QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 35, 8 March 1935, Page 47

QUESTIONS AND ANSWERS

OYAR" (Auckland): It would be quite in order for you to change the detector portion of your set to the "Tiny Tim" circuit. However, as you are using a .0005 mfd. reaction condenser, when: winding the coils use only approximately three-quarters of the turns specified for the reaction windings. You could take 14.5 mills.-from the 12 mill. tapping on your "B" eliminator with negligible chance of damaging the latter, but if you wish to be on the safe side, use a biag of -16.5 volts on the pentode. To reduce the high note response of the latter valve, connect a .005 or .01 mfd. fixed condenser from the plate to earth. Strictly speak‘ing, you should use a pentode output transformer between the last valve. and your speaker to give correct matching. Iiowever, results should be quite good without this. I.’ S. (Dunedin) : I have a five-valve bat= tery set operated by an eliminator. A. few weeks ago the first audio transformer burnt out and I purchased a new one, not of the same make, and put it in. Since then the tone has been harsh. "I nave tried varying the bias and also applying more voltage to the last stage, but with no improvement. When the speaker is plugged in after the first audio stage the tone is excellent, so I believe that the -last stage is at fault. Is the transformer of the wrong type? ' A.: This is probable, though, as you ‘have stated neither. the make nor type of the new transformer it is hard to say. Also, if it is of indifferent quality it would account for the poor tone. Try reversing the connections to the primary or the secondary, but not to both, and see if any improvement results. If not, then the only solution is to exchange the new transformer for one of a more suitable type. The fact that the tone is good when the speaker is plugged in after the first audio stage jndicates that the trouble lies after that, and also js not in the eliminator. Is your bias battery in good order, and also. what is equally important. have you had the output valve tested? If it has lost its emission it would be responsible for the poor tone.

2. I have a commercial dual-wave set covering the American police band as well as the broadcast band. I would like to tune-in foreign short-wave stations, Could I get the waveband altered? A.: It would not be practicable to attempt this. Your only plan. is to tise an a.c. short-wave converter such as that described in the "N.Z. Radio Times" for July, 1934, 3. Is there,a right and wrong way to connect the power cord to the wall plug? At present the large pin is connected to the red wire. A.: Evidently your power socket is of the "T" type, and if this is the case the red or phase wire should. be connected to the horizontal top pin and the black wire to the lower yerticaj. one. PRs. (Ohaupo): Suitable eoil details . for your set ace as follow: R.f, coil, aerial 24 turns, secondary 115 turns; detector coil, primary 45 turns, secondary 115 turns, reaetion 45 turns Use 3+ gauge d.s.c. for all windings, 2. When I rotate ihe r.f. tuning condenser it makes very little difference to the volume, Could you give me a remedy for this? A.: If the sensitivity of the set is normal, then evidently you have too many turns on the primary of the aerial coil. Try connecting a fixed condenser of about -0001 mfd. capacity between the lead-in and the aerial terminal. This should sharpen up the ‘tuning appreciably. However, you cannot hope to expect the tuning of the v.f, staze in a regenerative set to be as sharp as the detector tuning. The xf, control is always much broader. "PDuUsty BILL" (Hamilton): What size power transformer could I make with the stampings I have on hand? Also, what are the turns, details, and — sizes of wire to use? A.: You could built a 150-watt transformer from the stampings you have on hand, The core would be 1ftin, x 12in. Full details as to the number of turns, size of spool, etc., wili be found on page 133 of the 1934 "Radio Guide." od FI. (Ashburton): Could I use some * of the parts of iny five-valve battery set to make a four or five-valve shortwave receiver? Would I need to buy any other parts? . A.: You have not stated the model of your set, but no doub: you could use quite a number of the resistors and fixed condensers, Also, the tuning condensers could be used by taking out sufficient of the moving plates to reduce the capacity to approximately .00012 mfd. per conden- . ser. The "Explorer Shortwave Four," described in the December, 1933, "Radio Times," would be a good set for you to build. ° G E, (Dunedin): I wish to receive the * main European’ and American short-wave stations on my set at medium speaker strength. I do not wish to go to too much expense, so if possible would like to use a single-valve superhet conyerter; Is this possible, and, if so, where can I obtain a circuit? A. A yery simple circuit of a converter of this type which has given excellent results with a veceiver of the same make as yours will be described: in the, 1935

"Radio Constructor’s Guide," to be ‘published at the end of this month: B W.B. (Dunedin) : I live in a suburb * of Dunedin and have been informed by a dealer that my set-a six-valve t.r.f. -will be of little use when 4YA is rebuilt and the power is increased to 10 k.w. To what extent will the selectivity of my set be affected? A.: Your set, though it ig a t.r.f. type, bas a band-pass filter in the aerial circuit, and so the tuning should not be unduly .broad. Also, as it is understood that the new station is to be erected several miles from the city, it is quite possible you will not notice any increase in spreading at all. At the worst, an efficient wave-trap would confine the station. to within a few degrees of the dial, and would enable you to get outside stations. A I'.G. (New Plymouth) : The "Comet * Superhet Five," described in the November issue of the "New Zealand Radio Times," is just the type of set you are looking for; it is of up-to-the-minute design, is simple to build, and is a great distance-getter. There is a band-pass filter in the aerial circuit, followed by: a 2A7 mixer oscillator; then a 58 i.f. amplifier; 2B7 diode detector, delayed A.V.C. control and pentode audio amplifier; 2A5 output pentode; and an 80 rectifier. Full constructional details, together with a free blue print of the circuit and under chassis wiring, are contained in the jssue of the magazine mentioned.

Information Coupon (o he use(i with a I requests for information.) ame O f set Model Name 4 ddress Nom de plume To be kept i0 subsequent inquirieg ['lease Note : (1) Be specitic aln d briet. tabu: lating if possible. (2) Write legibly a n d on one side of the paper_ (8) We do not design circuits. (4) Limil three questions ules8 1/ is enclosed. (5) Posta) queries limil 3 ques- tions: Enclose stamped and addressed envelope and 1/- fee: