Questions and answers

Radio Record, Volume III, Issue 48, 13 June 1930, Page 26

Questions and answers

bd ‘GPARKS" (Christchurch) is intending to rebuild his set, and wishing to incorporate a wave trap, asks if this need be shielded. A.: Yes, you may have difficulty in eliminating the local station if the wave trap, when situated near the coils of the set, is not shielded. 2. Would it be an improvement if 1, shielded the last stage from the second? ‘ Av: No. Not unless you are getting audio frequency howl because of the design of the transformers. 3. I am using 5-volt valves, and a 6volt accumulator. What number amperite must I use? ‘ A.: You have not stated the name of the valve you are using. For each different valve there is a different amperite. Ask your dealer to supply the appropriate amperite for the valve you are using. , §6C1.L.¥." (Dannevirke) has built the Browning-Drake 3-valve set, but ean get only very weak signals. The radio dial has, very little effect upon the tuning. A.: Bring the aerial in to the fixed plates of the detector valve (if these are connected with the grid, otherwise the moving), and compare the. signal strength with that when the aerial is connected to the aerial terminal. In all probability you will find that the strength has improved. This will indicate that the fault is in the radio stage, and you will have to undertake a very systematic search. You say you have tested the voltage, and each valve is being supplied with the correct voltage. Check up the radio stage again and make quite certain the voltage. is getting to the plate of the valve, as an open circuit in the primary of the regenaformer is one of the most common causes of weakness in this type

of set. The vanes of the R.F. condenser may be touching or the dial not moving the vanes. There may be dry joints, the spring of your valve may not be contacting, you may have made a wrong connection. If the signals do not improve when brought to the detector condenser, your systematic-search will have to be in both stages. So far as we can see, from your description and diagram, everything is correct. "HH L.W." (Auckland) wishes to * puild a powerful set, and has decided that the 5-valve Browning-Drake is his mark. He wishes to know, however, if this circuit can be employed without the shielding. A.: Not without great inconvenience, and spacing the coils to prevent feedback would introduce further troubles that were not bargained for. Shieldng is employed because the reaction makes the circuit made very sensitive, and is close to the oscillation point. The absence of the action, although it does away with the need for shielding, lessens the sensitivity of the receiver, 2. Would a formica panel do equally as well as a metal one, and would it entail any alteration in the wiring? A.: The formica panel would quite well be used, but you will have to be careful to see that any returns that are taken on to the panel are connected to their respective destinations. **R D." (Dunedin) submits a plan * suggesting the application of the screen-grid valve to the 4-valve Brown-ing-Drake. He wishes to make the set more selective and a little stronger on distant stations. A.: The proposed alteration has very many defects. It is well known that the screen-grid valve, to realise its intrinsic value, must have a specially-

designed. circuit. You are merely placing a screen-grid valve in the old circuit. Furthermore, one stage of S.G. is notoriously unselective. We think, therefore, that your: suggested alteration is not wise. If you want greater sensitivity add another stage of radio frequency. "Oo O.H." (Auckland) has a_ set * which developed cracking noises like static. This noise persists when

the aerial and earth are disconnected. The valves and set are 19 months old. There is an amount of dust in the set. A.: Quite a number of causes of crackling are dealt with in the 1980 "Radio Listeners’ Guide." The vanes of the condensers may be touching, a noisy grid leak, or resistances, or

NX ‘broken-down condensers. Furthermore, it may be due to the age of the vaives. Twelve months is long enough for any. valve, Give the set a dusting. "BE B.D." (Pahiatua) has a six-valvye * factory-made receiver with resistance coupled audio. He is using a pentode in the last stage. Until recently this combination worked splendidly. Now when the set nears oscillation point, it breaks into a terrific howl. The valves and resistances are O.K. . A.: Probably a fixed condenser has broken down. You can test the condensers by the ’phones and cell method, fully deseribed in the "Radio Guide." "J A." (Palmerston North) has a five- * valve B.D. with R.O. audio. Reception leaves much to be desired. General purpose valves are used in ali stages except the second and third ‘audio, where .B605 and A615 respectively are used. A.: General purpose valves should be used in all stages except the detector and the last stage. Use 615 as detector, and 605 in the last stage. 2. Is it feasible to fit a pentode in the last stage? A.: Yes, the auxiliary grid is merely connected to B+-, but much disappointment has resulted from the use of this valve. . 8. Would you two transformers in place of three stages of R.C.? A.:; There would be very little difference in volume, although if you substituted the last two impedaformers with transformers and making .one stage of R.C. and two transformers you might have better results. 4, What is the correct way to nenutralise this set? A.: It has been fully deseribed in the 1930 "Guide." Select a station ahout the middle of the dial, and ad--vance the reaction condenser until the set oscillates. Adjust the neutralising condenser until this whistle loses its * intensity and takes on a loud breathing sound, The set is then neutralised for that particular wavelength. 5: I have a B and C-eliminator and there is only one grid bias tap. Would

Questions and Answers BADERS of the "Radio Reeord" who are in difficulties about reception or set construction are invited to write to our "Questions and Answers" department for help. We particularly wish to assist those who know little about radio, as very often there is some very slight trouble which spoils completely one’s enjoyment of the _ programmes. Correspondents: are asked to observe the following courtesies: 1. Write legibly. 2. Make your questions briet and to the point; do not make apologies for writing, and, where possible, tabulate. : 3. Do not ask for a reply by post unless a stumped anu addressed envelope is _ enclosed. Even in these circumstances, we reserve the right to answer any question through our columns, 4, Do not ask us to design circuits or send detailed tay-out diagrams; but we ean offer ad vice regarding circuits. 5. Address all technical corres: pondence: "The Technical Kditor, P.O. Box 1032, Wellington."

it improve the set to bias the other stages?-Yes, slightly. (HOILS" (Cambridge) asks for particulars for ree-inch coils to cover the shortwave pands.

"P J.K." (Lincoln) has made Round "the World Two and is surphised because 2MH can be tuned in on four coils, i | A.: These are probably the harMonies. The original wave being found on the eight-turn coil. c Could you supply particulars of Vv da base eoils to suit a .00015 connser. Is any alteration necessary | A.: These particulars were published in our "Questions and Answers" page a little over a month ago. Add about 10 per cent. to the number, of turns, for a .00015 condenser. "S D." (Hikurangi) has taken two * leads from a three-valye set under the floor to another room. I find that the speaker at times will not go, and at others is very weak. I separated the wires and the speaker went for a while and is now very quiet again. A.: It seems that one of the wires is broken, or there is‘a high resistance ' joint. ‘Test each wire separately by the phone and cell or speaker and cell method, and if this is all right, tug the wires’ and try again. x 3a. When I tune in 3YA there is a a2 Pe ATES EMAL Ee TN Oe

whistle behind the speech or music. It ‘does not happen on any other station. A.: It seems as though 3YA is’ be‘ing hetroydned. 3. I was listening to 2¥FO and I could hear 2YA and yet I can tune in 4QG without interference. A.: If all three stations are on their allotted wavelengths, which is likely, the phenomenon is puzzling and cannot be.explained. "F. W." (Whaton) has a_ six-volt generator with an ammeter and cut-out and he wants to charge a fourvolt battery. A.: Place a six-ohm rheostat capable ‘of passing two or three amps. between the negative of the generator and the ammeter and. connect this to yuor bat_tery. Adjust until the charging fate is about 4 amps. If possible, put a voltmeter across to make quite certain the 4 volts are being delivered. C." (Westport) has a factorybuilt receiver, the dial of which is graduated from 1-100. He wants to know how he can calibrate it to read metres, A.: In our issue of January 24 we prepared a report of a receiver in which a calibration curve was given showing the relationship between the two for a similar set. The method of finding the relationship is to plot a graph like the one shown with dial readings along the side and preferably frequencies horizontally and. plotting the position of several stations known. The frequencies can be obtained from the 1980 "Guide." Connect these up by a curve. To use this chart find the frequency of the station required, travel along the vertical line running =

from this number until the curve is encountered, then go horizontally and the approximate dial reading can be found. "KEEN" (Eastbourne), wishing to make ‘‘Pentode’s"’ Crystal and Amplifier, has a quantity of lin. standard stampings and asks: 1. How many turns can I use for the primary, secondary, filament and rectifier windings if I am using a 4volt valve for rectifier and a 2-yolt for the power? A.: Primary: 34 8.W.G., 1850 turns. Secondary: 86 gauge, 1850. Filament: 20 for 2 volts, 37 for 4 volts, 58 for 6 volts. 2. How many turns and what wire shall I use for the choke, using the same stampings? A.: Wind on 10,000 turns of 386 S.W.G. wire, using a .08 gap. This will give a 100-henry choke. 3. Later on TI will construct a dynamic cone speaker, Would the pot magnet eliminate the choke and would this necessitate alterations to the circuit? A.: The transformer as described will not supply sufficient power for the job. You would need a much more ambitious one. 4. Can volume be controlled properly without detuning? A.: Yes, put a 500,000 ohm potentiometer between the aerial and the earth terminals, "LJ NION" (Pukekohe) encloses a cirecut diagram and asks several questions. 1, Do you consider this circuit the best to use a screen gride in this manner? — at 2 a rrr,

A.: Yes, it is quite good. 2. Is it efficient for short waves? A.: It’ appears to be. . 3. What increase of efficiency should I expect from this circuit over that ° using the ordinary triode? A.: We can assure you only that you will have more trouble. 2. Does the anode voltage method of controlling reaction compare’ favourably with the condenser method with regard to general efficiency for short and long waves? A.: It is more suitable for short wave, but is efficient only when a very good resistance is employed. The "Frost" seems to be one of the very few of the resistances that are suit-: able. 8. Should I use any form of shielding with the.screen grid circuit? © . A.: Yes, it should be enclosed in a suitable screening box, e.g., the "Radio Record" standard. "RMD " (Rakaia) asks if a power transformer’ can be employed to step-up 5 volts A.C. provided from a rotary convertor from a battery. A.: Yes, but you would have te add about 10 per cent. to the stated secondary (primary in the original specifications). 2. How long would a battery last working this motor six hours a day, A.: It depends upon the drain it imposes on the battery, which to a certain extent depends on the load placed on the secondary. It would be necesSary to put an ammeter in series with the convertor to ascertain how much it will require. If it takes over an amp and a half, it is not worth while. ee

Band Secondary ‘Tickler Below 10 2 2-3 3 15/30 4 4 28/45 8 5 42/65 14 2-3 6. 60/100 26 2-3 8