QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 13, 9 October 1931, Page 29

QUESTIONS AND ANSWERS

FRANK

KEE

The Technical Editor will, through these columns, be pleased to help readers experiencing trouble with their sets. Queries are limited to three-for more than this a shilling fee is charged, and 2 similar fee is payable for queries answered by post. Supplying layouts, circuits and solutions of intricate theoretical problems is beyond the scope of this service. A coupon must accompany all requests for information. Nomnappearance of the coupon in any issue cannot be regarded as & reason for its mot being used. Address all queries, The Technical Editor, Box 1032, Wellington.

x ‘ battery will do, but a 45 CHRTAIN (Christchurch).-I cannot get the "Sparrow Hawk" One to oscilover the whole broadcast band. A.: Have you tried reversing the connections to the reaction condenser? Often the connections to the coil are back to front, and reversing them at the condenser will generally have the desired effect. If this does not have any effect, you must add more turns on the reaction coils, or try the effect of a midget condenser in series with the aerial. 2. I am using 201A as detector. Would this be suitable? A.: This would be quite satisfactory, although 615 type is possibly the better oscillator. 3. I am using an old-fashioned .0005 condenser with four plates removed. Is this now the correct capacity ?-Yes. KANtt (S. Westland).-My set has stopped. The "A" and "B" batteries are quite in order. All I can get is a howl. A.: This sounds like a transformer. Use the phones and cell test and see if the primaries of both the transformers are in order. To carry out this test, take a pair of phones and connect one tag to a terminal of a 45 volts Any other s usually the handiest. Attach a piece of wire to the other terminal of the battery, then touch this wire and the free terminal of the phones together and you will hear a click. ow disconnect the primary wires from your transformer and put the two wires one on each terminal. You should get a fairly strong click in the phones. The absence of this click indicates that the primary is burned out. This method of testing can be used for other components. 2, Can I do anything to improve daylight reception? A.: It is doubtful. Daylight reception is very uncertain anywhere. 8. My set is really an all-wave one, but I cannot get short-wave stations. A.: Your set should be able to get them quite satisfactorily, as it was designed to o 80. However, if you could never get them, the chances are that there has been something wrong with the set all along. Wili the set oscillate on the short-wave? If so. then, with a little care, you should be able to pick up short-wave stations. C, (Auckland)-Can you suggest ¢ anything to minimise interference rom an 11,000 volts line and two power ransformers ? I have two thirty-foot asts, but I cannot get past the local station. A.: Your aerial wire should be pointing at right angles to the lines. Instead of the ordinary lead-in take a length of armoured flex, connect this with the top wire and bring it into the set.. Earth the shielding of the wire to an earth separate from the normal one, that is, your water pipe. It would not be a bad plan to use a separate earth to the set, other than the water pipe, as these are not always satisfactory when interference is bad. If you could raise your masts another ten or fifteen feet you would probably get better results, but it would be necessary to shield the lead-in as suggested. 350 (Taumarunui).-Would there be anything gained by changing earth pipes \, to copper sheet ?-No. 2. Would there be a gain by using 2 shielded lead-in wire from the extreme top of the aerial to the terminal of the receiver ? A.: Only if you are troubled with interference from nearby electrical apparatus. It is possible that by using a_ shielded Jead-in the amount of signal pick-up will be lessened, so, unless interference is bad,

don’t do it. You appear to be getting excellent results. PYRO (Lower Hutt) -We think you would gain very little by removing the insulator, but your aerial could be improved by the addition of ten to fifteen feet to the mast. The lead-in is not too long, although it is always advisable to bring it as direct as you can. if it has to straggle round the walls, power will always be absorbed. Do not worry because your results are not quite up to the other fellow’s. It is not always a difference in sets, but in most cases locality is the deeiding factor. Your set is an excellent one. We have had very good results with

D*uc (Picton).-Do not use PM26 in | the second-last stage. It is purely a last-stage valve. Q08 (U. Wutt).-The condenser is a .00025 SS and 100 turns of 20 enamelled wire on a 3in. former will match this. The coil would not be as efficient as it might be. You should use finer wire. Say, 26 d.s.c. Ms (P.N.).-Is it in order for the lightning arrester to have a separate ground, or should it be placed between the ground terminal of the set and grouné, as it is between the lead-in and the set? A.: It can have either a separate ground or could use the ground of the set. Your present installation appears, from your illustration, to be correct. KF C.E. (Dunedin): As far as we can ° ascertain your set has a tuned first stage, and in this case you must make the following alterations to the h.f, adapter. in the place of "LS" use an ordinary radio frequency choke. "C4" is not required. "C5" as before. 2. As I would have to take my power from the mains, is there a transformer on the market giving the required "B+" and filament voltages? A.: No. But you could very easily have one made up to the specifications given in the "Radio Guide." : X74.'W. (Wellington): We think your arrangement No. i is the better. Stand off insulators will not decrease the efficiency of the antenna in any way. pro- , viding, of course, they are kept clean.

yePF° (Oamaru): Would you supply me with the method of calculating the eorrect transformer ratio to match a pickup to a commercial receiver? A.: We regret there are too many factors involved for you to be able to work out the correct transformer ratio, but, with your receiver, easily the best manner is to connect the P.U. without a coupling transformer. Tf this does not give you enough volume, use 2a $ or 33-1 transformer of a good make.

ae RAvrI0 (N.P.): In the technical diagram of the "Kestrel Three" the filament vheostat ig in the r.f. lead, while in the layout it is in the detector stage. Which was used in, the original? A.: In the vf, stage, where it acts a8 & volume control. 2, You state that the .0001 fixed condenser in series with a .00085 produces 2 tuning capacity of .0001. This is evidently wrong. How should the answer have rea A.: We meant what we said, but gave our answer in round figures, Actually the tuning capacity would be about .000075, but a tuning condenser of this capacity would be extremely difficult to obtain. .0001 ig the nearest practical value. 3. I have a number of .0002 variable condensers. What extra capacities are required in series or parallel to make (a) .00015, (b) .00025, and (c) .00035? A.: .06 in series, .00005 in parallel, and .00015 in wvarallel.

X.82.H.B. (Napier).-Since employing a new valve osciliation cannot be controlled. A.: We suspect the bias resistor in the first stage, or a defective choke. Try a different value of grid leak, and if all these fail try another grid condenser. If you could manage to do so try another valve in the first stage. 2, How many Plates would i take off my condenser to reduce size from .0005 to .00035 ?-Hight,

HRHH-VALVH (Wanganui).-I intend to conyert my present set into the "Kestrel Three," and wish to use .0005, variable condensers instead of .00085. What are the coil specifications? A.: Secondary, 85 turns of 84 gauge d.s.c. wire, tickler 48 turns of 38 d.s.c. You need not worry about the short-wave coils. UDIO (Blenheim).-I am_ greatly troubled with hum in an a.c, set. Blectricians have looked it over and seem to think it is all right. It appears not to be present in the first audio stage. A,: An article was recently published in; the "Radio Record" outlining the steps to; be taken to prevent hum. it might be advantageous to you to look it up. Hum is sometimes caused by one or more microphonic valves. To locate such a valve, operate the set so that the hum comes in very loudly. Then place your hand firmly on each of the valves in turn. When you come to the offending valve the hum will decrease in strength or disappear altogether. You should either replace it with a new valve, or use one of the lead caps made for the purpose. The rectifier is often an offender as far as hum is concerned. Bxtra by-pass condensers from the plate lead of the audio stages may cut out some of the noise, but the elimination of hum in a commercial receiver is really a job for a radio technician. 2. have made a wavetrap, using the circuit described in a back issue of the "R.R.," and have not had success. I used 2 .00035 instead of a .0005 variable condenser and 100 turns of either 16 or 18 wire on a nm former. Are the specifications correct ? A.: Yes, but a more efficient coil could have been made by winding 60 turns of 26 gauge enamelled wire on the coil and tuning it with a .00035 condenser. We cannot explain why your wavetrap will not work. All we can say is that it should. Constant RBADER (Gisborne): Can you give me some information concerning tungar bulbs? I ain anxious to build one into a five amp. charger. A.: Owing to the high filament current taken by the tungar bulbs they have rapidly gone out of date, being replaced by more economical rectifiers. A five amp. charger has a filament which requires 20

amps. of current 2. volts. The secondary would have to be wound to deliver 20 volts, and this would allow for the 14 volts drop through the bulbs. To make a "B" charger, wind on sufficient turns to give 90 to 100 volts with wire sufficiently heavy to take current at a tenth of an amp. It is unlikely, however. that you could get an efficient tungar charger from itin. stalloy. Twenty amps. is a big hurdle, isn’t it? 2: The gauge of wire sent in is 36 dec.

N Z. 10TW (Wellington): Would you supe ply the correct number of turns for -0005 condensers with .0002 differential condensers, to be wound on a valve base? A.: Secondary, use 30 d.s.ec. and wind on 90 turns, or 28 d.s.c. with 100. Tickler, 40 and 50 respectively, with 32 or 34 ds.c. wire. Primaries, 30 and 42 respectively, same wire ag tickler. A B. (Napier).-Write International e Radio, Blair Street, Wellington. Ft M. (Christchurch)-We are working © on a three-valye receiver, incorporating multi-mu valve, 224 and pentode, in a Loftin White hookup. Watch our constructional columns for further announcements. Your step-down transformer would not be likely to suit the pentode valve, However, try it. B 8.D. (Wanganui).-I have constructed ° the Advance a.c. receiver, and although I have had good results on certain bands, have had difficulty with oscillation on others, A.: Try the effect of a Smaller reaction condenser, say, .0002. Check the yoltage on the detector valve, and if necessary reduce this slightly. Reduce the numbers of turns on the coils which do not oscillate as they should. . 2. What do you think of the idea of .0005 condensers for broadcast? What would be the correct numbers of turns for 2in. coils? A.: For the Secondaries use 72 turns of 8 d.s.c, wire, and for the tickler 25 turns of 26-30 gauge d.s.c. wire. .0005 condensers are perfectly satisfactory, Wt: (Bay of Islands).-tIs it possible to attach an r.c.c. amplifier to get more volume for special occasions? I am using a five-valve set. -: No; you would get howls and other forms of audio instability. 2. What is the usual value of audio chokes ? A.; Anything from 15 to 50 henries. X TAL (Auckland).-What valves would you recommend for the "Crystal Set with an A.C. Amplifier," described in "All About the All-Electric," Osram or Philips referred? 7 P A.: Philips E409, in the first stage, output. Osram, M.H.4 and M.L.4. . 2. Could a s.g. valve be added in front eg a erystal detector to get distance?-No.

the text of "Ui 8. Could you supply fig. 5 mentioned in ise of thode Type of Valves" ? A.: We are publishing here a circuit of a crystal set with an a.c. amplifier, using the cathode type of valve. Full constructional details have been given in the 1931 "Radio Guide," LP (Auckland): How can the maximum voltage be applied and the maximum current delivered by a triode used in a half-wave rectifier be computed ? A.: The maximum amperage delivered is’ that shown by the valve curve, or maximum plate voltage without bias. It will not, however, deliver this for very long, and it, will goon drop. The maximum voltage to be applied is that recommended by the manufacturers, It can be exceeded slighy. 2. What is meant by saying "this resistance is 75 watt rating?" A.: That it will dissipate 75 watts of energy. A watt is equal to a volt multiplied by’ an amp, or W equals IH, Bliminating BH by the formula H — IR (R is the resistance) we get W — I2R. We now have two constants that can be supplied. We know the watts dissipation and also its resistance. Let us see how much current our resistance will pass. We will transpose the equation to make it read: I -or I _ R = Let us take an example. We have a 4000 ohms, resistance, 75 watts dissipation. How much current will it ‘pass? lem yr R =z Y -- amps. =: 1/7 amp. approx. — 148 mills. We now know that our resistance will pass a maximum current of 143 mills and that this current will breakdown 572 volts. This, multiplied by the milliamps passing (148) will bring us back to our 75 watts. V HAT ig the best combination of valves for my six-valve American set? A.: Use 221’s in all stages except the last. There you may use any type of power valve; C603 will be quite suitable. You would have to make no other alteraftom to use the 221 valve instead of the wVLO, 2. I am using an output transformer, but there is yet distortion in the set, even with the volume control well down. A.: This may be due to the last valve being inadequately biased, or possibly to your valves being almost done, . Cs. (Auckland).-I am using a five-valve three screen-grid midget set. Can I, without interfering with the set, improve selectivity ? A.: The easiest way to improve your selectivity is to use a midget condenser in series with the aerial. This will knock the volume back a little, but with unselective sets, it cannot be helped. The of the condenser should be about 0001 to .0003. When tuned in to a low-powered Australian "RB" or'New Zealand "B," the set sets up a violent hum when the volume control is turned up? . A.: Your set is not properly or balanced. It may improve matters if you used a better earth. If these things fail we should advise you to consult with the dealer who sold you the set. WITCH (Tolaga Bay): Can two .0005 | condensers be. used for the ‘Kestrel Three’? If so, what should be the number of turns? . A.: You could use a by winding ten turns less on the secondary coil, The tickler will remain the same,

2. Could 2in. wooden ecol!l formere be screwed to the baseboard instead of valve : base and socket ?-Yes. 3. Would the .00026 differential condenser i have to be changed ?-No, * OOK-UP (Matamata): We cannot see the reason for B4. BS, of course, goes to the audio transformer or resist-ance-coupled unit which, by the way, would be the better. Try it without B4. It is rather difficult for us to criticise a sketch when it is divorced from the remaining part of the circuit. 2. In some audio amplifiers I notice that ‘bias is obtained through a resistance coninected between the transformer and earth, ‘whfie in others the bias terminal on the transformer is connected directly to earth. Why is this? A,: Bias is usually obtained through the filament drop and the bias terminal connected to earth. The other system, by inserting a resistance in the return from the bias, achieves the same end, but differently. The former is the more usual practice. 8. Where is the best position to connect a pick-up to a set using a screen-grid detector and rec. in the first audio? A.: Arrange a jack so that the grid lead between the first audio valve and the rece, unit is broken. The valve ¢’ gf of the grid lead and a return to ear@¥ are used for pick-up terminals. We note your request for information regarding the multi-mu valves; we devoted an article to this subject a short time back, but we shall probably have more to say about, them in the near future. (Otahuhu): I have acquired a pair of telephones, resistance 6000 ohms, and am using a "B" and "C" eliminator with B406 in the last stage. Would I injure the phones if I used them on my set without any alteration? A.: With very sensitive phones such as you are using, it would be advisable to use an output filter in the last stage. Such a filter ‘is illustrated in a sketch accompanying your query. 2, Would the eliminator be suitable for

use with the "Sparrow Hawk" one on shortwave? A.: Yes; you might have to use an extra choking condenser in the detector plate lead, but it may not be necessary. 3. Using a .0005 condenser could I use 28 d.s.c. wire for Li and L3 for the broadcast coil? What will be the number of turns? A.: Yes; wind on the number of turns specified. GPARKS (Christchurch): Where could I obtain an inexpensive one valve shertwave circuit and the approximate cost \of building it up? ry A.: The "Sparrow Hawk’ one, descrid®d in the "Radio Guide," is an excellent one. Use a .0001 condenser instead of the .00035 and shorting condenser. The total cost would be between £3 and £4. 2. What is the procedure for making honeycomb coils? A.: To give a description of this is quite. beyond, the scope of Q. and A. In any case, honeycomb coils are well out of date. 3. Have all detector valves ave prongs. A.: Avec. detectors.-Yes. J AUSPANKA (Hawke’s Bay).-We do not think your set would be a worthwhile improvement on the Browning Drake. If you want to build a small super-hetero-dyne receiver, watch for an excellent one we shall be describing within the course of a few weeks. ‘ FY"? (Wanganui).-I have a commercial battery set which I want to convert to acc. What Philips’ valves do you recommend? . A.: None. We do not advise you to convert a five-valve battery set to an a.c. one, Conversions are rarely satisfactory,