Questions and answers

Radio Record, Volume IV, Issue 44, 15 May 1931, Page 18

Questions and answers

M M. (Mount Eden): When will you * publish the a.c. version of the Outspan Five? A.: We shall not make any rash promises, but it will appear in the near future. YZ (Otago): Can I add another 45 volts "B" battery without making any alterations to the set or using dif: ferent valves? I use four already. A.: It: would not be wise; four is ample "B" battery for any set, as it represents 180 yolts. It would not place any extra drain on an "A" battery. Lorry (Linwood): There is an aud ible hum in the transformer of m) L.W. Would this cause a hum in the speaker? : * A.: It is possible that the hum may be picked up by the -amplifier, but it is unlikely, 2. If I connect the earth to the sub panel it causes loss of volume and distortion. A.: This is due: probably to some slight mistake in the set. However, so long as you get better results by leaving it off, continue to do so. You are not likely to do any damage. ‘8. I have two fransformers of govil make, 4-1 ratio. Is this too high toa use with push-pull transformers? A.: No, they are quite satisfactory. CREEN GRID (Ngaio): I use a switch to connect my L.W. to the tuner. When I break the contact when changing over from radio the set be comes unstable. .: Do you mean that the set is un stable while the switch is in neither position? If so, it is quite understandable as the L,W. is most unstable when

there is no load on the grid circuit. If not, could you. make your question a little clearer? D E.C. (Waikato): What is the best * valve to use in the "Sparrow Hawk" One? A.: Any two-volt valve, though one of the new 230 type would. fill the bill excellently. 2. Would the 2-valve a.c. amplifier cescribed in the "Guide" be suitable to use in the "Sparrow Hawk" One?-Yes. 8. I have been troubled with static for the past two weeks. What is the best device to reduce this? If you connect a .025 or there abouts condenser in series with a 50,000 ohms variable resistance and connec this combination across the phones, you can by-pass much of the higher frequency signal, and as static is of high frequency it also will be by-passed to a large ex tent. OwL (Greymonth): Could the -Radio gram Five described in the 193! "Guide" be successfully changed over fo1 shortwave work ?. A.: Yes, by adapting it along the lines deseribed for the Differential ‘Three a few weeks back,. You would make the same coils and couple the set up in the same way. We have not tried this ar rangement, but no doubt it would work 2. What ratio of push-pull transform er do you recommend ?-About 34-1. H W.Y: (Awanui N.): What briefly * is the difference between 66R and 66K speakers, and which is- the more suitable for a battery set using 90 volts A.: 66R will’ take 50 mamps of d.c. current against 66K 30. It is a heavier

speaker which will take greater volume, without distorting. A 66R would be the better speaker for your purpose. . 2. Which is the better combination of valve for my five-valye American set -the 119 type or ordinary four-volt valves? ; A.: The ordinary four-volt would be the better, but do not use A425 as first audio. Use A409. This valve is totally unsuited for the work you are requiring it to do. . 8. Which combination is the most economical on "B" batteries? A.: There is little difference. The possibility ‘is in favour of the first-mentioned combination. DE ‘DUFFER (Dunedin): I have a five-valve set, the first audio of which is B605, followed by two 256’s in pushpull. I use the set only 12 hours a week and the valves went out in three or four months. A.: You should not use B605 in the first stage. This valve draws far too much anode current to be passed through an ordi .ry transformer, and when operated with only 44 volts grid bias you are shortening the life of the valve considerably, 2. I have 224 volts in the push-pull stage. Is that. right? A.: Try increasing it to 45 volts. with an idea of double bias (as explained fully in the "Radio Guide’). This will reduce anode current and possibly lengthen the life of your valve. 8. My six-volt acenmulator is charged ’ by an old type tungar 2-amp. charger. It has never heen fully eharged, and T have never run it down below the red figures (1200). Is this good practice or should it be run down occasionally ?

A.: It is very good practice. If everyone did this their accumulators would last twice as long as they normal]. do. 4, On one or two occasions I have left the "B" supply on for some time with the "A" switched off..Would that shorten the life of the valves? " A.: Most decidedly, yes. It is very bai practice, and tends to ruin not only the valves byt the eliminator. It is a wonder you have not had a flash over before now. WESTER (Westport) : Could 26 d.c.c, wire be used in the 100 watis transformer described in the "Radio Guide?" A.: Not without very seriously increasing the bulk of the transformer and »ossibly spoiling its efficiency. The numher of turns would be the same. How does one calculate the length of wire used in these power transformers?

A.: Take the average diameter of the winding, the length of which is to be calculated. If it is the primary and you are winding on a 1tin. square bobbin, the mean diameter will be about 1%. Multiply this by 3.1-7th and multiply this by the number of turns. This Will ‘give the length of wire required in inches. ERVICH (Waitaki): What is the standard colour code for resistance and wiring as adopted by the R.M.A.? A.: Drake’s Handbook gives the following :- "A definite schéme of colours for receiver wiring has been adopted and should be followed wherever: possible. Wires are made either with covering of a single solid’ colour or are made witha cover mainly of one colour but with a tracer

K or a single strand of some other colour woven in."

In general the following plan is used :-- Blue for the antenna circuit or loop circuit, Reds and maroons for B battery or plate power supply circuits on their positive sides, ' . Yellow for the A battery or filament circuits on their positive sides. Green for the grid bias or © battery circuits. . Brown for the loudspeaker circuit. Black for negative sides or low voltage sides of various circuits. The colour is chosen according to the point from which the circuit starts. Thus, circuits starting from the B battery or power unit would be given red or maroon, regardless of the other point to which connection is made. The following list shows the colours used for the body of wires and also for the tracers in the circuits of receivers employing this scheme :- Colours for Receiver Wiring. Antenna or grid side of loop-Blue., Ground or filament side of loop-Black with blue tracer. A plus filament-yYellow. A minus, filament-Black with yellow tracer. A plus'and B minus-Yellow with red tracer. A minus and B minus-Black. B plus, highest voltage-Bright red. B plus, intermediate voltage-Dark red. B plus detector-Maroon. . B plus to speaker cireuit-Red with brown tracer. ‘ .B minus-Black with red tracer. C plus-Green, C plus and A minus-Green with yellow tracer. C minus, grid bias-Black with green tracer, Speaker positive-Brown. Speaker minus, plate ‘side-Black with brown tracer. Speakers, interconnections.-Brown with white tracer, When tracers are used to indicate that the wire forms a part of two different circuits, the solid or principal colour is chosen for the circuit having the higher viltage or the more positive yoltage. Thus a wire in the A+ circuit and the Bcircuit would have yellow as its main colour because the A battery side is of the higher voltage. The tracer would then be of red or maroon to indicate that the wire also enters into the B battery or platé circuit, What books on a.c. set servicing do you recommend? : For a comprehensive treatment of Set servicing see "Rider’s Practical Set System." You should get the American trade publication "Radio," also. the American "Radio News." A handy annual is the "Radio Trouble Finder and _ Service Manual," prepared by "Radio News." The "Radio News" for Decem- _ ber, 1930, was a special serviceman’s ‘number, and contained articles on building one’s own testers, ete. C.A, (Pio Pio): I have reason to be- * lieve that I do not have my eliminator adjusted for correct grid bias voltage. A.: Try moving the pin with the Oterminal along to the right, switching the set off each time. Actually it should be in about the last hole, but move it around until the best results are obtained. Keep it in the first row. unless, of course, the C- connection is taken to either the second or third. terminal. N YSTIFIED (King Country): My American set flattens the B battery rapidly, and oscillates when tuning in to a station? A.: This is due very likely to 2 broken down condenser. This is a small, flat component which should not pass direct eurrent, and_which is situated between one of the B-+- terminals, usually B+ u.f. and earth. If this breaks down your B battery is connected directly to earth, and of course the battery becomes flat and the set squeals, Better get someone

who knows something about the set {o replace it. GYD. (Otahuhu).-1. What is a megger test? A.: A megger is an instrument to measure resistance, consequently when you apply the megger test you merely measure the resistance between any two ‘points. 2. What is a tuned circuit? A.: A coil (inductance) and a condenser (capacity). In more technical terms, a tuned circuit is a combination of inductance and capacity, which reacts at a certain frequency, determined, by the

amount of capacity and the amount of inductance. In other words, a coil and condenser. If a coil or condenser is yariable, the combination is spoken ‘of as a yariable tuned circuit., 3. What are low loss coils? A.: Strietly speaking, coils in which there is a-low distributed capacity; in other words, 2 high efficiency coil. F, CHOKE (Wellington): Will a * 0003 variable condenser make much (difference to my set. which specifies a .00025 condenser, and ean I change over my .00035 reaction to a .0003? A.: You could change over to a .0008 ‘if you removed about ‘10 turns from the

secondary coil. You could use .0008 --for reaction, and if it is fierce take- oft a few of these turns. Will this set get New Zealand stations on headphones ?-Yes. 8, How long would a standard B batkery last if it were used two hours a day? : About nine months. GNORANT (Wellington).-I. have a two-valye set. Can IL use two..00035 condensers instead of a. 00025 . and a .0005? A.: If you want to have an ill-wave seat you will need 2 smaller condenser than .00025 for tuning. Say ygh use a on page BD),

{Questions and Answers | (Continued from page 19.)

40001 condenser in series with your .00035 | variable condenser, and then construct coils such as those described for the Diff. series, or those described for the Sellens Shortwave Set, for controlling reaction of .00085 would be satisfactory for broadeast, although for shortwave the control would not be fine enough, and again you would have to reduce the capacity by a .0002 condenser. 2. I have made a one-valve set, but it oscillates when the station comes in loudest. Why is this? A.: A station always comes in loudest when the set is oscillating, but the signals are very rough and unpleasant tao listen to. Furthermore, you are annoying your neighbours. : 3. Can I alter the capacity of .00035 condensers to .00025 and .0005 respectively? ; A.: You can reduce .00085 to .00025 by connecting a .001 condenser in series with negins can bring it up to .0005 by cona .00015 condenser in parallel, but thi is unwise because you will find that the * condensers will not cover the tuning range correctly . This is because you cannot get down below a minimum capacity of about .0002. HORTWAVE (Marton).-What should be the proper dimensions of the coils for the Diff. Two A: They were given in the article, but if you wish to make a very efficient broadcast coil you should increase the diameter to 2in. . 2. Is there a wave trap described in the 1931 "Guide’?. If there is, I have not located it. A.: A wave trap is a rejector. Perhaps this makes things clearer. (See page 61.) 8. Using a .000385 tuning condenser for the "Night Hawk’ you specify 28 d.s.c. wire for the coil, while in the Diff. series you specify 30 gauge. -Which is really the better? , ‘A.: There is little cr no difference, but the 30-gauge makes a slightly smaller soil, and for broadcast purposes this would e better. For shortwave the wire can be larger. . H. M.S. (Waihi) -When I turn my set * on, it does not operate until I dis-

»onnect the C. The set is very heavy on ‘A" babbery. Some time ago there was trouble with the set, and one of the resistances burned a littl e. A.: Something has burned out within the set and is partially short circuiting your battery. Better get someone who ows something about the set to look. to W AR. (Martinborough) : I construct- * ed a battery version of the superheterodyne short-wave adapter. Grid return was taken to a potentiometer across the "A" battery. he set then oscillated, but I could get no stations. I was using the 1875 k.e. coil. A.: The coil for the 1875 k.c. is to cover the 160 m. band and actually spreads from about 120 m. to 200 m., but there is little to be heard on this band. In the de. version the grid return should be taken to a potentiometer across the "A" battery. ® In the absence of a potentiometer a return should be made to A+. 2. What is the value of C3? A.: .0005 mfds. fixed. 8. Is the choke a short-wave or broadcast? I am using the latter. A.: The choke should really be a shortwave one, but.a good broadcast choke .will usually operate satisfctorily. 4, Should the adapter howl when brought into resonance? A.: It should not howl unless the receiver is oscillating. This trouble can almost invariably be cured by adjustment of the B voltage on oscillator and first detector. Reduce these progressively until the howl ceases. Where a potcntiometer is used for grid return an adjustment of this will often effect a cure. 5. Could you make the windings for L2, 3 and 4 more clear? I have wound the coils for the 60 to 100 metres band. Results have been splendid. I ‘later wound coils for the 7500 k.c. band, but results were poor. : Since the 80 metres coil is operating’so well, copy the connections exactly for coils for the other band. The coupling coil L2 sometimes requires variation when an oscillator valve having an unusually high or: low output is used... So try varying the number of turns on this.