OCR Text |
Show (Edited by G. Douglas Wardrop, Editor of Radio Merchandising.) By H. S. POTTER Experts have recommended strongly to the amateur that he take up experimentation experi-mentation In short wave radio, and a tendency lias been noticed lately toward to-ward the construction of simple shortwave short-wave receivers. They ere not only economical In construction, but are easy to operate. Inductive or conductive coupling, if properly operated, will give about equally loud signals, so there Is no objection ob-jection from that point of view to the latter system. An objection which Is frequently advanced to conductive coupling Is that there Is much broader tuning, but If the apparatus Is properly proper-ly constructed very fine tuning can be obtained. An Inductively coupled set, however. Is very much more difficult to operate, due to Its many controls. An Increase In selectivity Is gained by using a variometer In the plate circuit, cir-cuit, but this advantage Is more than balanced by the Increased difficulty in I r,s.i ANTENNA CONQ RHEOS-v. I rig 2 tuning In distant stations. As to signal sig-nal strength, I have found very little difference to result from using or omitting omit-ting the plate variometer. The advantages of the set here described de-scribed may be summed up as follows. The controls are very few and simple; the appearance Is such as to recommend recom-mend it; the operation is highly efficient effi-cient and the cost Is very low. The following Is a brief description of the construction : The panel is of 3-16-lnch bakelite, 6 by 14 Inch. The polishing Is done with a sheet of No. 1 emery, moistened with olive oil, followed by a clean, dry piece of flanneL i The holes are drilled as shown In Fig. 3. The drilling can be done with an ordinary hand drill., The holes for the mounting screws of the condenser must be drilled to fit the type of condenser con-denser used. The antenna series condenser Is mounted at the left end of the panel, and controlled by the knob and dial. Any .001 mf. condenser may be used. A balanced condenser would probably prove advantageous In fine tuning, but It is not necessary. Inductance Mountings. Probably the most difficult part of the construction of this set Is the making mak-ing and mounting of the Inductances. The main, or stationary, Inductance is wound on a piece of bakelite tubing 3 inches In diameter, and 5 Inches long. Using No. 24 D. C. C. wire; start V2 Inch from the top, wind on 44 turns In IV2 Inches, skip Inch, and wind on 78 turns more. This winding wind-ing should be tapped at the 7th, 18th, Slst, 44th, 64th, 84th, 104th turns, commencing com-mencing from the top. At the blank center space two V-Inch V-Inch holes should be drilled, diametrically diametri-cally opposite to one another. These serve as bearings for the ,-lnch rotor shaft. Two small holes are drilled In the form Inch from either end. and directly above and below one of the rotor bearings. The inductance Is mounted on the panel by means of two 6-32-lneh nickel plated, flat-head machine ma-chine screws inch long. Great care f T 1 t 1 f.g.S . PANEL 7 SHELL 1 RESISTANCE ) B element POINTER! 1P WWCT ARM " f's-4 must be exercised to get the two bearing bear-ing holes In the rotor In line with the one on the panel. For the moving part of the Inductance Induct-ance system a hardwood ball rotor 3 Inches In diameter Is used. They are. In most cases, drilled for a 3-16-Inch shaft, so It becomes necessary to drill 1 out this hole for a 4-lnch shaft A 3-32-lnch hole Is bored through the 1 rotor and shaft, at right angles to the shaft, and a short piece of 3-32-inch , brass ro driven Into this hole, to lock the rotor to the shaft. The rotor should lie wound with 18 turns of No. 24 D. C. C wire on each side, or 30 turns in all. On the rear of the panel, near the center, Is a single pole, double throw r.vltch. As may be seen by a glance at the hook-up this switch serves to connect tbe ground to either the first tap switch or the Inductance side of the grid condenser. When In the latter position the receiver Is more selective I I and Is particularly useful for receptloD of radiophone. With the changing over switch on the opposite side and the antenna connected the tuning Is broader broad-er and better adapted for general reception. re-ception. The Rheostat. The rheostat In this set Is homemade. home-made. A strip of bakelite 1 by 2Vi by 3-16 Inches Is wound with reslstunce wire to a resistance of 6 ohms. This resistance element Is mounted on the panel by means of two small brass brackets, and machine screws, as shown by the drawing. The edge of the resistance element Is Inch from the panel. The brackets, which are homemade, measure i-lnch lengths of brass tubing used as sleeves to slip over tbe machine screws. This means of mounting may be best understood by consulting the drawing of the tube controls (Fig. 4). The shaft of the rheostat Is a 1-lnch length of 8-32 threaded brass rod. The contact arm Is a piece of phosphor bronze 14 by Inch, secured as shown In Fig. 4, by a pair of locknuts. On the front of the panel Is a pointer, held between a nut and a knob threaded thread-ed 8-32. For the tube socket a homemade base Is used, only the metal shell being be-ing purchased. A piece of Vs-lnch bakelite 2 by 3 Inch Is used. 1-Inches hole Is bored In this piece of bakelite, centered 1V4 Inches from either side and from one end. Contact springs of phosphor bronze are attached In the proper positions po-sitions to make good contact to the tube. (See Fig. 2). The base Is supported sup-ported to the panel by brass brackets Inch from angle to hole on one leg and Inch on the other. The grid condenser has a capacity of about .0002 mf. Two strips of tinfoil tin-foil 1 by inch are separated by a piece of thin mica, and are so arranged as to have an active surface of inch by Inch. The mica and foil are assembled between a piece of -lneb bakelite and a piece of fiber, each Inch by 1 Inches, clamped together by small machine screws which pass through holes In the ends. These screws serve the double purpose of holding the condenser together and . acting as terminals. ter-minals. A pencil mark on the fiber, between the screws, serves as a grid leak. This can be varied until best results are obtained. Tubes. Almost any tube now on the market can be used. Although a soft tube requires re-quires a critical adjustment It gives better results than most hard, non-critical non-critical tubes. If a soft tube Is used an adjustable "B" battery, made up mot- UC I W i3l. r of a dozen two-cell flashlight ceils soldered sol-dered together In series, should be employed. This arrangement is more economical and satisfactory than a potentiometer. Almost any kind of knobs, dials, switches and binding posts can be used. If all bright parts are nickel plated a very fine appearance will be obtained. A composition knob 1 Inches in diameter was used on the rheostat, but If a ready-made rheostat Is used the knob supplied with It can be employed. em-ployed. The cabinet has Inside dimensions of 14 by 6 by 5 Inches, and Is made of Vi, Inch quartered ouk. The cover Is hinged so that It may be raised to Insert In-sert or remove the tube, throw the change over switch, and to Inspect the Inside. Strips of -Inch oak are fastened on the Inside, 3-16 Inch from the front edge and the panel secured to It by screws, so that the panel surface sets flush with the edges of the cabinet 1 The cabinet can be finished up In any desired manner, but the simplest Is to merely stain it with oil stain and then varnish or wax It Although the set will work after a fashion when wired In almost? any manner, good results cannot be obtained ob-tained with poor wiring. No. 14 bare copper wire should be used. All leads, particularly In grid circuit, should be short as possible. Close, parallel grid and plate wires should be carefully avoided. If It Is desired to obtain lond signals It Is a simple matter to connect an amplifier to this set. The "A" battery bat-tery posts on the amplifier should be connected In parallel to those on the set, and the phone posts of the set connected to the input posts of the amplifier. Radio News. |