QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 42, 26 April 1935, Page 46

QUESTIONS AND ANSWERS

C K.M. (Brighton): I built up the * "®mpire Shortwave Four," described in last month’s "Radio Times," and found it very satisfactory on the short waves. However, I cannot pick up any stations on the broadcast band. There is ample reaction. A,: The chances are that there is a break or a faulty connection in one of the windings on the coils. You could try a continuity test of the windings by connecting a pair of headphones in series with a 43 volt bias battery, and touching the two pairs of pins in turn on. each coil base with the two free leads from the improvised tester. A loud click in the phones indicates that there is continuity. "PfhD-UP" (Palmerston North): My set has developed a squeal when tuned in to any station between 250 and 300 metres. Reception from stations over any other part of the dial is all that can be desired. A.: You have not mentioned whethe: the squeal changes in pitch as you pass over each station. If it does rot, then the trouble lies not with your set, but with the stations on that part of the band causing heterodyne interference.

There is nothing you can do to cure this interference, which, anyway, should be considerably lessened’ after the frequency reallocation of the Australian stations takes place in about six months’ time, "CURIOUS" (Timaru): On all the YA stations my set develops a loud buzzing when the volume is turned full on. This noise is not present when I tune in weak stations. A.: BPvidently you are overloading your speaker, thus causing the buzzing of which you complain. If the volume is not more than the speaker should be capable of handling, however, it probably needs recentring. "Fea" (Christchurch) : Evidently the main reason why your special interference-reducing aerial is not working correctly is that you have not got the flat top high enough above the interference field. In this respect there is some useful information contained in the article entitled "Curing Man-made Static," appearing in the November "Radio Times" of last year Also, there is n comprehensive article on aerials of this type in this year’s "Radio Guide" which is now on sale. "BEGINNER" (Waimate South): Here are the coil details you require: 20 metre band, secondary 4 turns, reaction 3 turns; 40 metre band, secondary 10 turns, reaction 5 turns; 80 metre band, secondary 18 turns, re action 7 turns. However, you will find that using such a large value of tuning condenser for shortwave reception that tuning will be very. sharp. You will need to use a high quality slowmotion vernier dial. C FLW. (Invercargill): As your set * takes approximately 14 miils. "B" current, the value of the bias resistor for the output stage should be upproximately 850 or 900 ohms. To obtainthis, join another 400 or 450 ohm carbon resistor in series with the 450 ohm bias resistor at present in your set. H.W. (Papanui): ‘The following are ~* the coil details you require.

G T. (Murchison): Sorry, but through * a misunderstanding we thought you were operating the seven-valve model of the same make. The, circuit diagram of your set does not show the value of the potentiometer across the bias battery. If the potentiometer itself is taking 6 mils. it has a value of approximately 2500 ohms, which is far too low. In this case it should be replaced with avother of from 50,000 to 100,000 ohms. bringing the "C" battery drain down to from .6 to .8 mils. However, fore making this replacement, first make sure that there is no short-circuit or partial short-circuit to. earth in the circuit between the moving arm of the potentiometer and the grid of the controlled

valve. An easy wuy to check up on this would be to connect a milliamm: ter in the circuit to measure the drain flowing trom the "C’ battery when the set is on, and then to d'seonnect the lead from the moving arm of the potentiometer. If} the drain falls uway, it is a. sure sign that the fault lies not with the potentiometer, but in the circuit between the moving arm of the latter and the grid of the rf. valve. preesw (Wellington): I have recently completed the 1983 "Hight Star -Five," and find that though it brings in the stations well, the volume control is very ineffective. Eyen with a short indoor aerial it is impossible .o reduey the yolume on the locals’ sufliciently for comfortable listening. A.: The two most likely reasons why the volume control is not operating. satisfactorily are, firstly, that the side of the .volume control potentiometer which should be earthed is really "up -in the air," and, secondly, that the first tap ou the voltage divider, i.e., that. to which the other side of the potentiometer -is connected, is set. too far down towards (Continued on page 48.)

Questions and Answers (Continued from page 46).

the earthed end of the divider, and f thus does not give sufficient control. Try moving this tap up towards the positive end until it is about 40 volts positive to chassis. ‘This should give you adequate control on the locals, even with an outdoor aerial. C.A: (Napier): The "Sky Hawk * Shortwave Four," described in the 1935 "‘Radio Guide," is an up-to-the-minute four-valve shortwave battery set which you will find gives excellent results. OULD-BE — CONSTRUCTOR" (Waimate): I have built up the "Tiny Tim" described in the 1934 -"Radio Guide," and think it is a wonderful little receiver on the broadcast band. \ I am a little disappointed with its per-. formance on shortwave, however, as so far I have only been able to pick up Morse signals. Would it be advisable to add an audio stage, using ‘another type 30 valve? A.: First of all, to get good results with such a small set you must have an efficient aerial and earthing system. The fact that you can pick . up Morse signals shows that the set is ‘oscillating. Secondly, do you know when and where .to listen for overseas stations? You could obtain some valuable help in this. direction by watching the shortwave page of the "Radio Record" each week. If you wish to add another audio stage you could do so as shown in the "Midget Two," described in the "Radio Times" dated January, 1934, 4

band. * Metres. Primary. Secondary. Tap. 12 to 19... 2 33 3 21 to 89 ... 3 63 i 41 to 79... 4 134 4 81 to 100... 5 263 1}