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1947-48 Theatre Catalog, 6th Edition, Page 348 (334)

1947-48 Theatre Catalog, 6th Edition
1947-48 Theatre Catalog
1947-48 Theatre Catalog, 6th Edition, Page 348
Page 348

1947-48 Theatre Catalog, 6th Edition, Page 348

same in diameter on the film face. The upper is acting as a feed sprocket. The lower is acting as a hold-back sprocket. Except for a difference in diameter, the sprockets are similar, and are held on their shafts by an end screw and key washer.

The upper sprocket shaft carries, on the non-operating side, a large fiber gear, the driving chainwheel for the bottom take-up, and a small fiber gear. These two gears and the chainwheel are screwed together and rotate as one, and are keyed to the slotted end of the shaft by a key washer integral with the large gear wheel. The whole assmbly is held on the shaft by an end screw.

The main driving pinion on the layshaft engages with the large fiber gear on the upper sprocket shaft, the gear reduction being such that the 990 rpm. of the driving pinion is reduced to the correct speed of 360 r.p.m. on the sprocket shaft.

The large gear wheel is of such a diameter that its upper part is just proud of the top face of the soundhead. The projector is driven from this gear wheel.

The lower sprocket shaft carries on its non-operating side only a small steel gear and an idle guide sprocket for the chain drive to the bottom take-up. The gear is driven by the small fiber gear on the upper sprocket shaft, the ratio being, of course 1:1. The gear on the loWer shaft is keyed to the slotted spindle by a key washer integral with the gear wheel, and retained by an end screw.

All the gears in the train described are robust and should give many years, service.

The gears and shafts already decribed are mounted on the soundhead casting proper. The scanning drum shaft, together with the optical system, photo cell, and exciter lamp, which together comprise the scanning unit assembly, are carried on a resiliently mounted plate attached to the soundhead casting.

The housing that carries the Scanning drum shaft and fiuid flywheel is offered up to its bore from the non-operating side. It is held in position by three screws passing through' the flange of the housing into tapped holes in the plate.

The fluid flywheel is retained on the shaft by a hexagonal end nut and a shouldered washer. There is also another washer on the other side of the flywheel.

The lay on roller, which holds the film in contact with the drum, also runs on ball bearings. An enclosed springloaded plunger, with non-adjustable tension, applied a predetermined thrust on the lay on roller bracket. An engraved disc permits of tracking the lay on roller in respect to the scanning drum so as to correct for possible displacement of the sound track.

The optical system of the soundhead is one of its most interesting features. The exciter lamp is mounted in a compartment on the extreme left of the soundhead. Immediately in front of the lamp is a large condenser which projects the light horizontally forward to a prism mounted partly within the scannirg drum. The prism reverses the light path and directs it back through the sound track, through the objective lens and on to the window carrying the mechanical slit. The window is in a hous

ing containing a prism, which directs the received light vertically downward on to the cathode of the photo cell. The optical magnification is six times, which means that an enlarged image, six times that of the actual sound track, is impressed on the window. With the film stationary, it is immediately possible to check whether the focus is approximately correct, and with the film running, it is immediately evident if either sprocket holes or the edge of the picture is being projected onto the slit. The window has fixed masks to accept the internationally accepted scanned width of sound track of 0.084 inch. The adjustable tracking of the lay on roller centers the scanned sound track on the window. The slit is correctly adjusted for azimuth (horizontally) at the works and locked with an Allen screw.

A detail refinement in connection with azimuth adjustment of the soundhead deserves mention. The Window upon which the mechanical slit is engraved is locked by an Allen screw immediately below the window. Just in front of the large condenser lens is a screw head, which is, in fact, the end of a fine worm which meshes with a worm wheel on the mount of the window. If the Allen screw be loosened, the window carrying the slit can be rotated by applying a small screw driver to the screw head.

As the slit is correctly set at the works, the adjustment provided is not intended to be used by operators or during ordinary service work. Once adjusted and locked, it cannot vary, but a supervising engineer may need to satisfy himself that the setting is optimum. The only satisfactory way to do this is to employ a long length of focusing film, or an endless loop, and with a voltmeter, a power level indicator, coupled to the output terminals of the power amplifier, alter the azimuth adjustment while the film is running until a maximum meter reading is obtained.

Focusing of the soundhead objective lens is also best done by the same method, focusing film and meter, as recommended for azimuth adjustment, although it can also be done by inspection of the image on the window, or by the tfblink" method. If the inspection or the blink method be used, it is essential that a sufficient length of focusing film be employed to ensure that it lies on the reproducing drum in exactly the same way as would a whole reel in normal running. This means that the focusing film should be laced through from the bottom sprocket of projector to fire trap of lower spool box, allowing the usual size of loops, and the film should be induced to adopt the normal running position past the scanning point by turning the inching handle.

The optical system is of such a design that even with maximum light on the window, filament structure is not pros jected. The correct adjustment of exciter lamp, condenser lens, and supplementary lens is that which gives maximum light on the window, and consequently greatest output from the photo cell. The exciter lamp should be set so that the horizontal bar of light from the condenser falls squarely across the entry side of the prism. The condenser should be adjusted so that the maximum intensity of lightis projected on to the window, taking care to preserve horizon tality of the condenser mask. Any deviation from the horizontal of the mask

twill immediately be apparent on the

window. Optimum position for the condenser lens will be found to be that when the front edge of its barrel is level with the front edge of the mount. The supplementary lens, on the exit side of the prism, will usually be found to give greatest light when screwed right home. It is locked by an Allen key. Once the condenser and supplementary lens have been adjusted, there is no need to alter them. They do not require re-adjustment when an exciter lamp is changed.

When an exciter lamp is changed, it is only necessary to check that the bar of light is across the center of the prism, and that in consequence the light projected onto the bridge is evenly disposed above and below the slit. The height adjusting screw on the lamp holder will immediately correct for any difference between one lamp and another.

The photo cell*a gas type with caesium.cathode, with standard (British) four-pin base-is carried horizontally in a cast aluminum box below the condenser lens. Access is obtained by removing the two cheese-headed screws in the front cover. Access to the pins of the cell holder is obtained by re moving the screws which hold the dished cover on the front of the cell box.

Various types of slits can be used with the soundhead, depending upon the purpose for which the head is used. For re-recording a very fine slit is used so that a straight line frequency response may be obtained from the cell. For all normal reproduction purposes a comparatively coarse slit is used, because the over-all frequency response curve recommended by the Academy of Motion Picture Arts and Sciences entails serious curtailment above 2,000 cycles. The standard reproducing slit is 0.009 inch high, and taking into account the six times magnification of the optical system corresponds to a slit dimension of 0.0015 inch. This dimension naturally results in a considerably increased amount of light being passed to the photo cell, with a gain in cell output and an increased signal voltage on the grid of the first tube. Its effect on the frequency response curve is progressively to attenuate the response curve above 2,000 cycles, giving a maximum of -12 db. at 8,000 cycles.

The photo cells are coupled by low capacity coaxial cables, not directly to the grid of the first tube, but to the outside contacts of the changeover switch on the control panel. From the moving arm of the switch, the signal is conveyed to the input tube of the voltage amplifier. The switch is noiseless in operation, and will remain so because it handles no direct current. Isolating condensers prevent the arrival at the switch of do. from the cells, and another condenser prevents microscopic grid currents arriving from the input tube. Even though the switch were to develop high contact resistance, it would cause neither loss of sound nor contact noise.

The efficiency of the optical system, due to the use of optical components of large effective aperture, and to the coating of all the surfaces of condensers, lenses, and prisms, is high.

The exciter lamp is a normal 8-volt,

1947-48 Theatre Catalog, 6th Edition, Page 348