This article describes one of the latest forms of sound-box for a gramophone. The details can be slightly varied to meet requirements, but the general design and dimensions should be adhered to, as the instrument is the product of much experiment. The whole of the work can be carried out with hand tools, but the slide-rest is desirable. The illustrations are reproduced full size.
The body (Figs. 1 and 2) and the ring (Figs. 3 and 4) are brass castings; the patterns must allow for shrinkage and machining. Mount the body on a face plate or in a recessed hardwood chuck, bore out the centre to 9/16 in., and face up. Remount on a truly turned spindle, and finish both sides and rim to the dimensions as follows: Greater diameter 2 1/16 in., lesser diameter 1 1/2 in., thickness of latter 3/8 in., thickness of central flange 1/8 in. Then scribe lightly a circle exactly 2 in. in diameter on the face of the flange at A (Fig. 2). Remove from the chuck, grip between lead jaws in the v ce, and file the flat B (Fig. 1) to within 1/8 in. of the lesser diameter. The holes indicated should be bored, the two at the back of the body C (Fig. 1) to a depth of 1/4 in., and tapped 1/8 in. thread (see Fig. ,5), and the three D, in the flange, 3/64 in. clearing The ring (Fig. 3) is a light casting, and is conveniently chucked in recessed hardwood and the face machined. Then reverse and tool out the recess E to 1 3/4 in. diameter and 1/4 in. deep leaving a flange which when finished is 1/64 in. to 1/32 in. thick, its inner diameter being 1 5/8 in.
Re-chuck on a hardwood mandrel that fits the recess accurately, and turn the rim to 2 in. in diameter, its width being 1/4 in. full to 9/32 in. While still in the chuck, file the flat F (Fig. 3), leaving 1/15 in. substance to the recess. Then remove from the chuck and hard-solder on the fitting G (Figs. 3 and 6), which is' of 3/16 in. brass, 7/8 in. long, 1/4 in. wide, and 3/4 in. over the lugs. A plan and elevation of the fitting G is shown by Figs. 7 and 8. The small knife-edge projections H (Figs. 3 and 6) are 1/8 in. wide and 1/16 in. high. They can be formed in the material by filing or planing, in which case 1/8 in. brass must be used; or they can be riveted into the 1-16 in. metal and trimmed up. The holes, tapped 3/64 in., are spaced 3/8 in. apart, in line with, and 5/8 in. apart across, the ring. The body (Fig. 1) and the ring (Fig. 3) are now assembled by the scribed gauged circle A (Fig. 2) the screw holes D marked, bored, and tapped in the ring 3/64 in. by 1/8 in. deep as at J (Fig. 3).
The fiting, shown by Figs. 9 to 12, is now built up and hard-soldered together. The base, of 1/16 in. brass, is 7/8 in. long, 3/16 in. wide, and 7/16 in. over the central lug. It has a shallow saw-kerf centrally on its under side as at K (Fig. 9) to engage with the knife edges of the ring fiting G. The needle socket is of 1/4 in. brass rod 3/8 in. long, bored to suit the needle shank and tapped at the side 3/64 in. for the set screw.
The diaphragm finger M (Figs. 11 and 12) is 1 1/16 in. long from the under side of the base to its central hole; it is of 1/16 in. brass, and tapers from 3/16 in. to 1/16 in. at its terminal disc, through which, and the talcum diaphragm N (Fig. 6), passes a small screw, nutted at the back of the diaphragm. Two pieces of steel, or hard brass, spring 1/2 in. long by about 1/8 in. wide are riveted to the base, as shown at O (Figs. 9 to 12), equidistant from its centre and 3/8 in. apart. In the ends 3/64 in. holes are bored, to correspond with those in the ring plate G. When attached, these springs are bent equally towards the needle socket L, so that when the screws are passed through them, an equal tension can be exerted on the diaphragm finger M.
A dise of rubber P (Figs. 5 and 6) 1 1/2 in. in diameter and 1/4 in. thick, with a 3/4 in. hole in its centre, is attached to the back of the body with two 1/8 in. cheese-headed screws Q 1/4 in. plain under the head. Into the central hole is cemented a 7-16 in. length of brass tube R having a 11/16 in. bore. This tube must not be in direct contact with the body, the object of the rubber being to insulate the vibrations of the soundbox from metallic connection with the tone-arm or trumpet.
Fig. 6, which is in part section, is intended to facilitate the assembling of the parts. Before this is done, the work should be polished and lacquered, or nickel-plated. First secure the rubber buffer P and the tube R to the back of the body (Fig. 1). Then loosely attach the finger fitting (Figs. 9 to 12) to the ring (Fig. 3).
diaphragm centre. The small screw is then passed through the finger end and diaphragm, and secured with a small flat nut at the back (see Fig. 6). A speck of hard wax can be melted into the joint, to arrest possible vibration at this point.
The sound-box is now complete, and will be found to weigh between 4 oz. and 5 oz., which is about correct for modern disc records. The tuning-up must be carefully attended to by actual trial on a machine, the tension screws being used for the purpose. Theoreti-
Select a good mica disc N of 1 11-16 in. diameter, and bore a fine hole in its centre. Cut a length of 1/8 in. rubber tube, with scarfed ends, to neatly fit the recess in the ring. Lightly dress the edge of the diaghragm with adhesive, and place it on the rubber within the cell, working it carefully to a central seating, not touching the cell at any point. Apply light pressure and set aside to dry; do not stick the gasket to the flange. When set, fit a second rubber gasket to the back of the diaphragm, without adhesive, and assemble the body and the ring by the three screws to fit the holes D, when the two rubber rings S will evenly and closely grip the diaphragm N. For the sake of clearness, the rubber tube gaskets S are shown conventionally, in Fig. 6, as black circles. In practice, of course, they will flatten under pressure, and spread considerably.
The finger fitting is now gradually screwed up, half a turn at a time on each side, until fairly rigid with the main body, the terminal disc just bearing on the cally, the diaphragm should be submitted neither to much pressure nor tractive strain, its own elasticity determining its normal set.
If facilities for brazing and silver-soldering are not available, the joints can be made with soft-solder, using zinc chloride as a flux; but this method is less satisfactory.--"Work," London.