QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 44, 10 May 1935, Page 47

QUESTIONS AND ANSWERS

NS) J.Q. (Carterton): The battery, model * of the "Sky Hawk Shortwave Four," described: in the 1935. "Radio Constructor’s Guide," is just the set you want. It will out-perform any. four-valve shortwaver we have yet heard. S M.A. (Hawera): My set covers. the *. medium and long waves. Could I substitute short-wave coils for the longwave ones? A.: No; to do this the set would have to be completely rebuilt and it would not be worth while. Your best scheme would . be to: use a shortwave converter such as that to be described in the May "Radia Times." — O P.R. (Picton): I have a four-valye * battery kit-set, and the "B" Dattery ‘consumption appears to be very heavy. ©On an average running of three to four hours a day, three heavy-duty "B" batteries last only four‘months, During the last month reception is poor. How long should these batteries last? A.: With a set such as yours,- from nine to twelve.months. You could soon tell which valve.is taking excessive current by connecting a milliammeter in the "B+" leads in turn. The tota] plate drain should not exceed 11 or 12 mills. am check up on the bias battery and make sure that the bias voltages are correct. 2. I notice that there are three wires joined together and connected to the "B+. 185-volt" terminal, This is not as indicated on the chart. Does it make much. difference to the "B’"’ battery consumption? : ‘A.: It depends on the destination of the . wires. For instance, if one of them is intended to provide the screen voltages for the r.f. valves, then this would largely explain. the excessive current drain; 674 volts is ample for the screens. Note: The unit. you mention would operate quite satisfactorily with your set. "REACTION" (Christchurch) : I. wish . to build the "Comet: Superhet Five" described in the 19385 "Radio Guide." 1 have on hand a dynamic speaker matched for a single 2A5, but with a 1000-ohm field. What alterations would be necessary to use it in place of the speaker specified? ~ A.: You could use an extra Smoothing choke, but you would find it difficult to accommodate this on the chassis designed for this set. Your best plan would-be to get. your speaker field rewound. to: the correct resistance, ‘" AMATEUR" (Ashburton): You ap-’ pear to have made a very thorough overhaul of your set, andit is difficult to suggest. where the trouble lies. However, you state that you have tried fresh valves in practically every. socket. If you can borrow them, it-would :-be well worth while substituting new valves for those yo have not yet replaced. Another possibjle cause of the trouble is a defective vaive socket, while still .another is that there is a partial or intermittent break in one of the coil windings, due to electrolysis. Take the set out of the cabinet, .invert it, and then when signals fade away quickly test the plate voltages’ of the mixer-oscillator, and J.I', amplifier,’ While

without.an actual, examination of the set it is. very difficult to suggest exactly where the trouble lies, the strongest. possibility is that it is in ‘the. coil kit or speaker, ar me E R.. (Auckland): The 160-metre and *’ broadcast band coils of the "Viking Shortwaver" should’ be close wound with 32 gauge enamel or 84 d.s.c.. You would find that with wire of an appreciably heavier gauge, you would not be able te accommodate the winding on the former. L.D, (Mayfield): 1 am building a * short-wave set, and intend using a 00025 mfd. condenser for tuning and a .00035 mfd.-for reaction. Aré.these suitable for’ the circuit I have enclosed? A.: Both- condensers are a little on the large size, and you. will have to tune yery carefully and usé a good ‘slow-motion ver: nier. dial for best results. Suitable coi! details are. as follow:- | :

Use tie 4 gauge €.¢c.c. you..haye on hand for the primaries and secondaries, and 82 gauge for the reaction windings All secondaries can be space-wound.* W J.A.C. (Wellington): £ havea "B" * eliminator on. band which is practically new, so I feel reluctant to dispose of jt. . Could it-be used for the plate cupply of a set like the "Haglet Two" or the short-wave converter described iu the. 1935 "‘Radio Guide," if a separate transformer were usc" for the filaments? The .maximum voltage of the eliminator i: about 180 volts. Poo _ Aw: It would. do admirably for the purpose ;-you .will find that 180-volts. is quite: sufficient. __

2. How is. reaction controlled .in the short-wave converter referred. to: above? A.: There is no control-for ‘the regeneration, as the valve shouldbe constantly oscillating. It. serves. the . purpose of the mixer-oscillator in the ordinary ‘modern broadcast. set? "CURIOUS" (Picton): Try decoupling the supply voltage .to the plate circuit of the detector in, your short-wave set. Todo this connect a resistor of from 10,000 to. 20,000 ohms between the B-+- lead and the tapping on the eliminator, and then connect a fixed condenser of from .5 mfd.-upward from the set side of the resistor to earth. As .your tuning condensers are of very small capacity, you will need at least two pairs ‘of coils to give complete coverage ofthe broadcast, band. Details of the first pair ‘of coils are:.’. Primary 25. turns, ‘secondary 170 turns;. primary 50 turns, secondary 170 turus,, reaction -45 turns, -'The*second pair of coils are wound as follows: Primary 30 turns, secondary ‘220 turns; primary. 60 turns, secondary. 220. turns, re-action-50 turns, -You have-not stated the capaeity, of the reaction condenser you are using, and so’to obtain smooth reaction it may be. necessary to do a little-experi-menting by putting on or taking off a°few reaction turns;. a : ; "(CCABELLA" (Marton) ; You. could use your phones as av improvised microphone by removing the grid ledd fromthe 57, which is evidently the second detector in your set, and connecting the phones either between. the cap of the valve and the grid lead or the cap and:earth, . "PuSBE" (Waikino) : Evidently one of ' the valves following the LI. amplifier in your set is‘a little "down," and should: be replaeed: ‘ se

Band. Pri. Sec. . React. 20 metre ..cssses 2 3 40 metre ..eoeees 4 10° 4 80 metre .:....:.- 5 18 6