QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 22, 11 December 1931, Page 14

QUESTIONS AND ANSWERS

The Technical Editor will, through these columns, be pleased to help readers experiencing trouble with their sets. Queries are limited to three-for more than this a shilling fee is charged, and a similar fee is payable for queries answered by post. Supplying layouts. circuits and solutions of intricate theoretical problems is beyond the scope of this service. ; : A coupon must accompany all requests for information. Nonappearance of the coupon in any issue cannot be regarded as ‘a reason for its not being used. sa Address all queries, The Technical Editor, Box 1032, Wellington.

B C. (Waitara).-Would the tuner de- * scribed in the recent issue of the "R.R." be an advantage, as my set now employs a midget condenser in the aerial? I have a .00015 variable condenser on hand. Would this be all right to use with.a coil.of 180 turns? A.: The apparatus described in the "R.R." would be quite satisfactory for > -------

your set. You could use it with a .00015 condenser, and the number of turns you mention. , D L. (Gisborne)-A short time ago a ° . series of articles appeared in the "R.R." explaining how.one became an amateur, transmitter, A few copies of

these are still available. The cost, of course, would ‘depend upon the outfit you intend to, built, but you could start off. with a very moderate outlay indeed. : »J ESO. (Christchur'ch).--Providing you use a grid suppressor of about 500 ohms in‘ series with the grid of the first valye,. your circuit will be quite in order.. You should. not connect the plate of the. rf. valve direct With the aerial unless it noes through a suitable condenser, say a H. H.M. (Dunedin) : Could a carborun- * dum stabiliser be. used with. any. effect om a Small valye set? = A.: ‘Only in connection witha crystal,. so as. to turn the combination, into, a crystal and one-valve amplifier, which. on local stations, would possibly sive better results than a one-valve a.c_ set, 2. I wish to use a two-inch. former for the broadcast coil for the "Snarrow Hawk." What would be the turns with a .00035 condenser using 26 to 30 d.e.s. wire? ; A.: Secondary 75 turns, tickler (for a .0002 differential condenser) 30 to 35 turns. EF M.P.: (Nelson): I am using a five- ° valve American: neutrodyne .. set which distorts when volume is turned up. Would this be due to my ‘valves. two of which are 14 monhts old? A.: It is due probably to your not having a sufficiently large power valve. You have not stated what valves you are using, but we presume you have 201A type in all stages. The one in the last stage should be replaced by-a 605 tyne. which should be biased by about 12 volts "C" (with 90 volts ."B"’). Renew the valves which are 14 months old, as when a valve ages it loses its amplification factor and in consequence cannot handle the signals as it could before. We. would recommend 221 type valves in al} stages except the last, where you should use a high-gain power valve of the BGO class. C D. (Christchurch): We regret your > query has not received attention. It has been referred to: the technician

who ‘designed the battery, ° and we ar awaiting his reply. _ We shall publish it as sooh as it comes to hand. WIRELESS (Dunedin):: If you wish to turn: your set into an ‘all-wave one you should construct one of: the Differential -series which. was. -deseribed in the "Radio Record" ‘during the year. You would find the "Night, Owl Three" would make an excellent: all-wave set, and all coils for long and short-wave were given. M S. (Alexandra) : Your case is serious, * and we are placing it before the proper authorities. You Will. hear more ater, I H. (Manaia): We cannot tell you much about your eircuit from the lay-out diagram, -We need the theoretical, for it would take us hours to unravel | it as it is at present. You valve’ conibina- ° tion appears ‘to be quite a good:one. We. do not like the layout, and think it could possibly be improved. Some of the wires seem unduly long. Pt£kYeorus (Dunedin)-TIn building a set employing. one or two r:f. stages and using reaction, would it be possibie to gang two or three. stages, as the case may be, and .put midget condensers in parallel with the main condensers. to tune each stage properly? A.: Yes, but would be the gain of putting all on: the one: dial if you have to have. midget condensers. to trim them up. Reaction quite spoils ganging. We have found that most of the gain due

to reaction is lost when several conden_8eFs are placed on the one spindle. 2. What is the best method of yolume control on a two-valve (det. and a-f.) set? ‘A.: Dimming the filament is usually a quite satisfactory process, though you cou',l use a 500,000 ohms. potentiometer across the aerial and the earth terminals, bringing the aerial into the moving arm. 8. What value potentiometer should be used to plaeée actoss a "45-volts "B" battery to get the necessary: yoltage for grid biassing ? ; Be RE os A.: About 100,000' ohms. Owt (Greymouth) : You can isolate the "B" supply from the moving vanes of the second tuning condenser by inserting between the moving vanes and_the connection between B plus.:power and the bottom of the tuning ‘coil a .006 fixed condenser. You would, of course, take the connection to the speaker‘from B plus power and not from the.;moving vanes of the condenser, as is now shown in the diagram, The moving ‘plates can then be earthed. Me, . A. (Napier): When I turn on my * electric set a loud screech is heard. On removing one of the power valves it stops. They can be interchanged and tried in each socket, but so long as one is out of the circuit, everything is normal. A,: You have a defective resistance or condenser associated with the bias of the valves, If you cannot locate this combination yourself, you had better get a service man to do so. GIMPLEX (Waikanaé): Do not be so ) impatient. .Everyone must wait his turn. We do not undertake to answer questions the same week in which they are sentin. Your queries were answered in last week’s "R.R."

VELOX (Auckland).-We de not recommend your using an untuned stage, rather use an extra tuned stage of screen grid. If you want a good set, why not build the "Outspan Five’? You could still use your own type of regeneration and your coils. G W. (Wanganui).-Is-there any age * limit for the.amateur transmitters’ examination? A.:. Yes, 14 and over. How much does the license cost? A: The-‘same as a receiving license30/- per annum. ‘This entitles one to re--eelve and transmit, so-you do not have to have an extra license. X46.MC. (Banks Peninsula).-Sorry we do not know the output of your eliminator. We think, however, that it will be quite satisfactory for your purposes. : ANGER (Timaru): The gauges of your wire are: 22 dicc,, 30 d.c.c., 36 d.s.c, Construct coils to specifications _ given in table 2 in the "Radio Record" dated ‘April 2,1931. Your circuit is quite anwordinaty one, and should be perfectly It would, however, be impreved if you used a fixéd condenser in series with the aerial. ‘While it would not stop the set from radiating, it would’ improve the oscillation,.making it more easily controllable, move any dead spots. and ehhance the selectivity. .

M P. (Kumara): Coil specifications for * the " .ight Hawk Two" have been published luany times in the "Radio Record." They also appear in the booklet "Questions and Answers in Radio," which is procurable everywhere. : Z . D*4M0 (Waitomo Caves) :.If I put .0001 fixed condenser in the place of the specified grid condenser, resulta are better, Am I doing any. harm? A.: No; it is not a bad plan to try changing the value of your grid condenser. One often gets far better results than with the condenser specified. 2. When .connecting "B" battteries I leave the negative terminal until last, and when it I get a spark each

time the wire is touched. Is this correct? A.: In all probability, yes. It merely indicates that your set is functioning, as_ it takes current from the "B" battery. If the spark were excessive and your batteries run down very quickly, the indication would be that there is a brokendown condenser in the set. . 8.: A red streak appears in one of my sereen-grid valves. . A.: If the valve functions quite well there is no need to worry about the streak. xou are not harming your set by using it. Mo! WHARDE (Feilding): To get. the maximum from my "Outspan" I have to break the connection between the bottom of the primary and secondary of the first r.f. coil, taking the secondary to "A-." This gives me an increase of about 20 per cent. Can you explain why? A.: Yes; if "A+" is earthed, you are putting a positive bias on your screengrid valve, and, of course, it should not operate under such conditions. Another veason may be that a primary coil is reversed. . 2. I am using 221 first audio, and two PM6 in push-pull. I ean jack out _ the 221, but with it in the circuit I ¢annot put more than 34 volts on the filaments'of the valves. ; Le A.: Why cannot you? Does the set become unmanageable? It seems that by the rather elaborate jacking arrangement you must necessarily have to cut-out: the 221, you are introducing instability. Try reversing the primary connection to the second audio transformer, although we think you will not get real satisfaction until you have cut out the jacking device. h UG (Khandallah) : The volume Contro} * of my a.c. commercial set, hroke down, I replaced it with a 20,000 ohms

one, but this one functioned for about 24 hours and then it broke down. A.: Obviously you did. not get the right type of volume control, or there is some fault in your set which is causing a high potential to. be placed on the volume control, thus causing it to break-down. If your set is model 34, 36, or 38, the yalue of the volume control is 10,000 ohms. These models employ 226 type valves, a 227, 171, and 180. X46MC (Banks Peninsula): The ~ under baseboard wiring of the "Hagle shows four 1 mfds. condensers. According to the theoretical sketch, however, it appears that one of these should be .l mfds. Which is correct A.: In the case of doubt, always follow the theoretical diagram. ‘This will show you that the correct value of the condensers is .1. The decimal point was on the original drawing, but did not come up in the block-making process. C J.S. (Auckland): I find that altering /* the position of the plates in the neutralising condenser in the "Ranger Three" makes no difference whatever. What is wrong? A.: a 1 mfd. condenser between "B+" rf, and earth. Keep this condénser as near as possible to the point where the

connection from the battery enters the coil. This will probably help in neutralising the set. 2. Should the set operate with the r.& valye removed? . A-:: This often happens, indicating that coupling is taking place across the valve socket, Make quite certain that it is properly insulated from the, baseboard. 3. Your sketch is not quite correct in that you should not use the same grid bias on both first and second audio valves. Take them to separate tappings on the bias battery, placing about 3 volts on the first valve and the correct bias, as recommended by the manufacturers on the last valve. our theoretical sketch is correct. The addition of another audio stage should not only increase the power of the local, but should bring in more stations. Increasing the number of turns on your primary may possibly help you a little in dx; you should be able to bring in the Australian stations quite satis factorily.