QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 39, 5 April 1935, Page 41

QUESTIONS AND ANSWERS

815. (Invercargill) : I have a six-valve a.c. t.r.f. commercial set and would like to have it modernised to take type 58’s in the r.f. and detector stages, Would results justify the expense? A.: It is purely a matter of opinion, though the change-over need not be a very expensive one if you use adapters. In this way there would be no need to make any alteration to the wiring. It -~would be better to use a 57 for detector as it was designed for this purpose. , 2.: Could the change-over be’ effected without alteration to the present-.andio system? : A.: Yes, there is no need to interfere with the audio side of the set at all. 220A (North Auckland): Would the "Empire Short-waver" described in the March, 1935, "Radio Times" give better results than the "Wleo Three," published in June, 1984? I am particularly interested in picking up amateur transmissions. . A.: The first set you mention is slightly the more powerful of the two, but then it has an extra valve. The 1935 "Radio Constructor’s Guide" will contain a: description of a-four-valve battery shortwaver which is more efficient than either of the two sets you have in mind, because it uses a type 15 indirectly heated valve s detector. This is a screen-grid cathode ype valve, and it enables electron-coupled reaction to be obtained just as simply as with an a.c. set. The valves used are 32 r.£ stage, 15 detector, 30 first audio and PM22A output pentode. The tuning condensers are ganged. . "PUZ4ZLED" (Hukapapa): At ‘night when stations are coming through strongly on my "Simplex Superhet Four," a loud low-pitched rumbling often mars reception. Turning the volume control slightly one way or the other eliminates ‘the trouble, but unfortunately the noise occurs just at the point where volume is best. The "A"? and "OC" batteries are in good condition, though the "B’s" are down somewhat.

A.: You are evidently troubled either by a microphonie howl or by audio insta--bility, due to the run-down condition of the "B" batteries. Try placing the speaker on a separate table so that the vibrations from it are’ not transmitted to the chassis and valves. If the noise disappears, then evidently one of the. valves has become microphonic. As a cure, try cushioning the chassis.on rubber cushions. If this does not cure the fault, try fitting additonal de-coupling in the plate circuit of the detector, by connecting a resistor of from 5000 to 10,000 ohms in series with the plate resistor and by-pass-ing the junction of the two with a condenser of from .25 to 1 mfd. Also, make sure that the speaker cord is well away from the aerial lead-in and that the audio witing under the chassis is away from c.f. wiring. . 2.: Where could I fit a 60 milliamp. fuse-bulb in my set? A.: Disconneet the "B" negative lead in the set from the "A" negative, and connect it to one side of a fuse-holder mounted on the back of the chassis.. The "A" negative lead is connected’ to the other side of the fuse-holder, and from thence a contiection is run to the switch terminal where the "A" negative and "B" negative leads were previously attached. : "PHAR LAP" (Levin) : I recently purchased a 1935 all-wave receiver. Should I instal a special doublet antenna recommended by the makers? A.: It should give you far better results and less noise than an ordinary "LL" type aerial, particularly if you are in a location which is electrically noisy. 2.: I have not been able to locate any stations on the 16 or 14-metre bands, A.: This is because there are no stations of any account operating below 16.88 metres (PHI. Holland), and 16.86 metres (GSB, Daventry). The latter operates daily between 1 a.m. and 3 a.m. DBA. (Mata Mata): With a Beverage aerial, is it necessary to have it running in a straight line from. the’ set or could the main length be run at ‘right angles to a 30-yard "lead-in"? Also, in which direction does the Beverage aerial have the greatest pick-up? 6 A.: It would be quite in order to insta the aerial as -you have outlined, though the directional effect will be a little marred. With a normal Beverage, stations located in the direction to which the far end is pointing come in best. "TTOTARA" (Stratford) :' Of the three _ receivers you mention, we would strongly advise you to build the "Comet Superhet Five." It is simple and cheap to build, and is far more sensitive and selective than the "Scotsman’s Superhet," which uses only three valves and a rectifier. . "MAYFATR" (Wellington): I have a +f nine-yalve all-wave radio, but am not able to get the best from the set beeause of QRM. The interference is particularly. bad at night, with spells of noise like the running of an electric motor lasting about 25 seconds each time. On Sun-

days the interference caused by buses and motor-cars-is particularly bad. Could (Continued on next-page.)

a LATE ETS fap Ete Gee BEE LED OND bed O1G TE PPE-ALL TEE OAT? OT -- Information Coupon (To be used with all requests for information.) Name of set eee eececcccccesccncesee Model See eeeWeeesseecccscesececeseee Name SHOP OCS CREE SCC eDeeee EL Ee TED Address Seecceweoeecesescsosceseeeees Nom de plume Per eres ereeseesesesese To be kept in subsequent inquiries. Please Note:(1) Be specific and brief, tabulating if possible, (2) Write legibly and on one side of the paper. (3) We do not design circuits. (4) Limit three questions unless 1/- is enclosed. (5) Postal queries limit 38 questions. Enclose stamped and addressed envelope and 1/- fee, PTE TE EEE TEE TE tts Ui

I fit a line filter or special type of aerial which would minimise this.noise? A.: The conditions you’ outline are roughly duplicated by those described in a special article on reducing interference published in the November, 1934, "Radio’ Times." Whether you would be able te erect a flat top for a special noise-reduc-ing aerial high enough above the noise level to improve reception we cannot say, but if you can, then the installation of a special aerial would be well worth trying. Incidentally, the intermittent noise you can hear at night is evidently from the lift motor, I K. (Wellington) : While listening-in to * American short-wave stations, I have often heard the announcers ask for reports on the transmissions. How does one prepare these reports, and can they also be sent to. broadcast stations? A.: Yes, they can be sent to all stations, and they are generally acknowledged by a verification of reception. Collection of these verifications from stations all-:over the world forms the very interesting hobby to which the 1500 members of the

N.Z. DX Club deyote most of their spare time. Full particulars of this club, by the way, can be obtained from the secretary, Box 1680, Wellington. Regarding the reports, there is a special form prepared for the use of club members which simplifies this task considerably. All that is necessary is to fill in the number of blanks according to the instructions, and then the report is complete. | "(ARID" (Riverton) : I have a "B" eliminator .which, when it is switched on, gives a reading of 10 volts, rapidly falling away to no reading at all, What ig wrong? A.: The most likely cause is a worn out rectifier, though a shorted bleeder registor er smoothing condenser could also cause the trouble. "Ts KOON" (Gisborne): I have built _ the "Air Ace Short-waver" without the 2A5 and am using it with ’phones. The volume obtainable is quite good but I think I should get better because I have to keep the reaction potentiometer on full to obtain the volume. . What alterations do you suggest to allow more flexibility with this control? , A.: Carefully slide the reaction winding a little closer to the grid winding. If the improvement this effects is not sufficient, then replace the .0001 mfd. by-pass condenser from the plate of the 57 to earth with a .0002 mfd. . 2.: I propose adding another 56 and a 59 (leaving the 2A5 out) so that the run of the valves would be 58,%57, 56. 56, 59. What values resistors and condensers should I use to couple the second 56 to the first? . A.: Plate.resistor 50,000 ohms, coupling condenser, .01 mfd. and grid resistor .1 meg. Slight variations to the values given will not. affect results. Couple the second 56 to the 59 with the same value resistors and condenser as specified for coupling the 56 to the 2A5 in the original model. 8.: Do you ‘think this arrangement would give more volume? A.: It will give a little more volume than the.original arrangement, but probably scarcely sufficient to justify the expense of. adding an extra valve in the way you have outlined, Our correspondent adds: Congratulations for designing such a hum-free set as the "Air Ace." I have used only two 8 mfd. electrolytics in the power supply, but there is absolutely no trace of hum, "TT UNDUREFOOT" (Whakatane): I have just completed the "Silvatone A.C. Seven" described in the October "Radio Times." Volume was very poor at first and I found that by earthing the cathode of the 56 direct instead of through the 50,000 resisor, volume became very satisfactory. I have compromised on this by replacing the 50,000 ohm resistor with one of 2900 ohms and I have by-passed it With a 25 mfd. condenser. ‘

A.: With. your present connection, only one of the output valves is working. By making the alterations you have outlined, . the 2A5 which has its grid coupled to the cathode of the 56 driver is not receiving any signal. The circuit used in the audio side of the "Silvatone A.C. Seven" is identical with that developed for one of the most popular kit sets in Australasia today. In rare instances, trouble such as you have outlined has been encountered, and in all cases it has been overcome by providing normal bias for the 56, which is done in the following way: Leave the 2900 ohm resistor and the 25 mfd. bypass condenser in parallel with it connected to the 56 cathode, but unsolder the other ends of these components from earth and take them instead to one sidé of the 50,000 ohm resistor you took out of the circuit. The other side of this resistor is earthed. Now disconnect the .5. megohm grid leak of the 56 from earth and connect it to the junction of the 50,000 ohm resistor and the pair of components mentioned earlier.’ From this junction, also, the grid of one of the 2A5’s is fed through a .l mfd. coupling condenser. 2: I have a 55 on hand which I would like to add to this set to obtain delayed A.V.C, Could I use this valve for A.V.C. only or would it improve the set to use one of the diode plates as second: detector and one for delayed A.V.C.? . , A.: You could do either, but if you followed the latter course you would not be able to obtain reaction, and whether you desired this feature or not, we cannot teil. If you wish to use the 55 for separate automatic volume control, then the series of articles on "Modern Automatic Volume Controls" now running in the "Radio Times" would be of great assistance t@ you, . 2: In the 1984 "Radio Guide," a description is given of an electron-coupled oscillator. In the diagram the leads from the r.f. B. to "B+" is intersected by a cross. What does this indicate and are any of the valves used in my set suitable © for this oscillator? A.: The cross can be disregarded. The 57 could be used. in this circuit, with the suppressor grid tied to cathode. R G. (Millerton): I have a battery set * which uses a 221 valve. Ig there a more modern type of replacement I could use? ° Se A.: Yes, you could use a 82, though as the 221 has a 3-volt filament, you will need _to include a resistor in the filament circuit of the 82 to drop the voltage to 2. As the 221 takes .06 amp. filament current, a 15 ohm resistor would be required. You could use one-half of a 30 "PENTODE" (Otorohanga): Could electron-coupled reaction be used in battery sets? A.: Yes, the-"Dleo Three," a $-valve battery shortwaver described in the June, 1934, "Radio Times," illustrates a suitable method. However, there is now. available a type 15 screen-grid indirectly heated cathode type of battery valve which enables electron-coupled reaction cireuits as used with a.c. models to be employed for battery sets without modification. A four. valve battery short-waver using this new type valve will be described in the 1935 ohm centre-tapped resistor. 2 "Radio Constructor’s Guide,’ to be pub-: lished in about a week’s time. 2: I built the "Quality Five" as a fourt" valver with a pentode and its performs ance js little short of marvellous, considering its limitations, However, I was troubled with bad rf. instability and all

the usual cures failed to eradicate it. Ultiymately, I: remedied it by connecting up a .0005 mica condenser between aerial and earth. The set operates now with full screen and plate voltages entirely without any instability whatever. ‘That the sensitivity .has not suffered can be gauged from the fact that sixteen Huropeans can be tuned in during the early daylight hours and 65 stations are available at good strength by 9 p.m. in the evening. ‘Why was the method I discovered for curing the instability successful? A.: Evidently the coils you are using have high impedance primaries, and by connecting a .0005 mfd. condenser across the aerial primary you have added sufficient extra damping to the first tuned circuit to cure the instability. A smal! proportion of the signals would be bypassed to earth, but this would be of small account if the set was stabilised in consequence. "QTUDENT" (Auckland) : Is it legal to connect the mains straight on to a rectifier without using a transformer? A.: In the case of a.c. mains, no; d.c., yes, ~_-s 2: If in servicing a set a serviceman has to replace some parts for which he has to charge, is it necessary for him to have a dealer’s license? A.: No, N A.T. (Wairoa): I have an eight- * valve commercial set which is not as selective as I would like. Could you suggest a remedy? A.: As your set has five tuned circuits it}should be fairly sharp, and so if it has aj} tendency to be broad it must need realigning. "MAURICE" (Lower Hutt): By connecting the charger to your accumulator the wrong way round the latter would be run completely down, and if it were left in this condition for long, sulphation would result, largely ruining the Sattery. However, if you gave it a good charge with the charger connected correctly immediately after you discovered your error, there should be no ill effects.