Questions and Answers

Radio Record, Volume IV, Issue 51, 3 July 1931, Page 16

Questions and Answers

S. L.M. (Greymouth): My differential adapter will oscillate over the whole range of 40 and 80 metre coil, using a .00035 condenser, but it will only oscillate over a small range when the . 0001 is in series with it. A,: There are two things to try, (a) increasing the number of turns on the tickler coil, (b) the use of a small series condenser in- the aerial cireuit. . 2. The differential condenser has a very effect on the tuning. : This is quite unusual, but it would be to attribute it to a definite ¢ause without experimenting with your set. If you get the reaction number of turns correct: you may find that this froable will disappear. ‘In a tuned plate cireuit, at what

frequency . band would the: impe dance of an average coil and condenser most nearly match a-s.g. valve? A.: At medium wavelengths. J ATEM (Kelburn) : I built the Radio- ’ gram Five, but am not satisfied with the result. When I use the detector valve and audio frequencies, the local stations tome in well, but with the r.f. stages I eun just hear 2YA and 2Z¥F. A.: We are afraid we cannot tell you what is wrong. Obviously there is some wrong connection in the radio stage, and it is a case of hunting round and experimenting until you find it. Have the valve tested in another set. 2. If I have too much reaction the set develops a howl, which stops when I de-

tune the condenser in the detector circuit. A.: This is due to the voltage on the detector being wrong. Hither reduce the voltage by "the tapping on the eliminator or put a higher resistance in series with the detector lead. 83. Is it quite in order to use grid bias supplied from the eliminator instead of bias resistance? A.: Yes, instead of earthing the grid returns carry them: to an appropriate tapping on the eliminator, and earth the. centre taps on the filaments. ROGRESS (Pahiatua): Can I use a tungar battery charger. ¢o charge both "At" and *B" batteries, and how can it be done? A.: Most tungar chargers are like the one in the diagram and "employ a resistance Jamp to break down the charging rate for the "B" batteries. If yours is

not wired like this the only way you ean charge the "B" battery without pulling down the transformer is to fiud out the maximum voltage that the low tension

winding will supply, and break the "B battery up info units of this voltage charge them in’ parallel. In other words. if it will deliver a maximum of 12 volts and you have a 90-volt "I" battery you would have to divide it up into eight sections, and each section will contain eight cells. These are arranged in parallel and the charger connected, The, diagram illustrates the idea. . 2. How ean I overcome harshness in & Browning Drake? A.: If properly constructed and a good speaker and power valve used, the B.D. will not be harsh, It is all 4 matter of adjusting your valves to operate under the best conditions..Use a power valve of the B405 class and bias it with about 12 volts if you are using 90 yolts "B." You have not stated what type of speaker you, are using. This may be at fault,

GPEAKER (Auckland): 1 have a erystal. set with a 2-valve amplifier. When I use a 15 feet extension cord on the speaker it still whistles at 880, but when I earth one of. the terminals the noise ceases. Is this detrimental to the batteries? . ; A.: Unless you are using an outppt filfer of some description, yes. Try "the use of a one or a half mfd. condenser petween one of the leads and earth.’ This should by-pass the squeal without shortcircuiting your batteries. If, however, you are using an output filter, you are doing no harm. UNK BOX (Petone): I have built a shortwave adapter which goes satisfactorily, but when the plates of the aerial condenser touch, 2YA comes in. Why is this? , A.: According to your circuit everything appears to be in order. The system will be fairly broad in tuning, and is sharpened up by-the aerial condenser. When, however, this ,touches, the tuning becomes very broad, and what is probably -one of 2YA’s harmonics comes. in strongly. 2. My set is a three-valver, and as T have been troubled with audio howl, T have tried all the usual eures without success. I believe the trouble is in my transformer, but I do not want to be rasin«« and buy a transformer until I know certain. A.: We agree with you on two counts: (1) That the trouble is in one of the transformers. and (2) ‘that you should not buy until you are sure. Try this: Reverse the positions of the transformers, putting the low ratio first and the high ratio second. Having done this listen-in to the first stage and see if the howl persists: If it does and you still cannot find the cause you will have to pull the set down and rebuild it. By the way, a jacking system for the first valve frequently introduces audio howl. (Note: We have noted your request for tips and jottings, and will try to get this column going again.)

NEW CHUM’ (Hastings): I have a well-known midget set. Is it nossible to get the American stations on this re"Yeciver? A.: That is rather a difficult question to answer. American stations have been received on three-valye sets, and yet owners of 10 valve super-sports receivers cannot hear a whisper of them. So much depends upon your locality. It is impossible to lay down any definite rules in respect of their reception, | ; 2. My aerial is about 80 feet long and about 50 feet high one end and 40 feet the other. Is it any good?-Yes, excelent. 8. Is my set a good make for reception? ¥ A.: We do not know anything about eo P.U. (Auckland): I intend making * _ shortwave coils for my set, but the middle of the primary of the existing broadcast coil is tapped. Will this be necessary in the shortwave coils? A.: It sounds as though the set is one of the h.f. det. audio types neutralised by the-split primary method illustrated herewith. This type of set does not lend itself to shortwave, (Note: We regret we cannot give you the specifications you ask for. Very many specifications for shortwave coils are published in the "Record," and we cannot continually be designing coils for individual requirements. Already our Ques- . tions and Answers section is out of proportion to the size of the paper.) oA (Taihape) : I intend to construct the Outspan Five. Can I use a metal panel and connect "A-" of the last tuning condenser to it, biasing the detector valve with the leak return to "A+"? A.: Yes, this method is quite in order and ig preferable under those cireumstances. : 2. Would h.f. chokes be advantageous in the screen-grid leads to the h.t.? A.: They would be of use if the set

were likely to be unstable. We found them unnecessary. 3. I have a set of lin. diameter coils to match a .00035 variable condenser. What would be the correct number of turns for the primary to match these for the Outspan Five? A.: "Cathode" went into this point about matching a week or so back. Roughly about a third of the number of turns on the secondary will be sufficient, but if you experiment you would find out the most suitable number. . SPARKS (Midhurst): A continual hiss is heard through the phones, and it stops if I eut down the _Yolume by the rheostat or touch the grid terminal on the transformer. A.: Try reversing the turns to the primaries of one of the transformers, Failing this, place a low value condenser about .0001 across the primary of the transformer. 2. Sometimes when the set is working full strength it cuts itself out, and it does not come on again until the rheostat is turned back and then on again. A.: This seems to be one of the valves, probably the detector. Have them test-: ed and at the same time examine your rheostat very carefully for any defects Hi HLS. (Lower Hutt): How many square centimetres in an inch? A.: Centimetres are a linear measure, in other words they correspond with inches, not with square inches. Square centimetres correspond with square inch. There are roughly 24 centimetres in every ack and 63 square centimetres in a square ine 2, About what is the size of my condenser with seven moving plates and eight fixed?-A.: Probably .00015. 3. Is the short-wave power-pack in the "Radio Guide" suitable for my set, and what will be the cost? . A.: It would provide more power than necessary for your set, but it would

-- be a good investment. If you made everything yourself it would cost you about £5. . T N.G. (Waipukurau): Can you give * me the name and address of a dealer from whom I could obtain spare parts for the Cossor Melody Maker? A.: As these components are fairly readily obtained we would advise you to write to any of the city dealers who specialise in country orders. Quite a number of them do these days, and you should not have any trouble in getting what you require, D K.W. (Gisborne) : Would the valves * PM4DX and PM8 or Radiotron 230 be suitable for the Ranger Two using dry cells? If not, please suggest a combination. A.: Both combinations are quite suitable, the new 230 Radiotron valves being 2 volts. 2. I would like to add a stage of rf. Can you supply a cireuit diagram of such using an ordinary r.f, valve? : Yes. We will shortly publish a circuit that should suit you. 8. Would the Ranger Two give loudspeaker reception on the Australian and New Zealand stations using the combination of valves specified, or would a detector and two a.f. be better? A.: If you want really satisfactory results from Australian and New Zealand stations you should use a three-valve set with a radio stage, such as that which we will describe in the near future. Such a nomen is better than the det. two audio. WinpswEpr (Hataitai) : A plopping sound has developed in my commercial eight-valve set. It was first noticed when the aerial was on, but now it ean be heard still faintly with the aerial removed. All valves have been renewed recently with the exception of the 280, which is only about four months old.

eaten A.: It sounds to be a condenser puncturing, although it may be your rectifier. It is a case where the serviceman should be called in. But first check up on a neighbour’s reception to see if he notices it, for quite possibly it is outside interference, A P, (Otahuhu): I have built the Out- * span Five, which is working well, (Concluded on page 80.)

CoOPRDSPONDENTS must attach this coupon to all queries sent to the Technical Hditor (Rox 1032, Wellington. Limit three questions, uniess letter is accompanied by 1/ee, Name of set ..... eves eer scepeoesNumber of valves Ce eesceesasorecees Name Address SPC OSE TOS SCOHOSOSSHSOSBEEOSS PSOSCSROSSHSISOSC CSO SSOSOOSE® POPP OOOH TOTS OTSSOSSSHSSESSOOESESESSE® SPO CHO OOS SHEE EHS SESESOSEOSCOOSSESEHEESS Nom de plume Coevecarsesesesoces To be kept in subsequent inquiries. Date ose 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 eannot be supplied’. without:a specialist’s fee, eo

Questions and Answers (Continued from page 15.)

~~ om rxcept that it will not cover the whole x€ the broadcast band. 1ZM on 247 m. tomes in on my dial reading 13, 13, 5, md 7ZL on 517 m. on 95, 86, 82. Should vot the set tune between 200 and 550 metres? A.: Not necessarily. You seem to be covering the bands where there are any stations worth listening to. Take a few turns off the first two coils. 2. You mention that you hope to match the coils so as to bring out a single pr two-dial receiver. Has anything been done yet? A.: Plenty has been done, but still we cannot give specifications that we could guarantee would give you perfectly match coils. You see how difficult it would be in your case to put the three coils on one condenser. It seems almost impossible to get the balancing properly (when using reaction) over the whole jand. We think our readers will have to let-us off that promise. FPUKEKO (Hinuera): My set has deid veloped a crackling noise, but if I disconnect the earth and let it rest on the metal frame beside the terminal a great deal of the crackling is eliminatIs there a short in the wiring somewhere? A:It seems very much like it, although the trouble may be due to a broken-down eondenser, We suggest you take the chassis out of the cabinet and examine the wiring, very carefully tugging at the different joints likely to cause trouble. If vou cannot get to the bottom of the trouble in this manner it will be a case of sending to the agents for repair, 2, Sometimes 2YA is not satisfactory strength, and the new Wellington station comes through at tremendous volume. I eannot fathom this, A.: Neither can we. Reports of the reception of the two stations conflict. They ean be explained only by atmospheric conditions in relation to their wavelengths.. Many people complain that 2ZW comes in anything but well, and others point out that it is one of the best stations they reevive. Probably neither station is at fault. QVERTONE (Christchurch) -Oan you give me advice on the purchase of a pick-up > guitable for my a.c. set? A.: We regret we cannot direct you to any particular make, as this would be against editorial practice. We have tested several pick-ups, and the reports have appeared in these columns. ‘he best and

most expensive of these is the Audak, which has a frequency response from _below 50 to well over 8900 cycles. Your set is probably limited to about 8000 cycles, with a sharp cut-off at about 5000 or CERTAIN (Christchurch).-Could you advise me as to where I could obtain sufiicient data on the construction of an efficient "A," "B," and "CO" eliminator using a metal rectifier? A.: You could obtain all the information you require from the 1931 "Radio Guide." W K.L. (Nelson).-Can you supply me ¢ with a circuit of the-latest and most’ satisfactory battery-operated receiyer? This should have three of four valves. A.: We advise you to wait for the "Kestral" Three. ‘nis is easily the best receiver of its type, and will be described in these columns within a fortnight, RYSTAVOX (Dunedin).-I have read about a loudspeaker which works directly from a crystal set, having incorporated in it a type of microphone bar amptifier? Where could I obtain one of these speakers ? A: We do not know anything about the agency for this set. Your best plan would be to purchase a microphone bar amplifier, and use this in conjunction with a good speaker. You would reach the same end, and would do so far more economically than if you went to the trouble of importing the speaker described; but microphone bar amplifiers, and the type of speaker you mention, as well as the book from which you took the particulars, are all long out of date. The best plan to get loudspeaker reception from a erystal set, if you cannot get it direct, is to make a simple one or two-stage amplifier. Modern valves are yery light on batteries, and consequently economical to run. . N W. (Dunedin).-I am troubled with * interference by a nearby lighting plant. ‘How can I remedy this? A.: See replies and diagrams published in response to a somewhat similar inquiry, published elsewhere in these columus. The most satisfactory method is to use chokes

and by-pass condenser. ‘The choke should have 150 turns of number 18 copper wire: on a three-inch former. Your log appears to be quite a creditable one, FAAROLD (Dunedin).-I have an 8valve combination electrie set, and f am interested in suortwave. Would it be possible to use either the Night Hawk or sparrow Hawk adapter? A.: Use the Sparrow Hawk adapter, as described in the 1931 "Radio Guide." It would cost you roughly #5/10/-. _ 2 Would this be as good as the super. Ket. adapter described in the "Guide"’? A.: It would be easier to build and work though it would net give you the same signal strength,

ANGER (Napier).-How many turns would I wind on a high-frequency choke for the Ranger Two? A.: A. suitable choke was ' described about three months ago in the "R.R." Tt consists of a thousand turns of wire on a former, of an average diameter of 2in., the wire to be wound in slot, about 86 or 38gauge giving the best results, 2. Will a .0005 and an 11 moving and 12 fixed plates condenser do for the Ranger Iwo? A.: Yes; the capacity of the eondenser you mention is about .0003, and will do quite well for reaction. 3. Can I use a Philips valve A810 and a Mullard PM3 for the Ranger? A.! Both these valves are out of date. The modern PM 3 has a filament voltage of 4, not 3.8 as you state. However, you eould use both valves until you were able to get the newer types. ol) D.L. (Itnawai).-A certain brand of ° eondenser is advertised as having a phese angle difference of "9/25 minutes. Phase angle I understand, but 20 minutes is an extremely long time when dealing with electricity, so would you please explain? A.: In all circuits of alternating eurrents such as those including condensers, there is a rise, fall and reversal of electrict pressure or voltage, and also a rise and fall of current or amperage. ‘he rise end fall of the voltage is not always m exact step, ot in phase with the rise and fall of current. In an ideal condenser, which does not exist in practice, the current rise and tall would lead the voltage rise and fall by one-quarter of a cycle or 90 deg. All condensers throw the eurrent and voltage slightly off from the 99 dee. ideat difference. The more the condenser throws the enrrent and voltage out from this iderl difference the ereater is the loss of energy. The amount of displacement is called nhase angle dif ference of the condenser. The phase angle (lifference is the measure of poorness in @ condenser-the greater the phase angle

difference. the poorer the condenser. In fixed mica condenser the phase angle difference generally becomes less and Jess as the capacity of the condenser decreases. A. small fixed condenser has a slightly less phase difference than a large one of the same type. Phase angle difference is therefore measured in degrees or in parts of degrees. A sixtieth part of a degree is called a minute. Therefore to say that your condenser has # phase angle difference of from 20 to 25 minutes implies that the angle is less than one degree, which indicates a good condenser. . 2. Is there an officially recognised for gadio servicing, etc.? . A.: If you supply us with the word between "recognised" and "for" we will try to answer: your question. 8, Is one allowed to have an hf, oscillator for set testing purposes, and not hold a transmitting license?-Yes. TT R. (-).-I wish to construct a 20/ ® 80 heavy choke and have stalloy already cut. A.: Your best plan is to make a o> so that it will exactly fit the windy ® that is 5/8in. x 1 7/8in. Fill the bobbin with 84 gauge wire."