A Corner for Beginners

Radio Record, Volume II, Issue 13, 23 November 1928, Page 11

A Corner for Beginners

By

Observer

Notice to Beginners and Correspondents. (1) Readers in difficulty or who have solved a difficulty which may be of interest to others are invited to write to us. (2) Questions of general interest will be answered in these columns, those referring to construction will be referred to "Pentode." (3) All questions inviting reply must be accompanied by a stamped and addressed envelope, for individual reply. (4), Address all correspondence "Observer," Radio Record, Box 1032, Wellington. (5) Where a query refers to trouble which cannot be located the following information should be given:(a) Name and model of set if factory made. (b) A diagram of layout if home constructed. (ec) Particulars of valves, voltage, and grid bias. (d) As many other details as readily available.

Correspondence. et LARGE number have written in " asking interesting questions. Owing to pressure we are compelled to hold these over till next week, when they will receive attention. While being perfectly ready to help anyone in difficulty, we ask that owners of factory-made sets refer these problems to their local agents for the set. Many correspondents are now sending set diagrams. This is a wise step and saves a great deal of trouble. Remember, be specific. Above all, don’t forget the stamped and addressed envelope. No charge is made; we make only this one request. The right to answer any question in the columns we reserve, so correspondents should watch the corner carefully. Quite frequently questions referring to constructional articles will be answered in "Tips and Jottings." Questions referring to the identification of stations and general questions will be answered in "Answers to Correspondents," under "Our Mail Bag." "Artificial "Static." T have a four-valve receiver, with. a good horn speaker. This set is in no way connected to the electric light in my house, yet when I switch on the night in any room I get a crackling in the speaker---J. Levin (Christ- church). | [This probably is caused through a slightly defective switch. When pressed into operation the bridge is not completely formed, and a little are of electricity jumps from one point to another. This are ‘causes vibrations, which are picked up by the coils of the ' receiver and amplified’ to give the noise you speak of. Examine or have examined the switches in the house.] Distortion in the Set. py nine cases out of ten, distortion of some form originates in the set. We refer particularly to sets which are home assembled, or good factory sets which have been in use some time without adjustment. A good set installed by a reputable dealer is not liable to cause trouble for some time, but, sooner or later, some small fault arises which needs attention if pure reproduction is required. The causes of distortion are many, but. the following hints may save a good deal of unpleasant reproduction. (1) See that the batteries are kept within the prescribed limits of voltage and amperage, not neglecting grid bias, ' (2) Make certain that the valves are suitable. The makers of valves recommend certain valves for certain places. Keep them in these places. In general, high amplification valves are in the Radio or High Frequency side, while the last valve has usually quite low amplification, but requires high plate voltage and correspondingly high grid bias. (3) Don’t exceed the amount of plate voltage recommended. (4) A receiver worked at oscillation pment to bring in a distant station is o%.worth while. Signals are invariably distorted. (5) See that the flexible lead from .Athe speaker running to the B+ output terminal is the one marked positive (+) or coloured — red. Speakers wrongly connected become gradually demagnetised and cause distortion. In our issue of November 2 a method of determining polarity of speaker leads ‘was discussed.

(6) To the constructor:-Do not place transformers or coils too close. The interaction will spoil the signal. (7) Overloading. There is only one thing can happen if very strong signals are fed to a weak speaker-chronic: distortion. Conversely, a powerful speaker used with a weak set will give unsatisfactory reproduction. Some constructors consider a speaker a permanent investment, to be used for the one valve amplifier and with every additional valve till they have a super six with maybe a pentode fed from an eliminator. No; it simply won’t do-this speaker must be capable of taking the volume _- supplied, likewise the final valve. Radio Reception. HE following advice from "Wireless Weekly" is for the growler who says radio is not what it should be, "Radio reception is better to-day than ever it was, but few listeners really take advantage of the improved condi-tions-and, of course, there is a great @ 1 more to be done. "Listeners, in general, comment most unfairly on our broadcasting stations and receiving conditions-whilst actually it is the listener himself who is at fault. "A great many people have receivers that have passed or seen their day in early broadcasting and still continue to think they should get good results such as those possible on present-day receivers. If reception is not good, these people blame ail noises due to loose connections, dust, dirt, rundown batteries, and other causes to poor old static, and yet do not service their sets in the least. "Listeners must keep up with the times as broadcasting stations do. Improvements and changes in our broadcasting stations are having a marked effect, but old receivers are not capable of the lifelike reproduction that is possible to-day, tt "Radio reception is what you: make Sizes of Condensers. (CORRESPONDENTS from time to time ask for the number of plates

in the different condensers. Here they are-the approximate capacities only can be given, as size of plates vary: It is to be understood that the spacing between plates is with one-eighth washers: 85 plates, .0015; 57 plates, .001; 43 plates, .00075; 29 plates, .0005; 19 plates, .0008; 13 plates, .0002; 7 plates, -0001. When a condenser has only five plates, it is generally termed ‘‘Vernier," and is used in conjunction with some other variable condenser. Fixed condensers run as follows, assuming the dielectric to be of mica: 002 inches thickness; 7 foils, each 2 inches x 1 inch, .001 mfd.; 14 foils, same size; .002 mfds.; 21 foils, same size, .003 mfds.; 2 foils, half inch by three-quarter inch, .001 mfds.; 5 foils, half by three-quarter inches, .0003

mfds.; 3 foils, 2 inches by 1 inch, .0005 mfds. Crackling Noises. DURING stormy weather, the aerial is likely to snap, and this will produce very weak signals or no signals at all. Intermittent signals accompanied by crackling noises may be caused by branches of trees coming into contact with the aerial, or by the aerial touching a wet building. The aerial must at all times be kept well away from trees and buildings, and all connections to and from the lead in insulator (if used) should occasionally be cleaned with sandpaper. Hand Capacity. MAN Y set operators, especially those who assemble their own outfits, are troubled with what is termed hand capacity. ‘It is found that on tuning in a station the signals come in at full strength but when the hand is remo, ed they are lost. Undouhbted, the easiest way to overcome this is to reverse the connections on the secondary tuning condenser. In other words the moying plates should be connected to the ground. By doing this the parts of the variable condenser that are near at hand will be reduced to zero. If, for some reason, this connection does not overcome the effect the panel should be shielded. Do not allow the metallic parts of the condenser to touch the shield, which should consist of a sheet of tinfoil fastened to the panel wi-h some adhesive, such as shellac. This shield should be connected to earth. Bypass Condensers. BYPASS condenser should always be across the "B" battery. This should be mounted within the cabinet and servés to reduce the _ effective length of the battery leads from the set to the battery. It is most important in a set using one or more stages of radio frequency amplification or where the battery leads are exceptionally long. A 1 mfd bypass condenser should be employed.