QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 18, 9 November 1934, Page 43

QUESTIONS AND ANSWERS

L F.B. (Aucktand): How could I add * a hum-bucking coil to my home-built dynamic speaker, constructed from details published in the "Radio Record" several years ago? A.: If you have sufficient room, a humbucking winding could be put on quite easily around the voice coil. It should be in series: with the latter and have exactly the same number of turns, though it should be wound in the opposite direction. (WWALLSEE" (Wellington): A friend of mine has an a.c, set-the "Link Three’-described in. the "Radio Times" about two years ago. He now wishes 10 build the "Skyline Five." Would it ve possible to use the by-pass condensers of the former set when making the latter? A.: You could use the condensers you list as follows: Three .01mfd., C2, C6, C8: two, .2mfd., 08, C7,: one .0d5mfd., C9: one .05mfd., C4 After the set has been: built you could use the 3mfd. condenser to replace either C2 or. C8, if a temporary substitution shows that results are improved. "QB" (Auckland): I have a_ fivevalve set which when first bought gave excellent bass reproduction. Now, however, the bass seems to be considerably weaker. The valves have all been tested and are in good order. A.: Is the tone control still operating? If not, then there is a possibility that the tone control condenser is "open." .In this case the treble response would be accentuated, thus making the bass sound weaker in comparison. Another possibility is that the fixed condenser across the 2A5 bias ‘resistor is defective. Note: Barly copies of the "Radio Record" are still procurable, from Box 1680, Wellington. D T, (Christchurch): If your receiver ° is giving you entire satisfaction, and is not unstable in any way, then its performance would not be improved by rebuilding it on a metal chassis, or by using a gang in place of the three separate condensers at present in the set. Single dial operation is, of course, much more convenient, though it is no more efficient. [Many thanks for your suggestion, but unfortunately the Post and Telegraph Regulations require that every page bears the day and date of publication. Glad to / know you enjoy "Questions and Answers." _ -Tech, Ed.] "r THANK YOU" (-): I have built up the "1934 Crystal Amplifier’ described in the "Radio Guide," and am very pleased with its performance, there being nothing wanting in regard to tone or volume, However, there is a rather too prominent a.c. hum in the speaker. How can I remedy this? When I first turned the set on, the hum level was low for a while. Then a click was heard, and immediately the hum became much more pronounced, while no signals came from -the amplifier. This has occurred on several occasions, but each time the fault has righted itself without any intervention on my part. A.: This sounds as though one of the electrolytics has developed a leak, but if this is the case, then you could expect a

spluttering from the speaker, the intensity depending on the degree of breakdown. Also, the click which occurs occasionally, followed by no signal, seems to indicate an open grid circuit somewhere, or a defective bypass condenser across either the 56 or 2A5 self-bias resistors, Did you make sure to connect the dry electrolytic condenser in the correct way? The end painted red should be connected to the cathode of the 2A5. Try earthing the . grid of the 245 to the chassis. If the

hum does not cease then this shows that it is arising either from the power supply or from the speaker. Incidentally, it might be worth while to try the effect of reversing the connections to the primary of the speaker input transformer. 2.: Should the earth lead -be connected to the chassis of the amplifier as well as to the earth terminal of the crystal set? _ A.: Yes, the chassis .of the amplifier should be earthed. You will probably find (Continued on page 45). .

Questions and Answers (Continued from page 48.)

this will make an appretiable difference to. the hum level. C II, (West Coast): If the dynamic * speaker you have on hand is of the permanent magnet type, then it would-give good resuljs with your set. "TNDUCTOR" (Owhango) : I would like to add a "B" class amplifier to my battery set. Could I: use my present speaker, and if so, what type of output transformer would I require? , A.: A Radiokes 3-8 class "B" pushpuil output choke would be most suitable. Each end of the winding should be taken to a

plate of the 19, and also through a .5 mfd. fixed condenser to one of the speaker terminals. The centre-tap connects to "B--" Ls. (Papakura): Your short-wave * converter is apparently wired up cozrectly. The trouble seems to be that it is . not oscillating. Try connecting a fixed condenser of .0001 mfd. to .0003 mfd. from the junction of the nf. choke and the 50,000 ohm, voltage dvopping resistor to the sereen, to earth, If oscillation is too fierce with the largest condenser specified, the value should be reduced. Also, have you tried reversing the connections to the reaction winding ?-Lhis is often the cattse of non-oscillation. The dial you are using should be quite suitable. R D.C. (+): It would not be worth e . : ‘ your set allwave model. It would be better to use it in conjunction with a short-wave converter, D.c operated short-wave cuuverters would no doubt be available from Auckland dealers. If you cannot obtain one, however, there is no reason why you should not use a battery-operated converter stich as that described in the August "Radio Times." LF: (Auckland) : Full constructional * details of an excellent short-wave converter which would work well with | your seven valve a.c. set appeared in the July "Radio Times." The total coat of a kit of parts is also given. "FIGMONT" (Midhirst): What are the capacities of nine and eleven plate tuning condensers? A.: You have not specified the size of the plates, of whether they are doublespaced or not; however, assuming the condensers are of the usual type, the capacities would be approximutely .0002 and .00025 mfd. respectively. 2,.: In my set, would the use of a threegang condenser with an oscillator tracking section be of any advantage? A.: (None whatever, ‘The use of one | would necessitate a special oscillator coil of a type not procurable commercially in New Zealand. Ww G.C. (Ohura): All the valves in your * set. could be replaced with two-volt ones. We advise you to write Philips Lamps, Lid., Wakefield Street, Wellington, for iiformation regatding replacement types. To add class "B" to your set, it would be necessary to omit the pentode output stage at present in your receiver. The A415 could be substituted with a B217 to act as driver for a B240 output valve. The improvement in efficiency which would result in changing over to the latest two-volt valves, combined with the lower operating costs, would make the changeover well worth while. . ASAXx" (Auckland): What would be the simpiest method of adding an amplifier to an existing two-valve battery set? A.: You have not specified the types of yalyes you are using, but assuming they are both general jjurpose triodes, you could add a class "B" stage, as described in the January, 1934, "Radio Times," using your present output valve as the driver. 2.: What would be the simplest method of connecting a gramophone pick-up to my set? A.: Your present set is not powerful enough for pick-up work, and volume vould be very low. If you decide to add the class "B" stage, then the pick-up can be connected from the grid of either the detector or the driver valve to earth.