to size of former) enamel copper wire, as described by "Megohm" in this week’s issue. Leave a space of about 4-inch and wind on about 25 turns of the same wire. Secure the ends and connect them as follow: One end of the smaller coil, the primary, is connected to the aerial, the other end to the earth. One end of the larger coil, the secondary, to the condenser and grid, and the other to the filament negative (or move ing plates of the condenser). The booster may now be connected as was originally described. If results are not quite up to expectations, reverse the connections to ee pe

the primary (or the secondary, if this is easier). . Electrical Interference, "RADIO" (Bay of Plenty) has encoun. tered #.type of interference which will be very hard to check. A few doors from where his set.is operating, a barber js operating an electrical hair-cutting machine. which travels along a wire stretched across the room, carrying current. Interference is very bad while the machine is operating. A.: The trouble may arise from any one of three sources, direct radiation from the electric machine, through the ground connections or through the electric lighting system (the set in question is an all-electric). If a condenser of 1 or 2 mfds. is connected between the wire on which the machine moves and the earth, the trouble may disappear. If it is travelling through the lighting system there will be little hope of preventing it. If through the ground a counterpoise aerial would probably prevent interaction. Audio Howl. "T H.W." (Tower Hutt) has been ¢ troubled with an audio howl. He cannot put the full voltage on his audio valve. A.: See reply to a correspondent last week. Another method is to reverse the connections to either the primary or the secondary of one of the audio transformers. I£ this cures the trouble it indicates that it has arisen through the windings of the transformers being in a different direction to the other. Quite frequently different makers wind their transformers in different directions, and the inducted currents, being out of phase, set up a hétrodyning note or howl. Cannot Increase Voltage. I CANNOT put more than 224 volts on the radio frequency valve of my set without reducing volume, write ""N.W." (Rongotai).’ All my components appear to be O.K., but I cannot get the volume outside Wellington. Reaction makes very little difference. A.: Test the valve used in the radio frequency stage, as this may have lost its emission. RKeneutralise the set, as has been described in the "Guide" and in the "Record." This will be facilitated by the introduction of a by-pass condenser capacity 1 mfd. between "B plus R.F." and earth. Use a grid leak of lower value and trace the circuit out so as to make certain that it goes to "A plus." If it goes to negative, as is probable, if it is shunted across the condenser, break this shunt and connect one end of the grid leak to the side of the condenser nearest the valve. The other end is connected to "A plus." An Extension Speaker. I HAVE an_ all-electric set, wires "G.J.H." (Invercargill), and I want to use a spéaker upstairs as well as downstairs. How might I do this?

A.: A diagram is appended showing the connections. Where a long lead has to be taken, a condenser should be connected in both feads. It amounts really to purchasing or making an audio frequency choke and connecting this between the. two speaker terminals, then, connecting: each speaker .terminal to one side of a'2 mfd. condenser. The other ter-

minals of these two condensers are #are nected to leads which can be takén to the set in the othér room. By connecting them to a jack the speaker can be cut off at will. ‘To use.the speaker nearest the .set take one lead from each of the terminals of the condensers farthest away from the set. Connect these with a jack, and the speaker may be used at will "Round-the-World Two." I Have just finished "Round-the-World Two," and have picked up PCJ REFN, and several amateurs, writes "D.J.C." (Petone). However, there are one or two points I would like cleared tp, 1, The set is very unstable, for when I bring my hands close to the neutralising condenser, which I placed on the panel, the set bursts out into uncontrolable oscillation. A.: The trouble is caused through the neutralising condenser in the ags yk lead being mounted on the panel.\] Was placed in the rear so that it sA\fald not cause this interference. It would be well in constructing a set of this description to follow explicitly the instructions given. 2. Should the coils, when plugged into . the sockets, be }in. apart--mine are a good inch. Would this affect oscillation. : The coils have to be placed just suiticlently close to make the set oscilate. The Screen-Grid Short-Wave Booster. "V ACK" (Johnsonville) writes:-I am. building a §.G.S.W. booster described in the "Radio Record," and’ in. tend to use commercial valve base coils. Will the old coils made to the 1928 "Listeners’ Guide" specifications do?-Yes. Will an A635 valve be -suitable, or. what other one could I use? A.: A screen gird valve will be neces-sary-A635 is then not suitable. At the present time A442 (Philips), PM12, or 1 6(Mullard), UX222 (RadiotroWh) RF222 (Ce-Co.), S215 (Osram), are Yertical mountings screen grid valves now readily obtainable. We have tested the Mullard and Osram valves, and reports will be appearing later. Will the different make of valve affect the set?-If at all, only in signal strength, . I am using a .0001 tuning condenser. Mb I duplicate or use another ?-Duplicate. . The Full-wave Crystal Set. BECAUSE he cannot ‘get good results | from his full-wave erystal set, "Try Again" (Devonport) asks regarding this set. (1) Should each condenser read the same on the same station? A.: Yes, the fact that the condensers | are not showing the same reading indicates that the coils are not matched. Add a few more turns to the secondary of the coil controlled by the condenser which has the higher reading. (2) The coils of 60 turns secondary maximum strength is obtained at 0 degrees on both condensers. What does this mean? : A.: It means that less turns. quired on the secondary coil. (3) A diagram indicates that the direetion of the winding coils are different, and this possibly is the seat of all the trouble. If the coils are not wound in the same _ direction the inducted current arising in the secondary circuit of the first coil will be neutralised by the inducted current in the secondary circuit of the second coil. | In other words, the effect of one crystal is being lost. Reverse the leads on one of the primaries or secondaries, and try the result. If then, the symptoms mentioned previously still persist, proceed with the suggestions made. The correspondent has stated that ‘he has used several combinations of erys-_ tals, and asks if we can advise him of any better combination. We cannot, his trouble no doubt lying in another direction, as has already been indicated. The Linen Diaphragm Speaker. ANY correspondents have recently " written in concerning the linen dia~ re re

ph ‘cone speaker which was published gume time ago. Unfortunately, we have sold out completely of the numbers in which the description appeared. However, in a future issue we shall publish a short description with diagrams. One of these corresnondents, "D.C.C." (South. land) has suggested an index to constructional items. We had this in mind, and intend publishing it with the completion of volume two next week. An Unsatisfactory Tuner. "BACKBLOCKS" (Glen Murray) writes stating that he has purchased a tuner to replace the coil already in the set. It was not satisfactory: the primary coil has 12 turns, and the secondary 52. The tickler 26 turns. _A.: The coil is of the wrong specifications. The primary should be of 24 turns at least. 2. I thinking of adding another RF. s r Would it be O.K. if I re hay primary and centre tapped it? -Yes. Is a moving tickler coil better than ¢ondenser controlled reaction? * Avi A few weeks back this was discussed fairly fully in the "Beginner's Corner," where it was shown that a condenser used to control reaction rarely alters the tuning, and is sometimes smoother than the moving tickler, How many turns should I use on a priv mary with 425 as detector? A.: 425 is suitable only with resistance capacity coupling. Try 409 and 415. UX 199 requires 25 turns. Improving’ Sets. AY RITING on behalf of the owners of -" several commercially-made neutrodyne receivers, "H.M.B." (Te Kuiti) asks how they might improve their sets, although the correspondent reports a quite good log. Sometimes he can get the Japanese on the Youdspeaker, but he is not able to get the"Americans. What can they do to go "D.X.-ing?" A.: "D.X.-ing" with factory-made receivers of the neutrodyne type witb Jess than six valves is very difficult. If a wireless enthusiast wishes to go "D.X.ing" he should construct a regenerative receiver. In this case, if tone is poor, ‘the valves may be at fault. In this connection it is unwise to use valves other than those originally provided in the sets, unless it is a power-valve in the last stage. A power-valve of the 171A type will greatly improve tone if correctly biased. The detectors used in this case are

' inclined in some cases to be harsh, so that a change back to the American detector for which the set was built may be preferable. Various Points. ‘Ss B." (Auckland), writes: I propose * 1o use the following valves in @ receiver, Screen-grid, PM4, PM3 detece tor, PM4 and pentode. Is this a suitable combination ? : A.: It can be improved. PM14 (screens grid), PM3 (R.F.), PM4D (detector), PM4 (first audio), PM254 (second audio), or PM14 ("Pentode"). While greatly increasing signal strength the pentode type of valve can be overloaded easily, and ruin the quality from a local station. 2. How many turns on the primary to suit the R.F. valve? A.: The screen-grid circuit should be tuned anode, which does not require a’ primary. PM83 requires 25 turns. Would a 7-plate neutralising condenser be all right for neutralising ?--~Yes. Choke Construction. I HAVE a quantity of soft iron, transformer laminations, which I should like to use in the construction of audio chokes ,writes ‘Choke’ (Hataitail). Can you give me: 1, A compensation between the magnetic stalloy and soft iron? A.: Two-thirds. Stalloy carries 60.000 magnetic lines of force to the syuare inch, soft iron 40,000, 2. The method of finding the value of a choke in henries? I have been placing a milliameter in series with the choke, and measuring the current taken when placed across a convenient voltage, such as 110 volts, or 230 volts A.C., 50 cycles, and then working out the value of "LL"? (inductance) from the usual formula. H/I is equal to the square root of (R squared plus P squared L squared). Where I! is the voltage, I the current, R. the resistance, P 6.2832 times frequency, and L henries. Would not the inductance of a choke vary at different frequency? If so, at what frequency is a choke rated? A.: The usual rating for a choke is 920 eyeles per second. Using this, the formula given ean be applied. I have an all-electric crystal and valve amplifier which gives splendid results. Are there any objections to my using a raw A.C. on the filament of the power valye. If not, what is the difference bebetween an A.C. power valve and a D.C. power valve? ; A,: As there is no difference a D.C. ' power valve may be used quite con- ' yeniently for A.C. Amplifying the Full Wave Crystal Set. I HAVE made the full wave crystal set, writes "R.L.F." (Wellington), and have found it much superior to the usual crystal set. I intend building another, and adding a stage of audio frequeney, and would like to know: 1. Is it possible to add a stage of radio frequency?-No, it is not practicable. 2. Would it be a disadvantage to bankwind the primary coil? A.: A bank-wound primary coi] would not be satisfactory for -this circuit.