QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 20, 27 November 1931, Page 14

QUESTIONS AND ANSWERS

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 seope of this service. A coupon must accompany all requests for information. Non-: appearance of the coupon in any issue cannot be regarded as a reason for its not being used. Address all queries, The Technical Editor, Box 1032, Wellington.

()MSK (Bay of Plenty): By placing my finger on the aerial terminal and moving my hand on the first power valve reception is increased 50 per cent. Is this injurious to the set? ‘ A.: Not at all; you are introducing a little regeneration, and this will do no harm. HWxamine your aerial equipment,

making quite certain that it is not at fault. 2. If I disconnect the earth and place it on the aerial terminal the volume is increased but a clicking noise is heard. A.: Usually by removing the earth the set is brought a little nearer oscillation

and consequently becomes more sensitive. Signs of instability, however, usually manifest themselves, and take the form of the clicking of which you speak. 8. My set, with the. aerial and earth connected, crackles above 1000 k.c. Is this outside interference? A.: We think so, particularsy as the noise is not present when the aerial and the earth is disconnected. ‘ D. B.C. (Whangarei): The instructions accompanying my set state that I should use four "B" batteries and two 224 volts "CG" batteries. I am using three 45volt "B" batteries and 27 volts "CG." Why are my batteries not as recommended? A.: It is probably a matter of cost. The increase in efficiency of the extra batteries would probably not warrant their cost, although some improvement would be noticed. EyXvVox (Timaru): You do not tell us what type of coil (r.f. or detector). and what valve you intend to use. 1 might be reduced to 1 megohm with advantage. © ig possibly a little too small for good reproduction, and should be increased to .01. J. F. (Kohi): The specifications call for a 32 mmfd. condenser. I have taken two stator plates off the 18 plates 50 mmfd. condenser. The rotors are immovable. Would this be satisfactory? .: Yes; in any case, without taking oft any plates, your own condenser wouid 2, What is the difference -between 200 and 200A valves and 171 and 171A valves? The former in each case are m9) amp filament current: valves, cand in the second $+ amp. G G. (Auckland): Could I use fixed * .gravity. type Daniells cells to keep a six-volt accumulator charged, or would I have to use six cells or eight suitably arranged ? A.: Your best plan would be to use eight connected in series with the ends attached to the battery. You need a higher voltage in the charger than that delivered by ‘the battery in order to overcome back e.m.f, OPEFUL (Wellington): I cannot overcome hand-capacity on the higher frequencies of my s.w. receiver. I have screened the detector portion of the set and have the "C" batteries within the set itself, A.: Have You done the following :- Connected all moving plates ‘to earth, tried another detector. valve, experimented with various voltages, tried another grid leak and condenser and potentiometer return? Dxtbw (Shannon): We do not adyive you to use the coupling proposed : your super. het. We shall be shortly a fair amount on super-hets, and you will be able to get all the information you require from this. The value ef the de-coupling resistance may. be. any-

oe at thing from 500 to 15,000 ohms, but the higher the de-coupling resistance, the higher must be the voltage applied to the battery tapping to which it connects. 8&8 (Hawke’s Bay): How many turns of 30 dis.c. wire on a 2in. former would I need to match a .00025 tuning condenser and the .0002 differential reaction condenser? A.: Secondary 90 turns, reaction 45 turns. 2. What is the correct value condenser to use in series with a .00025 to reduce its capacity to .0001?-A.: .00015. 8. I have a Crosley audio transformer. The ratio is not marked on it. Would it be suitable for .the ‘Differential Two’ ?- es. EKO (P.N.): What causes the blue glow in a 247 pentode valve? ‘This glow can be made to flicker by turning the volume control past a certain point. A.; The valve is probably soft; that i is, it contains a certain amount of air or gas which ironises under certain conditions, causing a blue glow. 2. When the second detector is touched a Joud hum is caused. A.: This is more or less normal.and is due to a slightly microphonie valve, BACKBLOCKS (Taranaki) : Wet cells for "B" battery use was fully described in the 1980°"Radio Guide." The Daniell Cells for "A" batteries were described in the "R.R." of November 7, of which a few copies are still available. Daniell cells as. "A" batteries are not expensive, but to be successful must be made up very carefully and attended to with assiduous care.