Question and answers

Radio Record, Volume IV, Issue 29, 30 January 1931, Page 13

Question and answers

G G. (Cheh.): My commercial set * will receive Australian stations from 250 to 850 metres much stronger clearer than those on the higher wevelengths, A,: There is nothing wrong-the design of the set is responsible. 2: What size variable condenser is required for the wavetrap described in the issue of January 9, by A.A.H,?.0005 mfd. ADIO" (Onehunga): My set will not work with a grid bias battery, and reception is very rough without one, bd A.: Are you sure you placed it correctly? It should be connected between the appropriate terminal on the transformer (GB-) and A-. The valve you are using in the last stage will require approximately 44 volts grid bias. ' "KTOT SURD" (Hunterville) :*I am using an eliminator my last valve, 8 PM254, with 150 volts. hat bias voltage is necessary ?--22.5 volts, J 2. Can I draw this from my elimina‘tor?

A.: No, not without seriously reducing the "B" voltage. i 3. How is this voltage drawn? A.: From a grid-bias battery, which is not expensive and which will last you. a year or more, "CURIOUS" (Cambridge): I have a fiye-valve commercial a.c. neutrodyne. My results, using’ an efficient ‘aerial and earth, are not very good. Would it be possible without much alteration to incorporate a screen-grid valve? A.: No, it would be impracticable. 2. Could you suggest any method of increasing its sensitivity. .A.; No. Perhaps it is not working efficiently. Communicate with your dealer. "FT IGHT" (Christchurch): I rewound the primary of my L.W. transformer as you directed. It was not burnt out, but when I. tapped the last stalloy strip in, it eut through the thin fibre and severed several turns of wire. had it tested by direct voltage. It delivers 450 volts and the following: Rectifier filament, 5.8 volts; four other filaments, 2.9 volts. Is this too high? A.: No, it will be quite all right when the load is on. .

2. Is it absolutely necessary to use on output transformer with a Blue Spot speaker? A.: You would certainly be taking a risk in omitting it. KF ©, (Auckland) desires to know cer- * tain coil specifications. A.: See the tables which are published this week. FE J.P. (Blenheim) ;,Where could I ob- * tain a full lay-out.and wiring plans for a two-valve differential set similar to the one valver published recently? A.: This week complete specifications and a full description, with photographs, are published, 2. What would it cost? A.: Approximately £7, including valves and batteries, " A MATHUR" (Nelson): I have a fivevalve commercial battery set, but it will bring in only 2YA on the ioudspeaker, My batteries are comparatively new, but my valves are of fair age. A.: Replace the latter, as the fault in all probability lies here. 2. What would be the correct valve combination? . A.: Four 221 or 201A valves and a 112A type power valve. L G, (Taupiri) : In the "Radio Trouble * Finder" there is a eireuit of a fivevalye a.c. receiver, Would this be suitable for short-wave reception?-Yes. 2. Could you give me the particulars of the components? . A.: No, it would take too much to work them ont accurately, but a_better set will appear in the 1931 "Guide.’ D KF, (Pukekohe): I am constructing "© a Daniell cell charger for a fourvolt accumulator, and would like to know how much of the sulphuric acid solution must be placed in each pot before the distilled water is poured in, A,; Mix ‘both beforehand, taking care to add the acid to the water, and not the reverse, Pour enough in each pot to } cover the zine,

Shortwayes With Broadcast Condenser THs arrangement shown in the accompanying diagram permits the use of .00085 standard condenser, and by shorting switch permits doubling range of coil wave-length, For instance, switch open, Coil range, 150850 metres. Switch closed, 190-550 metres, same coil. '

on Py ADIO" (New Plymouth): I have built the 1980 "Guide" short-wave receiver, but have not had much success with it. I have rebuilt it three times in the hope of locating the fault, but without success. Some nights I ean get two or three ’phone stations and a few Morse, but they are all very weak. Can you suggest anything that may be causing the trouble? A.: No, your particulars are rather vague as regards fault symptoms, We would suggest that you take the set to an expert for overhaul. 2. Do you know of a better circuit?0. 8. Should this receiver put up a better performance than this?-Yes, much, 4, Will there be a new s.w. receiver described in the 1931 "Guide’’? A.: Yes; a four-valve a.c. receiver, using sereen-grid radio frequeney detector. "X Y.Z." (Lower Hutt): Would like * some short-wave coil specifications, A.: See ‘the tables published in this week’s issue. 2. I cannot get my set to oscillate. I am using a 7 megohm grid-leak. : Try a smaller value-say a 4 megohm, and if your coils do not agree with the published specifications, rewind them. K. F.B. (South Dunedin): I am building the B.D. Four from the "Guide" and would like you to answer the following questions :- 1. Would an A225 be suitable ag an an valve?-Neutralisation may be difficult, . What is the purpose of the 2 "O-" terminals? A.: The two audio valves each take different grid-bias. 8. What "C" voltage would be required, using a 90 yolts B battery?

A.: The first stage requires 3, but we cannot tell you what the last valve will require unless we know what type it is, 4, I have a transformer on which the terminals are not indicated. How can I ascertain which is which? : Use a pair of ’phones in series with a dry cell, and you can pick out the two pairs, The pair giving the strongest click is the primary. It does not matter much which is B-+- and which P. 5. Is there any practical method of determining which of two transformers has the higher ratio? A,: You could place each in turn in the audio stagé of a two-valve set and determine which gave the greater yolume.. This, of course, would be the higher rati 6. What is the most suitable wire to use in wiring up the B.D. Four?Glazite. 7. Would formers with walls 1-8in. thick be suitable for the coils?-Yes, AINING EXPERIENCE" (Queenstown): My set has lately developed a howl and only cease when the reaction condenser is turned into oscillation point. JI had to reduce the plate voltage on the screen-grid valve to pre vent it, but this also reduces the efficiency. What is the cause of the howling % A.: It is probably due to feed back from the s.g. valve. This and its associ ated coils should be shielded, Slip a shield can, preferably an aluminium one, over the coil, and if this does not prevent the trouble, try a cocoa tin over the valye as we Bees’ (Pleasant Point): When yil you describe the two-stage BD It will ap sppear. in in our issue dated February 20, The set has now

ORRESPONDENTS must attach this coupon to all queries sent to the Technical Hditor (Box 1032, Wellington), Questions arriving without it are likely to go astray or be delayed. Name of set ....c.cccceccvccrecesse Number of Vales PTEVURTISiticiy iy) Name Cee c oer ee oer ses SB EOSZSRSLE8 Address Cee OEE BE aR VOR RESSCREHANE POC SO HEH SHEEP ESET AOS ERERETSERSO EP ESEG Nom de Plume cecoseccecccveccses To be kept in eubsequent inquiries, Date — cnacseccenss cececcccccnee Please Note:(1) Be specific and brief, tabulating, if possible. (2) Write legibly, and on one side of the paper, (3) We do not design circuits, but accept suggestions for feature articles, Solving trouble, as different from advice, is difficult by correspondence and while letters are given every consideration, answers are not necessarily correct-they are only our. opinion based on the matter’ supplied, which may be quite inadequate, , Intricate and involved specifications cannot be supplied without a specialist’s fee.

been constructed, and is doing great work on the distant stations. 2. Would a Bluespot cone speaker work well on the set?-Yes. . 8. Will it be battery or electric?Baitery. 4, Would a Daniell cell charger work satisfactorily with a six-volt accumulator with this set?-Yes. "LOwLoss" (Wellington) : Is a frame egal suitable to pick up 2¥A on a erystal set? I had suceess once, but have have. not been able to get good results again. ‘Av: A frame aerial is not at all suitable: for a crystal set, although you have made the to your erystal set that such an aerial requires. There is not enough signal pick-up from an aerial,: . is improved if the aerial is (a) at right-angles to the station,’ (b) toward the station? A.: It should run toward the station with the lead-in of the point most distant from the station, 3. What is the optimum capacity for a concetiner for 60 turns of wire? : On a 24in. former, a .0005. ey AKA (Hataitai): In the Differential Two: would 6 volt valves. be more efficient than four volt? A.: There will be very little difference, certainly. not worth the extra running expense of the six volters. 2. Is a rheostat necessary if four or six volts are used, with a battery of the same voltage? A.: No, but the rheostat makes a good volume control. 8. I extended the valve base with eardboard and covered it with a photograph negative which makes the circum-

ference half an inch greater than specified, I put on a fewer turns to counteract this, Would your specifications be satisfactory for the 60 to 100 metres coil? ‘A.: Yea, the slight difference you have made in the sizeof the coil will not have any material effect upon the tuning range of the coils. 4, Would it be. worth while to construct an h.f. choke and if.so what would be the details? A.: A short-wave choke would be better than a long-wave one if you are intending to use shortwave only. A deseription appears on another page. 5. Is a "C" battery necessary for shortwaye? A.: It would be better as it would save the battery consumption and give you better tone. HRHE-COIL TUNER (Christ- * church): I have made the circuit round the three-coil tuner and I enclose a diagram of it. It is not altogether satisfactory. Can I use three 201A. valves? ’ A.: Three-coil tuner sets of the type indicated are more.or less out of date. We are incorporating the very latest ideas in small sets in our- Differential series, and « would ‘ advise you to convert your. set to this. However, 201A’s would be. quite suitable except in the last stage, though we advise .the use of a high gain power valve. 2. I have wound the following turns on 3in. coils :-Primary, 36; secondary, 90; tickler, 12. .Are these correct ? A.: There are not sufficient tickler turns. ‘Try doubling the number.

3. 3YA comes in all over the dial, Why is this? A.:. With a set such as this used without wavetrap, the local station will certainly come in all over the dial. 4, What value grid-leak would be suit-able?--2 megohms, . 5. Would a .001.. by-pass condenser aeross the transformer make any difference to the circuit? ‘ A.: It would probably cause it to oscillate more smoothly. 6. I get a crackling ‘noise and then a rushing. Why this? , A.: It sounds like a defective gridleak, or grid condenser. ‘Try reducing the detector voltage and increasing the number of turns on the tickler as sug- ‘ gested. % Would a Ferranti 8-1 and Emmeco 5-1 do for this circuit? A.: Yes, they would be quite good.. NO&ktH BNDER (Dunedin). Is the Mullard. screen-grid valve quite as good as the 224? A.: No, they are about equal in efiiciency. 2. Is the 625 a better power valve for heavy work than the 605? A.: The 605 is a high gain power valve, and if you want to drive a large speaker and have 200 volts available, P625 would be the better valve. 8. How many turns would I put on the primary of an inter-valve_ transformer to match the 224 and S4V? A.: On a Lin. former you will require 40 turns. 4, Would a rheostat in the filament of A609 in the r.f. side be any use as a volume control?-Yes. 5. Would a decoupling resistance be needed in the plate circuit of the s.g. valve?-Probably not. 6. Where does the neutralising tap eome on an 80 turn 2in. diameter former? -At the twenty-fifth turn. S W. (Hawera): I'am experiencing a * great deal of noise with my set and: IT cannot turn up the "A" battery unless there is a strong audio howl. | A.: We could have sorted your circuit out better had you sent us a theoretical diagram, Glanecing over the one you have sent in we notice that you are using a 10-1 inter-valve transformer. This is far too high. _ Use at the most a 5-1, though a 34-1 would be better. This is probably the cause of all your trouble. OUR-450 (Rotorua): I. have been troubled with decreased volume and have had two sets of valves blown out. A.: It is not, easy to say what-is the matter with your set, and it probably would be a wise thing to have it properly overhauled. We suspect either a broken-down condenser or a "B" wire touching an "A" wire. Look over your set for any points where the insulation has been worn off, and then use one of your~old valve bases, attaching the wires that come up from the filament prongs to.a torch bulb rated, at 6 volts. Put this in one of your sockets, then jerk the set about, fiddle with the wires, and do anything likely. to repeat the cirenmstances under which the other valves burnt out. Repeat this with the torch cell in different sockets. If the torch bulb is blown and you cannot: locate the trouble send your set to a reliable radio technician. Y set works sometimes and then it refuses to make a sound. I have renewed the rectifier in the eliminator and notice that the voltmeter on the panel will first show 65 and then after about five minutes drops to 20, and later will fall right’ off ‘to nothing. A.: Have you tried another elimina. tor? It seems that there might be a broken-down condenser in the . power. pack, otherwise it would be a case of very carefully’ testing out the wiring and all the components to see that there is noth. ing broken: down.

S O.8. (Auckland): 1 cannot get myt * commercial three-valye set to work well. Do the parts in a valve set deteriorate after twelye months? A.: The grid leak deteriorates and condensers may burn out. Valves lose their emission and batteries need renewing. Other than that there is very little that can deteriorate in a set. 2. Do you think my set wants’ overhauling ?-Yes. 8. Would you advise me to discard e and replace it with a new electric set ?- A.: No; we think your set can be fixed up by a reliable expert. 4, Would a three-valve set get the New’. Zealand stations and the Australian, at the same time cutting out 1YA? A.: It will probably get the New Zealand and Australian stations, but itis doubtful if it would eliminate the loca GHUNTER (Mercer): When will you publish the new B.D. cireuit?-Feb-ruary 20. 2. When can the "Radio Guide ‘and Call Book" be expected? A.: Barly in April. 8. Will you give me the specifications of a high frequency choke? A.: They are given on another. page. 4, Would the former described in the yomie and Call Book" be suitable?-. 5. I have an eliminator. How can I step-up the output to twice the capacity? A.: You cannot without redesigning the transformer.

D*NAMIOC (Waitomo): What do you recommend as a: good system to build up my. set so that I could add another stage later? ae A.: See the differential series: now running. wage 2. Should a three-valve all-wave set. be _eapable of all stations on the loudspeaker and shortwave? : ; A.: Certainly not; you will be lucky to get more than the main New Zealand stations on the speaker, y Is the Blue Spot cone a good speak#r . : A.:¥Yes. A-laboratory report has already appeared on it.