Questions and answers

Radio Record, Volume III, Issue 49, 20 June 1930, Page 26

Questions and answers

/ . "J F.W." (Taumarunui) wishes to * purchase an all-electric chassis fitted with gramophone jacks, and wishes to use it in combination with a gramophone pick-up. If he uses it connected permanently, and places a switch in the grid side, will this be all right? . A.: Yes, but it really does not matter in which side you have the switch. "FE J." (Okatu) asks how he might * use his battery charger in place of the "A" battery. A.: You will require an audio frequency smoothing choke. This can be made from directions given in the "Radio Listeners’ Guide," or it can be obtained at least from Ferranti’s. Then Ae evens ere pueenthemessmenninmemenien ante thie eens

you will.need a 2000 mfd. electrolytic condenser. The smoothing choke is connected in series with one of the leads from the charger, and the smoothing choke is connected between this and the other terminal of the charger. The condenser is connected on the charger side of the choke, No transformer other than that embodied in the charger is necessary. The positive on the charger will be negative when it is used as a battery eliminator. " RS." (Invercargill) is not clear * about the batteries to be used with the two-valve Browning-Drake, and he wants further particulars about winding the primary coil. A.: We think the new "Radio Listeners’ Guide" will clear up all your difiiculties in this respect. There are three sets of batteries, the "A," the "B," and the "0." The "A" should be an accumulator, the plus is connected directly to the set, and the minus to the minus of the "B" battery to the plus of the "CO" battery. One wire from these three connections is taken to the set, and is the common minus lead or earth eonnection. .The highest B+ voltage goes to the radio valves, and 45 volts or even less, goes to the speaker. This is the detector voltage, for you have no audio if you are using the true Brown-ing-Drake Two, that is, a radio and a detector stage. The primary coil is wound on a separate piece of former. It may be a 4-inch narrow strip of the ordinary former, with a section cut out so that it becomes smaller when the size of the ents are drawn together, If now a sharp pocket-knife is run round this, and 2 group cut out, fine Wire can be wound in this groove, and it will not project above the surface of the strip. About 15 turns are wound on, and the ends threaded through small holes in the former to hold them tightly. This is now slipped inside the main former, and it ean be held in position by any convenient method. Stamp paper is quite effective; but there should be very little difference in diameter between the inside of the secondary coil and the outside of the narrow strip. "G L.B." (Wellington) asks how he * might improve reception from 2YA A.: The set, although it has two valves, is really only a one-valve, as one is a rectifier. Add another stage as we have indicated in the sketch forwarded to you. = © M." (Te Kuiti) asks for fuller de- * tails of the battery charger described in our article by "Hard-Up." A.: The charger consists of eight Daniell cells in series. A Daniell cell comprises a glass jar, a porous pot, a zinc and a copper rod, a saturated solution of blue-stone and a solution of epsom salts or sulphuric acid.

The former, which is less corrosive on the zine, was described fully in the article. Place the porous pot within the jar, mix up the blue-stone solution until no more will dissolve, and pour this round, but not in, the pot. Now mix up the epsom salts solution and empty this into the porous pot. Fill them as

nearly as possible to cover their respective rods, The copper rod (positive) rests in the blue-stone solution and the zine rod in the porous pot. Connect eight of these in series, and the resultant voltage should be about 8.5. or slightly more. By connecting in series, one negative rod is connected to the neighbouring positive and the positive of the battery so built up, is connected to the negative of the accumulator. This primary battery will need attention regularly. Fresh blue-stone must

be added almost every week. The copper strips, which should be as heavy as possible, should be scraped often. The zine will in time be eaten away. The system, however, works well. "7 EDDER" (Masterton) asks where he might obtain a syllabus ot fhe qualifications for the A.M.I.R.E, ex-~ aminations, and where there is dny college offering instructions in these subjects. ~ A.: There is no examination, but a eandidate seeking admission to the institute must have served a certain portion of his time in radio professionally, he must have operated a station and be nominated as being proficient in wireless theory and practice by six wellknown people connected with radio. The course given by Johnson’s Wireless School, Wellington, would adequately cover the ground ‘from the theoretical aspect. "FEW." (Taihape) in whose set there is a noise the origin of which he cannot locate, finds that when he removes the aerial the ~ noise still continues. Replacingy’ a 4 megohm grid leak by a 3, mad a slight. reduction in the noise level. A.: Was the grid leak replaced a new one? If not, it seems that the trouble is emanating from a defective grid leak, and this is probably the most common cause of noise within a set. We suggested several other courses you might adopt last time. Have you tried these out, and have they been successful? 2. Can I replace the 201A valve in my set with the new 221 valves without interefering with the resistances? A.: Yes, although the resistances are designed to drop the voltage from 6 to 5 when 4 amp. valves are used; there is very little drop when only .06 amp. is passing, and this can be neglected. 8. Is the 221 valve as light on "B" eurrent as the 201A valve? A.: If anything, it is lighter. "EGC." (Wellington) wants to kn where he can obtain particulars; of a short-wave adaptor. A.: The 1929 Radio Listeners Guide eontains an excellent one, but omit the 1lmfd. by-pass condenser. "F.C." (Bay of Islands) wants to improve daylight reception and wants to know if he can use & UX245 valve for this purpose. A.: We are afraid you cannot. The UX245 is in reality an A.C. valve, It requires a heavy filament current which would ruin your accumulator in very little time. Furthermore, you have not the requisite "B" voltage, and \. if you had you would probably be dis- & appointed if you wished to strengthen your reception. Although this valve has greater amplification than most super-power valves, yet its main func-

| nl tion Is to handle great amounts of | volume. , "PUZZLED" (Petone) has a six-valve . "~ « factory-built receiver, and has been, troubled of late by loud howls | which appear all round the dial. A | dexler replaced the valves and later | told him to adjust the neutralising © eondenser, . A,: The agent should not have advised you to interfere with the set, Neutralising, especially when valves other than those recommended by the manufacturers are used, is the job of 2 service man. It appears that your set is not neutralised, and we advise you strongly not to interfere further with the receiver, but to call up the man who replaced the valves, and to neutralise the set, as he should have dene when the valves were replaced. 2. There is a buzzing which ceases when- the aerial is removed. A.: hig appears to be outside interference which you cannot control. 7 "GjHORT-WAVE" (Temuka) can get only 2MH) on his adapted shortWave receiver, and the lower ends of the dial are greatly out of step. A: Put a midget condenser in the aerial and increase the detector voltage to 45 or more. "WAP." (Wellington) asks for a circuit to suit commercial coils. A.: It is not our practice to supply circuits. You should have obtained one from the dealer who sold you the tuner. But in this case, we are making an exception, and are sending you one. 2. I have a 5-valve neutrodyne the 2udio valves of which become warm. _A.: This is not particularly alarming, but try increasing the grid bias battery on She power and first audio stages. "P.W.T.". (Foxton) wants to know the life of three "A" batteries (dry cells) supplying .2 amp. A.: About 200 hours, if used intermittently. 2. Could I use wire such as that enclosed for an R.F. choke?-yYes. "J W." (Napier) has a five-valve bat- * tery set witha horn speaker, and would like to know if he could use one that would give better tone, say the Dynamic. A.: Yes, he could use a battery model , dynamic which draws its field supply from a six-volt accumulator or an inductor dynamic speaker which does not require field supply. e "N° SIGNALS" (Wellington) has a Round-the-World Two which will go in one locality but not in another. ‘The "A" battery shows five volts. A.: Your "A" battery is flat and this is probably the reason you are not getting the outside stations. Other than this, the aerial and earth equipment may be at fault. "G M." (Tikitiki) wishes to change "his three-volt valves to six-volt, and asks: 1. Would this mean better average reception? A.: In all probability, yes. ' 2. Would the set need re-wiring? A.: No. If any of the resistances ~heat up they should be shorted out. "S, What make of valves would you recommend? A.: As your set is American, use .Radiotron 221 for all stages except the last, where any medium power valve,

~ Oe such as Radiotron 112, Mullard 256, Philips 509. or Cossor Stentor. 4, Name a make of a reliable accumulator. : Standard makes are advertised in t. e "Radio Record." We cannot advise any one specifically as they are all good. 5. Would the battery connections be the same as for dry "A" patteries.Yes. "V A.S." (Khandallah), has a short"wave set and. gets Wellington’s harmonics on all coils except the bottom pair, which will not oscillate. Can

this be cured? The aerial is 110ft. long and 30ft. high. A.: The aerial is too long for ultra short-wave reception. Put a midget condenser in the aerial or increase the number of turns on the tickler. 2. I was disappointed that you have not included the amateur Australian shortwave stations in the 1930 "Guide." A.: It would have taken far more space than was justified. They can be obtained from "Amateur Radio," obtainable from Te Aro Book Stores, Wellington. " 8." (Whangarei) has built up an adaptation of the silk diaphragm speaker, but has met with very little success. The frame is circular, 6 inches wide, made by gluing .segments of rimu together so as to form a cylinder $in. thick. The resulting tone was thin and empty. A.: The speaker as described goes perfectly, and when adaptations are made like this there are so many chances of failure. Your best plan is to construct the speaker as it was described and you can be assured of good results. Are you sure there is not too much tension on‘the reed. This frequently causes trouble, the solution being to have the reed lengthened by a tapped rod. "L.-J." (Wellington) has found it difficult to obtain a circuit of a 1-valve receiver. A.: ‘We do not encourage the construction of 1-valve sets, because they can be a great'annoyance to everyone within a mile or two. However, in our

issue dated April 12, 1929, we described a combination model of a 1-valve and erystal set which can be used on the local station as. a crystal and amplifier, or as a 1-valve receiver, a A.RM" (Auckland) has constructed a power pack and can only get 180 volts at 60 mills., although it was designed for 250 volts. A.: Probably the air gap in the smoothing choke will have something .to do with your trouble. ‘The correct size of the gap is of considerable importance, when much current is flowing. The width of the gap should not be excessive, for the necessary inductance

could not be obtained to build up a sufficient reactance voltage to prevent fluctuation. On the other hand, if the width of the gap is insufficient, the coal will become saturated, and a large degree of variation results in a reduction of the filtering action of the choke. and consequently allowing a fluctuation in the voltage tc the plates of the valves, 2. Lam allowing 50 volts drop in the valve, and about 15 in the choke. Is this correct? A.: Voltage drop in the valve is greater than you have reckoned. BH equals 90, BA equals 45, B equals 140. The maximum anode voltage for BH valves should not exceed 350 volts per side. You should be getting, if you have provided 850 volts on either side of the centre tap, 250 volts at 60 mills. The resistance of the choke coil is about 500 ohms. "KK L.wW." (Auckland) asks if the * 6R would be a good unit for the silk diaphragm speaker.-Yes. 2. Would the 2ft. box baffle be better than the above? A.: Depends upon the construction, but there is really very little difference between them. 8. Would either of the above com- binations be satisfactory on 4-valve Hammarlund Roberts? All 201A’s with 90 volts "B’’? A.: Yes; but we advise you to use a semi power valve instead of 201A in the last stage. "SW. Adaptor" (Wanganui) asks what modifications are necessary

to convert "Round the World Two" into a short-wave adaptor. A: In 1929 "Guide" we described fairly fully how to connect the adaptor to the set. Take an old valvé base and solder wires into the two filaments and the plate prongs. These are then connected to the adaptor as follows: Filament + of adaptor to the prong that will go into the + side of the detector socket. The plate connection comes from P of the transformer in the complete set; thus the plate of the detector of the original set is connected with the plate of the adapior. The audio stages are omitted.