QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 27, 11 January 1935, Page 41

QUESTIONS AND ANSWERS

McC. (Wellington) : With regard , to your. query concerning the "Elfin Four" and. the "Modern Midget," there is very. little tv choose between these sets as regards distance-gettirg ability. Though overseas stations can ke picked up, provided a good aerial and-earth system is used, both sets are intended principally for local reception. However, you would be able to increase the distance-getting ability of your "Modérn Midget" by incorporating reaction as shown, for example, i2 ‘he circuit diagram cof the "Economy DX Three," described in the last September "Radio Times." You could cuite easily add a reaction winding to your present detector coil by winding 30 to 35 turns of 30 gauge d.s.c. wire «yer the bottem’ end of the secondary, separating the windings with a strip of brown paper or Empire cloth. A .9008 mfd. reaction condenser would be i suitable one to use. MM S. (Marton): It is apparent that * the noise suppression control in your set is not operaiing.as it should. Fave you tried shorting out the plate resistor of the 55? This should render the N.S.C. inoperative and the © set should then act as an ordinary "straight" receiver. Before the control. will act, however, it is essential that the three-volt difference between the 59 and 42-volt tappings be obtained accurately, and you have evidently not been successful in obtaining this, Addmittedly, it is rather difficult to obtain ~vith a voltage divider of the Radiokes type, but it can be dene by pinching 2 depression on the inrer sides of each of the two taps in question. Alternatively, you could try connecting a potentiometer across the 39-volt tap and the one higher up. taking the moving ‘arm to the cathode of the 57 N.S.O. valve. In this way you should be able to get a very fine adjustment of volt"age.

R. (Nelson): Why does a series -* aerial. condenser improve selectivity ? , A: An ordinary broadcast aérial can be regarded as being eqrivalent to a coiidenser in series with a resistance connected across the first tuned circuit of the receiver. The effecr of the aeriul resistance must be as low as possible jor good selectivity, and this effect wul Gepend upon the energy dissipated in the resistance-that js, on the current flowing through the resistance as well as on the actual value of the resistance. The aerial capacity in series with the aerial resistance is in parallel with the tuning condenser. If the aerial capacity is small compared With the yalue of the tuning capacity very little current will flow through it, most cf the current flowing through the tuning condenser. Consequently there is little energy wasted in the resistance. If the. aerial capacity is fairly large, the current through it, and therefore through the resistance, can. be reduced dy connecting’ a small condenser in series with it, thereby improving selectivity. It should be noted, however, that this series condenser may ‘have some effect on the signal strength, as it will affect the -voltege applied.to the tuned circuit. J H.A, (Palmerston): Will it be pos- * sible to pick up the new 1YA in daylight at Palmerston, Otago? ° : A.: The new 1YA came on the air unofficially on Monday, December 17, though the date of the official opening has not ‘yet been announced. Hence you should be able to judge by now just what results the new station will give you during the day in your locality. KK. ¥- (Paeroa) : I have a five-valve a.c. set which takes fully two minutes to heat up. Why is this? ‘A.: It may be due to the make of valves you are using-some take longer than others-but the most probable cause is that your valves are now getting old and need replacing. H.C, (Thames): I have built the * 4984 Crystal Amplifier," described in the 1984 "Radio Guide,’ and it is giving splendid results. With a crystal set of my own design, LYA comes in at ‘full volume. Could I-use a pick-up with the amplifier, and if so, how? A.: Merely by connecting the pick-up leads to the input terminals of the amplifier. You can use either the volume control incorporated in your pick-up, or the one in the amplifier, whichever is most convenient. Loud needles would probably give the best results. > "(COMET" (New Piymouth): I have built up the "Comet Superhet Five" described in the October "Radio Times," and am delighted with its performance, which, if anything; is under-rated in the descriptive article. Could I use a shortwave converter’ with this set, and if so, what type would be the most suitable? A,: Yes, quite easily. The Lekmek short-wave converter, described in the July "Radio Times," would give excellent results, Also, seyeral converters of

simple and up-to-date. design will be -described in the 1935 "Radio Guide;" which . will be on sale in several months. Any of these would be quite suitable to use with your set. . Q5N. (Nelson): A . tap in my voltage divider has burnt out, and so I have connected a resistance.from one of: the other taps to supply the screen voltage. As you will notice from the sketch, the r.f. plate voltage has been increased. Will this harm the set, which is. operating quite well? Bo, A.: No, the alterations you have made will not-damage your set in any. way. Note.-The receiver. you mention tunes from 75 to 200 metres, and from 200 metres to 550 metres. . T T. (Gisborne) : The second aerial you * have erected brings in more noise than the first because it is. appreciably longer and higher. Also, it is. possible that it is picking up a little more power interference than the first aerial, which is at right-angles to the power lines. Dvidently your set has not been lined up with: the longer aerial, and this would explain the lack of increased sensitivity, An interesting experiment, and one which would be well worth trying, would be to try tuning both aerials with a tuner such as that described in the 1934. ‘‘Radio Guide," or the -November, 1933, "Radio Times." ; "DRIMUS" (Dannevirke) and others: Copies of the "Radio Times" containing the description of a Daniell cell charger are completely sold out, ‘and hence the description is repeated else where in this week’s technical section.

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