Radio Broadcast (May 1928-Apr 1929)

36 RADIO BROADCAST NOVEMBER, 1928 FIG. 3. SCHEMATIC DIAGRAM OF TELEVISION AMPLIFIER (Continued from page )y) I he essential parts of a television reproducer are a tuner (any kind will do that is capable of providing a good loud-speaker signal in ordinary reception), an audio amplifier, a neon tube and a scanning disc turned by a motor mechanically arranged so that the holes in the disc scan the surface of the glowing plate. The amplifier used by the writer is shown diagrammatically in Fig. 3, while constructional points are suggested in the pictures, Figs. 1 and 2. The amplifier can be built from any standard apparatus; it employs the usual three-stage resistance-capacity-coupled circuit, APPARATUS REQUIRED THE following is a complete list of the parts required for the construction of the writer's television amplifier: Ri — 3 Wire-wound resistors, 100,000ohm ; Ro, R3, R| — 3 Grid-leak resistors, i-meg., 0.5meg., and 50,000-ohm, respectively; R5 — 1 Rheostat, 10-ohm; R6 — 1 Potentiometer, 200,000-ohm; Ci — 1 Mica fixed condenser, 0.001-mfd.; G> — 4 Filter condensers, 400-volt, 2-mfd.; C3, C4, C5 — 6 Mica condensers, 0.01 mfd.; L— 1 Pilot light; Si — 1 Toggle switch; -4 Vacuum-tube sockets, ux-type; 6 Resistor mountings; 10 Binding posts; 1 Front panel, 7 x 10 x Tf-inch; 1 Sub-panel, 7 x 10 x iVinch; 2 Sub-panel brackets. The pilot light and toggle switch are, of course, unessential, but were incorporated in the amplifier for convenience. The amplifier eventually will be mounted in the cabinet, and the pilot light will give some indication of current conditions behind the panel. The switch (Si) controls the filaments of the detector and amplifier tubes, while the other switch, visible in the pictures, was used to start the motor turning the scanning disc. The four filter condensers (C2) are connected in parallel to bypass the amplifier plate voltage, and reduce the tendency to motorboat. In the amplifier pictured the coupling condensers are built up in stack form, and consist of six 0.01mfd. mica condensers. In operation it will be convenient to be able to switch easily from the loud speaker to the neon tube, an operation that is facilitated by the double-pole, double-throw switching arrangement suggested in Fig. 3. By means of the two battery taps the voltage applied to the tube is practically the same with either the loud speaker or neon tube, thus prolonging the life of the 171 a tube and making unnecessary any variation in the C-bias potentiometer, R6. When the amplifier is operating the loud speaker about 200 volts are employed, while with the neon tube the full 450 volts is applied. These voltages can be sup Cross Bar 1 / ' Brass Strip, % Motor Shaft. Disk^ Looking down or up FIG. 4. MECHANICAL SPEED CONTROL plied either by a B battery or a power-supply unit. A Receptrad Powerizer was used in the author's laboratory as shown in the picture of the entire set-up, Fig. 1 . SCANNING DISC ASSEMBLY THE televising apparatus consists of the scanning disc driven by an adequate motor, some form of motor speed control, and the neon tube, combined in a convenient and efficient mechanical arrangement. A box frame, such as is illustrated in the pictures, Figs. 1 and 5, provides a simple and satisfactory unit. The motor shelf is so positioned that the driving shaft can be centered exactly. The shelf is clamped between cushions of soft rubber which reduce the vibration. The neon tube is mounted with its plates parallel to the scanning disc, so that the holes in the latter pass over its entire surface. The neon tube is a Raytheon Kino-Lamp and it should be placed in a horizontal position on the upper shelf as close to the scanning disc as possible. The Baldor type-MV2 variable-speed motor may be used for turning the disc. For observing the picture square hole, if x if inches, is cut in the face of the front panel exactly in front of the plate of the neon tube. The inside dimensions of the entire box, as pictured, measure 25 x 25 inches. It is built of half-inch wood (heavier material is desirable) and the front panel is 7 x 26 inches. Two ten-ohm rheostats, connected in series, are mounted on the panel, and these provide a very accurate motor control. The rheostats are in series with some additional resistors in accordance with the directions accompanying the motor. By means of the rheostats, in conjunction with a simple mechanical brake described in the paragraph on operating directions, it is possible to maintain the speed of the motor sufficiently close to synchronism with the transmitting disc. THE CIRCUIT ARRANGEMENT TH E detector tube has been incorporated in the amplifier constructed by the writer. V* hen used with the average receiver, posts one and three in Fig. 3 will be bridged, and post two will be plugged into the grid prong on the detector socket of the receiver. In the case of a regenerative receiver, the plate of the exterior detector tube will be wired to the plate terminal on the detector socket while post three will be led to the plus detector terminal on the set. In other words, the tickler or regenerative coil is connected in between the plate of the detector tube and the coupling resistor. If it is desired to use the detector socket in the receiver (and this may be the more simple procedure in many cases) post three is led to the plate terminal of the detector tube. The neon tube is connected in place of the loud speaker or output device. This is a simple series connection and is quite effective. While there are other systems of inputting to the neon tube, concomitant complications hardly recommend them for an initial attempt. The apparatus described was designed primarily for the reception of the television signals broadcast from wrny, New York City, employing a 48-hole disc at a speed of 450 revolutions per minute. A National Company disc is used in the illustrated apparatus. OPERATING DIRECTIONS TH E signal is first tuned in on the loud speaker in the usual way, a loud clear signal being the desired result. The motor should be started and the disc brought up rapidly to approximately the speed desired. This can be accomplished by means of a switch short-circuiting the speedgoverning resistors. The output of the amplifier is now switched over to the neon tube, which, when the television signal is received, should show a definite pattern when viewed through the disc. As the disc approaches synchronism with the transmitting disc, the pattern will resolve itself into more definite lines slanting away from the perpendicular. The lines become more and more perpen(Continued on page 37) FIG. 5. REAR VIEW OF TELEVISOR