QUESTIONS AND ANSWER

Radio Record, Volume V, Issue 25, 31 December 1931, Page 14

QUESTIONS AND ANSWER

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 a 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. Nonappearance of the coupon in any issue cannot be regarded as a reason for its not being used. Address all queries, The Technical Editor, Box 1052, Wellingion.

cC.'¢: McC, (Otira) --H..4 is designed for a special circuit utilising the jeentre tap of the rectifier. The maximum Woltage it will rectify is 180 volts 30 fnamps. One end of the secondary of the former -is connected with the centre , and the other end is connected ro tthe mid point of a two 4 mfd. condensers mnected in series. The other end of ese condensers are connected to the two ining terminals on the rectifier, and then become "B-+-" and "B-," and go to the smoothing apparatus,

O'CLOCK (Petone).-When I discon- . nect the earth why is it the volume inereases considerably ? A.: Because you ure operating the set nearer the oscillation point, and, consequently get greater strength This hap pens in some cases, but not in all. 2, How can I neutralise my set? A.: There are four neutralising condeusers, and these must be adjusted, starting with the one nearest the detector until the set is perfectly stable. We do not

advise you. however, to carry out this job yourself, To do it thoroughly requires an oscillator and extensive — technic knowledge. While neutralisation is an easy thing to do in a battery set, yet, with the modern ac. set it is not so eusy. and you might throw your set right out of balance. Lowever, if you can find those neutralising condensers you can adjust them until you stabilise your set. es ZL. (Inglewood).-Would it be satis- * factory to hook up my present audio amplitier, consisting of one stage of 201A followed by two 245's in pushpull to the detector of the Super Six? A.: It. would be quite satisfactory to do so. You should get good results, You should also be able to use a fair number of parts from the B.1). in the Super Six, It would not be possible to "convert" in the normal sense the BD. to the Super Six, but you could pull it to pieces and use the. parts. RED BIRD (Wellington) : You can use ® the valves mentioned quite well in your set. They are identical with the ones ut present in use. We can see no reason why the 280 burned out your line yoltage regulator, Are you quite certain you put it in the right way, and did not force it in with the filament where the plate and grid should be? 1t seems that some accident happened rather than that it was 9 characteristic of the yalve which caused ihe trouble. Asa matter of fact. we have interchanged the valves in our experiments many times without detrimental effect. JL NOCKER (Westport): What are the specifications for a short-wave frame antenna for the Super Six? A.: We shall publish them next week if possible. ® Phe yalves you mention are quile suitable for the circuit. For the last stage you can use almost any power valve you wish to. It all depends on how much voltage and current you have ayailable. The merit of the Super Six lies in the preaudio stages, and you can please yourself about the power valve. 3. Would two balancing condensers :n parallel with tuning condensers be a help in short-wave reception? A.: Probably. J YT, (Wellington) : We cannot promise @e definitely when the a.c. version of the "Night TIawk Two" will appear. 2) What is the value of the resistance shown in the theoretical diagram of the a.¢, version of the "Night Hawk." between the grid of the detector valve and the cathode of the same valve? Av: It is a grid-leak of approximately 2 megs, 3. Would two power chokes of 20 and 40 henries approx. be suitable for smoothing the h.t. output from a 50 watts transformer? A.: Yos, they should b2 perfectly satisfactory. R AS. (New Brighton) : What type ant * make of yalye would 1 use for the

"Sparrow ITlawk One," specially for shorty . a wave work ? ce A,: A415 type of valve is easily the best for this particular socket. 2. J have a condenser with four fixed plates and three moving. What is its cupacity 4 A.: It all depends upon the size of the plates. Jf they are standard size. (approximately the size of the ordinary .0005 and .00085) the capacity will be approximately .QUO1, 3. Could Lo use a dey "A" battery instead of a wet one? Av: for one valve a dry "A" battery would be perfectly satisfactory. S. G. (Auckland): We have not the h tafntest iden what your valves are. Your best plan would be to take them into a dealer and ask him to determine the characteristics of them. Ile coual probably then tell you their equivalents. 2, It appears that the plug in coils ave used for long-wave stations which are not heard in this part of the world. We cannot tell you very muen regarding your third question, as the information giyen is far too vague. We need to see the circuit to tell you anything about it. DPIAMOND (Lyttelton): I have built the "Outspan Five," and am pleased with its performance, but I get a rushing sound as T increase the volume on distant stations, Av: This is probably valve hiss due to the peculiarity of one particular valve. It would be hard to say which valve it is --probably the sereen grid or the detector, and can be got rid of "nly by changing the valve. 2, I am using 4DX as detector, 415 first audio, and 403 as a last yaiye. Is that combination correct ? : Perfectly. z What grid bias should T put on B+403? A.: That depends wnon the total valtage available. If it: 100-135 volts, you will need from to 15 yolts bias. UPER (Stratford) : Will it be satisfach tory if I add a stage of pushpull to iny Super Six?--Yes. 2. Would two .00035 variable condensers do in place of .0005? A.: No; you must use a .0005 to match the special coils recommended, If you use a .00085 you will not be able to cover the band. However, it will be worth while trying the .00035, and if they are not satisfactory, change over to .O00D. 3. Using the "B’ eliminator, would it be better to leave out the svaghetti resistances in the set. bringing the Jeads to difference yollage taps on the eliminator? ‘A.: It will be possible to do so, although the best plan would be to construct the set exactly as described, Particuarly with super-hets, it is not safe to make alterations and additions, — XOOA (Remuera).-TI have a 10-valve super het. Should not the multimu

\ \ valves in this.set counteract the intense fading of 2YA? — > A.: No. Multimu valves are not designed to do that. You are thinking of automatic volume control. If your set were fitted with this, then it would, to a cel tain extent, counteract the fading. 2. During an item the tone will often change suddenly: from base to treble with= out my touching any controls on the set. Why ig this? ~ A.: There is something wrong with the audio side of your set. Quite possibly there is a fault in the tone control system, but it would not be possible to direct you to do it. . 3. What would ‘cause a sharp click in the speaker,: particularly during an item or static? A.: It is quite possible that the noise to which you refer is being picked up on the aerial, and it is due to outside intezference over -which you -have no control. aad « enact "_y

Such troubles as two and three are very difficult to locate by correspondence; you would need to make a systematic search of your set. R. ELR. (Lower Hutt).-See our reply to a previous correspondent concerning the loop antenna for the Super Six 2. Can I tap the present broadcast loop for shortwave work? A.: No; it would not be satisfactory. . N. D.B. (ol aga Bay).-Will the coils for the s.c. "Cathode" super be the same as the Super Six, and will they be capable of being home constructed ? A.: They are quite capable of being They are entirely different from the Super Six coils. Will the "Cathode" super be adaptable for battery use? A.: It can be changed over the battery quite simply, OSCILLATOR (Thames): Could you supply a reliable book deseribing fully the super heterodyne circuit? : We advise you to write to Te Aro Depot, Wellington, who have quite a large supply of books that could help you, i in "The Principles of Wireless Telephony and Telegraphy" gives a very good chapter on the super het., but it is an.expensive book, ‘and contains many subjects other than’ super hets. Kt SET 3. (Dunedin): Could you describe 600 metres coils for my commercially-made set? «Ac: We cannot undertake. to design eoils. such as this, but if you add half as winy turns again to the existing number's,. you will probably be nest the mark, and this will give you a jweying off point from which you could mi" 2 any adjustments you wish. Your civcait is not really designed for all wave work, so we are not surprised that you cannot get good results on the S0-metre coil. A balancing condenser in the aerial may possibly help you. H FLM. (Southland): I have construct- * ed the Differential One with a fixed condenser between the plate and reaction coil, but when I put the aerial.on and turn on the set. there is a loud hum and frying noise. How should I remedy this? A.: Follow the instructions which appeared with the set, and do not make unnecessary alterations such as you hare done. NTEW CHUM (Christchurch): Your 4% aerial will be better with the lead in running straight down to the set. Generally speaking the total length of your aerial should be 100 feet, including the lead in, The poles are not too high, but it would be better if you could get the shorter one still higher,

R H.B. (Auckland): Can the audio * yalye obtain current from the same winding as the detector and radio. frequency valves? A.: Yes; providing, of course, you are not overloading the transformer. 2. What condensers should shunt ‘the bias resistance? A.: From .25 to 1. mfd. Your valves are correct. ;