QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 31, 12 February 1932, Page 10

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

7. H.S. (Normandale): Should there : * be’ a space between the primary, and reaction winding of the ‘shortwave coils used in the ° ‘Night Owl Three"? A.: Yes. Space about 1-Sin. to » 8-16in. . .2. What is the correct size of the " panel? . A.: Use a 15 x 17, though the size is, within limits, immaterial. R J.E. (Greymouth): I am _ desirous ¢* of building the "Picnic Portable." -but before proceeding, would like to be enlightened on the points :- 1. Could the circuit be altered and built up as a shortwaver ? : The circuit as it now stands would be quite suitable for shortwave use and we had this in mind when we decided

on yvalve-base coils. The coils are not very accessible, however, and you would probably find it necessary to hinge a portion of the cabinet near the carrying handle to facilitate coil changing. As you remark, an outside aerial would be necessary for distant shortwave reception. Provision is made for this in the set. J B. (Murchison): How can I use a * milliammeter to test the emission of my valves, and what readings should they show? A.: The easiest way to test a valve if you happen to have a spare socket, is to wire the socket up as is shown in the diagram. The readings should be as follows: With 90 volts on the plate, 44 volts grid bias, UX201A would show 23% mamps. Under the same _ conditions.

D.E.P. 610, with 90 yolts plates, and 10.5 5 volts bias, should show approximately 7 mamps. If you do not happen to have a spare socket place the milliammeter in series with your speaker, and adjust the yoltage in the last stage. This would give

you the readings that are shown, and then you could put in each valve in turn in the last stage socket. FLN.G. (Auckland): My set has de- * veloped a vibration noticeable mostly on the high notes. is due probably to a microphonie yalve or a troublesome speaker. Keep the speaker well away from the set, and at the same time cover the detector valve with a lead cap. 2. What type of battery set, commercial model, do you recommend as a start for experimental work, on a combination short-wave-broadcast band for preference? A.: There are really quite a large number of coy mercially- made short-ware-broadeast sets, but we regret it is not for us to make recommendations regarding commercial apparatus. 38. Will a short-wave adapter work with good xerults from my present a.c. set? ; es. X22NW (Tasman): What is the correct bias for B605 with 100 volts on the plate? A.: 12. You will be able to tell when the bias js right when the music is best and clearest. You ean tell accurately with a milliammeter. Adjust the bias until the needle stops flickering. 2. What capacity are heavy duty batteries. supposed to -be? A.: The milliampere hour capacity is usually supposed to be about 10,000 m,a. hours, ‘with a maximum drain of 15 milliamps. 3. I do not quite understand the neutralising of | ‘the set with the phasatrol connection; ""Doés the setting of the phasatrol alter the tone of the set? : For a set using regeneration you neutralise as you would vif you were

% using a balancing condenser. In other words, you tune to a station in the mide of the dial, adjust the reaction condenser until the set squeals, adjust the phasatrol until the saueal stons, or gets to its weakest point. If the squeal stops, further adjust the reaction condenser and adjust the neutralising condenser until it stops or weakens. The phasatrol will not alter the tone of your set to any marked extent, "UA (Hastings): How much eleectrie current should an eight-yalve a.e, set burn? A.: The average set will burn about 100 to 1380 watts. that is, as much as a fairly bright light. TOLESS" (Stratford): Is there any law or likely to be any law to prevent any persons not having passed in the electric examinations installing a radio recéiver, permanently or temporarily? A.: Very shortly legislation will be brought down preventing persons who have not passed the radio serviceman’s examination from installing any: radio receiver, either temporarily or permanently. C.D, (Auckland): Modifications of * the Loftin-White appeared in the "Presto Change-over Receiver," which was published recently. Apart from this we cannot undertake to give you further information. ‘The -Loftin-White was dealt with fairly fully in the Australian "Wireless Weekly" some time back. and many modifications were shown. If you could locate anyone with a file of this paper, or if anyone getting the "Wireless Weekly" happens to know the date in which these appeared, would they kindly let us know. and we would pass it on to our correspondent.: P K.B. (Oamaru): I have just added a special choke to my set, but cannot stop the hum. How could I remedy this? A.: See our article on the question of hum in i. recent issue of the "Radio Record." You do not say where you have placed the choke, and what kind of choke it is. 2, Could I add another audio stage? T have already added one, but would another still further increase the volume of stations? A.: No, it is not possible. If you add a further stage you will probably introduce audio howling and distortion. 3. What is the best combination of yalves for my set? A.: An ordinary screen-grid valve in the first stage, a high impedance detector. one, say, with an impedance of 20,000 ohms, or thereabouts, followed by a gen- eral purpose valve of the 221 type, and, finally, a medium power valve, type PM4 or B65, F.0, (Waipahi): I have a four-valve * battery set for which there is neither © battery nor grid-lexk. I get a little

distortion at times. Could I use a © battery and grid-leak? ; A.: You should use a © battery on the last stage. Find the last valve in your set and bfeak the connection between the grid return of the last transformer and A-. Take the grid return of the transformer, usually marked "GB," or probably, in your case, "A-," or. "OS," to about 9 volts negative C battery. Take the positive of the © battery to A-. This bias is about right for a medium-sized power valve of the PM4 type with 90 to 135 volts on the plate. Ww A.B, (Wellington): Could you give * me particulars of broadcast coils for the "Sellens" a.c. short-wave set? A.: You will need two sets of coils for each stage. The secondaries for the first band will be approximately 80 turns of 24 d.e.c. wire wound on a 8in, former. The primary for the first coil will be about 30 turns, and the second coil will be about 28, both of 30 gauge d.s.c. wire. or the next coils the secondaries will be 155 turns of 24 d.s.c. wire, the primary of the first coil 60 turns, reaction coil 50 turns, both 30 d.s.c. wire. These are aproximate only, and based on formulae. ou may have to amend them to get the best results, A O.McD, (Eltham): If you want to * know the cost of the "Cathode" super, why not write to one of the dealers who have advertised parts for this set in the issues in which they have been appearing? They could tell you more about costs than we can. Full particulars have been published in previous issues of the "Radio Record." V C.B. (Normanby): I am using three * PM6’s in my set. Are these valves liable to injure my transformers? A.: Yes, PM6 should be used only in the last stage. By using them in other stages you are using extra current, and are also putting a greater drain on your eliminator than necessary and reducing the voltage. 2. Yes, your eliminator has ample power for your set. 3. What value grid-leak and condenser should I use? Would a variable one do? A.: Variable grid-leaks are not satisfactory, as they are usually noisy. The value'should be approximately 2 megohms, and that of the condenser from .00015 to . 25, ‘Generally speaking, .0002 is considered to be the optimum, -*194 (Waikato): Will the efficiency _of the Pierce earth system be lessened if I connect up all the pipes with one wire? A.: No, provided you make really good connections to each pipe and see that the connecting wire is continuous, you will -then get quite as good results as by any other method. 2. Does it matter if the car radiator is left out? The soil round the pipes is damp, , A.: No. Provided the ground is kept damp; that is everything. Dial readings of my set have shifted about 15 degrees, after replacing the two variable-mu valves. The dial has not been touched. A.: This is a big jump. Are you sure you put in the right type of variable-mu At the same time examine your \

condensers and the dial to see that there is no slipping there. Drop a note to the New Zealand agents for your set and ask them about that short-wave adapter. They have had considerable experience and could make more sound recommendations than we could. "NEVA" (Wellington) : Oscillation and fierce motor-boating are prevalent in my "Radiogram" five. What is the cause and remedy? A.: Try reversing the primary connections to one of the audio’ transformerssay the second audio (first push-pull). Place an r.f. choke between B-+- of the second audio transformer and the common lead which goes to the r.f.c. in the plate circuit of the screen-grid valve and the battery. If necessary use a grid compressor, or an anti-motor-boating device in the connection in the first. audio

stage. (See accompanying illustration.) To reduce the oscillation try reversing the connections to the tickler coil and reduce the detector voltage until oscillation is smoother, but if this does not cure it, try taking off a few turns from the tickler coil. It may be necessary. to reverse the connections on the first coil, that is, making the top of the coil "HB" instead of "G," and the bottom of the coil "G." Of course, you will join the top of the coil at "EH? with the aerial coil as at present, This will reduce the field created by this coil and possibly get over your trouble. M (Seatoun): Could you_ supply, * through the "Radio Record," a diagram of a wave-trap using .0003 condenser? A.: Accompanying is a suitable diagram. On the secondary coil you will require 80 turns of 26 d.s.c. wire wound on a 2in. former. For the primary coil glue about half a dozen match-sticks round one end of the big coil, and over this wind 22 turns of the same gauge wire, allowing a slight space between each turn. Then anchor the wire round the match-sticks temporarily and use gum to hold -the windings in position. This is the primary eoil. The top, that is, the more distant end of the larger coil, goes to the aerial, the other to the aerial terminal of your set. The larger coil is tuned with a .0003 condenser, "TININY" (Auckland): My set performs erratically. Sometimes it goes splendidly and then volume drops, music and "speech becomes thin, the sibilants becoming accentuated. After a time everything is normal again. I have been over the set, with a fine-tooth comb, as it were, and everything is splendid. There apear to be no defects? A.: What about the speaker? Have you had that tested? Have you tried testing your valves by substitution Often on @ test bench a valve will appear quite nor-

mal, yet when put into a set it behaves in the manner you describe. When the set starts to distort then put your meter on to a different point of each valve and see if everything is all right. It may be that there is trouble in a transformer which shows up only occasionally, and when you test the set it may be all right. Carry out your tests, generally speaking, with the set in the unsatisfactory condition, It is not’ much use testing it when it appears to be quite normal, You would, of course, have to make your test with the set going, but with a litle care you could probably ‘do this. We do not think the trouble is due to outside interference. Have you had the lightning arrester out of the circuit to see if that is having anything to do with it? "*(NATHODE RAY" (Invercargill): I wish to use a short-wave adapter with my set. Does the valve base coil used with a 227 require the same number of turns for reaction as is shown in the table in a previous "Radio Record’? A.: You do not say which "Radio Record," but try the number given, and if any alterations have to be made, you can quite easily make them. If anything, put on more than required, as it is easier to take off than put on. Should the cathode of the adapter be taken to earth, or connected to the cathode of the detector valve of the set, as in the diagram? A.: Try both, and see which works the better. 3. Must the adapter be. built on a metal chassis, or is a wooden baseboard permissible?

A.: A wooden baseboard is quite pers missible. F H.W. (Taihape): The wire you en- * glose, which is 26 d.s.c., would be quite satisfactory for a coil. The value of the condenser is about .0002, but you do not tell us the size of the plates. They would be quite suitable for both tuning and reaction, although you will have te put a rather large number of turns on the secondary coil to compensate for thé low capacity.

F H, (Gisborne): Would the following * valves do for the ‘"Radiogr: Five’?: UY224, 227, first audio push-pull 226? A.: The 226 in push-pull would nog really be satisfactory. You should have 45's, You could use them, of course, int exactly the same method as is shown in the diagram, but remember they are 1 volts filament. If this should. not handle sufficient ‘volume for you, you will know the reason why. What size transformer stampin should be used for‘a filament transfor to supply these valves? .: Full particulars were given in the 1931 "Guide," which you have on hand. 8. Should I put on two filament windings, and what wire would carry nearly 5 amps? A.: Two windings of No, 16 gausd would be satisfactory. I want to use .00025 straight ling eapacity eondenser and 24 s.¢.¢. wit, What number of turns should I wind the coils? A.: The secondaries will need 160 turns. the reaction coil about 35 tur. and the primary about 50 turns, but f this latter you must use a finer wiresay, 30 gauge d.s.c. . Do you make the weekly "Radig Records" into volumes and sell them? A,: Yes. Drop a note to our accounde ant, and he will tell you all about it. "GROWLER" (Christchurch): I a . troubled with occasional in my set. A,: This may be due to many Causes, A defective battery, defective valves speaker, Generally the ‘first. 2. In the morning and afternoon the music and singing are not clear, but mulee : This is probably due to the same |We suspect either a defective valve, or a troublesome battery. It may be a burnt-out transformer, 8. What is the best kind of earth #@ one is not near a water-pipe or heater? One or more galvanised iron pipes driven into the ground and kept moist, The earth wire must be soldered or a tached with a standard earth clip, heater does not make a good earth. |