QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 19, 20 November 1931, Page 13

QUESTIONS AND ANSWERS

FRANK

KEE

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 cannet be regarded as a reason for its not being used. Address all queries, The Technical Editor, Box 1032, Wellington,

por (Ashhurst).--L am using an : aerial 40 feet high and 60 feet in length. I get K¥I faintly. Would it be better if I increased the length to 150 feet, and allowed 26 feet to pass over an iron roof? A.: The higher and longer your aerial the better it is for DX, Passing it over a roof will cause certain losses by absgorption, but we think even then the results will be better than with the more or less small gerial vou now have. 2. I am using single flex for the lower 20 feet of the lead-in from the aerial. Is this ‘satisfactory ?-Yes. WwW I. (Wairoa).-I haye a super hetero;Y e dyne receiver, and wish to use a shortwave adapter. Which of those shown in the "Guide" would be most suitable? A.: The a.c. super het, would be the hest, but the coupling of this to a super "vf heterodyne receiver is a rather ticklish problem. It is usually advisable to couple o the first of the intermediate stages rather than go straight into the aerial terminal. Our experience has been that home- constructed apparatus does not usually work at all well when nsed with modern commercial apparatus, and resultg are sometimes very disappointing. The adapter should cost abont £10 or £11. We suggest you write Mack’s Radio, Wellington.. He is a specialist in shortwave, and will tell you all you want to now.

1304 (Otago).-On moving the tuning dial, especially on stations aboye 1YA, loud bumps are heard in the receiver. A.: It seems as though your condenser yanes are touching at intervals. 2. Gan I add a tone control with littie alteration to my set? : Yes; quite easily. Determine the grid terminals of each of the two power valves, and slip a metal connection under each, Carry these out and take one to 4 50,000 ohms resistance and the other to a g@06 condenser. Connect the condenser "mW resistance together, and there yon have your tone control. You must make quite certain that the leads do not touch the metal parts of the set. 3, Can multi-mu valves be used to replace present screen-grid ones? A,: They could, but in order to get the fullest efficiency from the valves they should have their pwn circuit. However, the sereen-grid valves are really quite efficient, and we think yon would be disappointed if you used multi-mu valves in your circuit. BeRoADcasr (Auckland) -I cannot turn up the volume control of my set, because of a crackling noise which 1s ‘practically continuous, A.; Take off your aerial and then try rning up the set, If the crackling noise comes in with almost equal intensity, then there is something wrong within the set, probably a defective resistance, and you should communicate with the agent who sold you the set, If, however, the erackling does not appear when the aerial is off, it is evident that it is due to outside interference over which you have no

control, The problem of outside interference i8 one that is to receive the attention of the new Radio Board. pat (Cambridge) -Could I charge my 100 volts "B"’ accumulator with a 4volt 60-amp accumulator? A.: No; you eannot charge one accumulator from another unless you want to inflict damage, You could charge it from a 4-volt charger by breaking up the 100, Volts "B" accumulator into 4 volts sections, | eased (Mangaweka): You do not ¢onnect the centre tap of the af. 30 transformer to a resistancse to earth, but direct to earth. However, between the centre tap of the filament resistance and earth is a 1500 ohms resistance. 2. Is is possible to run 245’s with an a.c. 23-volt filament, the rest of the set being battery ?-~Xes. , 8. What must be the value of the centre tapped resistance across the filament lead? A.: Anything between 20 and 50 ohms. cops WY bere do I eonnect "A--" and A.: Io earth; that is, to a point common with the centre tan of af, 30 and the resistance from the centre tap of resistance "CO."

C M. (Lower Hutt): I have condensers | * with 11 plateseach. Would thoge be suitable for the "Radiogram Five?’ If so, what would be the coil specifications? A.: They would be rather too small in the "Radiogram Five." However, if you want to try them wind the secondary coils with 30 gauge 4d.s.c, wire, using 90 to 100 turns, The primary should consist, of from 40 to 50 turns of 34 gauge wire, while the reaction coil will be approxi- : mately 60 turns of the same gauge. W WW. (Gisborne): I have built the * "Night Hawk Two," using the "Kestrel" method of aerial coupling and grid return, Gradually insensitivity has set in, until now the coils are almost useless, I thought the batteries were at fault, so I took a couple from another set, but there was very little improvement in the short-waver, I changed the gridleak, but no better, . A,: We think the trouble is with the batteries, What have you done about the "A" battery? Although it may be an accumulator and freshly charged, yet an internal short may be taking place and the voltage rapidly dropping. Test your batteries with the meter while they are actually connected with the set. You can probably borrow a meter from your local dealer to do the job, Have your valyes ‘tested. Take off the earth, and examine the earth connection and the earth to make quite certain that they are all right. Try a smaller condenser in series with the aerial, ; 2. Can the coils have any de-fect?-No. . ; ; T W. (Petone): My "Ranger Two" is * insensitive, I am using the wrong value of coils aud condensers, the latter

being .00085. Could you give me the number of turns for these coils A,t Using a coil designed for a .0005 with a .00035 condenser will not diminish the sensitivity of the receiver as a whole; rather it will throw the tuning range

rr: >. oo slightly out of gear. For a .00035 you want about 10 turns more on the secondary than for a .0006. Insensitivity of your set will probably be due to a defective valve. Are the values of your condensers

and grid leaks correct, and do you have @ right amount of bias on the valve? oes your set oscillate? PHAR LAP (Napier): How can ‘I set about lining the trimming condensers of my eight-valve commercial receiver? A.: Without possessing special apparatus the best method would be to tune to a station at about the centre of the dial. say 1YA, then adjust the screws, commencing with one of those on the extreme. Move this in both directions, and note the effect on the volume. Leave it at the point of maximum sensitivity. If there is no change, return the screw to the position in which it was found. Repeat the process with the others. : 2. Could you recommend a good book on the servicing of commercial ‘receivers? A.: Servicing of commercial receivers is a job beyond ‘the amateur. Furthermore. it wil] not be long before interference with a commercial is prohibited by regulations. When a set loses sensitivity and the valves are still good, one should test for voltage on the screen, plate and grid of all the valves. Without this voltage test one more or less works in the dark and may do more harm than good. To test these voltages, special testing appara tus is necessary, and as the amateur rarely has this at his command we do not recommend interference with commercial sets. unless it is, of course, to trim up the condensers. Have you checked over your aerial and earth connections to make quite certain they are in order? Have you tested your rectifying valve? There are very few good books on the servicing of radio sets, unless it be Reyner’s "Testjing Radio Sets." He deals mainly with battery sets. L H. (Waikanae) : I have been troubled ° with persistent crackling in the set, and have taken my whole outfit to a radio dealer, who says everything is alright. As soon as I instal it in my home the noise starts again. A.: We cannot give you much help under these circumstances. We would

imagine the fault to be within the set itself, probably due to a loose connection or to &@ defective resistance, which, although it did not show up in the test was, nevertheless, present, and taking the set to your home disturbed it again. It may be a broken down condenser in your power pack. 2. Where can I obtain an article on ean Armstrong three-valve circuit? A.: There are really dozens of Armstrong circuits, for Armstrong was the man who brought about the regenerative method of coupling. A regenerative circuit was shown in last week’s ‘Radio Record," and used with the "Eagle Five" this week. A typical three-valve circuit was the "Ranger Three" described in the "Radio Record’ some months ago. iD O.S. (Rotorua): I added an extra * valve to my three-valve set, leaving the power valye, which was in the last stage, in the set. and using a PM+4. Is this correct? A.: No; you should have taken the nower valve from the last stage and employed a general purpose valve such as PM38 in what used to be the last socket, using PM4 in the last stage. By using the two power valves you are consuming far more battery current than you should. Furthermore, you are saturating the primary of the audio transformer between what, used to be the last stage and the existing last stage. You may possibly burn it out. 2. What kind of speaker would be suitable? A.: A light cone speaker. JX (Te Kuiti) : We do not have further particulars of the valves to whicb you refer. If we come across them, however, we shall notify you. From what we told you the other week you should be able to approximate the characteristics you reguire.