Radio Broadcast (May 1928-Apr 1929)

NOVEMBER, 1928 JAMES MILLEN'S TELEVISION RECEIVER 37 quietly. The swish of the disc through the air constitutes the major portion of the noise, and this is quite insignificant. Special rubber vibration absorbers are supplied with the motor for mounting purposes. MOTOR SPEED CONTROL THE diagram (Fig. 3) shows the method for speed control. For the variable resistor R>, a 75watt, 4 to 100-ohm wirewound resistor with a sliding contact is used. The other resistor may be a 10-ohm 10-watt resistance. This is labeled "Ri" in the diagram and is shunted by the push-button speedcontrol leads. The resistance Rt is so adjusted that with the push button released, the motor runs at slightly below the proper synchronous speed. Then, when the push button is depressed, the disc tends to speed up. Do not mount the television receiver in the same cabinet with the disc. Vibrations of the motor will introduce a synchronous noise that will result in a series of horizontal lines being drawn across the picture. Therefore, it is im FIG. I. GENERAL VIEW OF TELEVISION LAYOUT inside the tube are placed in a plane at right angles to the axis of the "Pin" of the base. If the pin, therefore, is pointed toward the disc when inserted in the socket, the plates inside the tube will then be parallel to the disc. The tube should be mounted at the proper height to cover the ij-inch square scanned by the revolving disc. The plates are connected to the "plate" and "filament" prongs of the tube base. sor (Continued from page 35) visable to use a three-stage amplifier using National transformers — see Fig. 3. Then again, the pictures being transmitted by 3XK at present are merely silhouettes, which do not require an amplifier with as wide a frequency range as if half-tones were being transmitted. As a rule, with a three-stage a.f. unit, the amplifier noise will not be very gTeat. Vibration from the receiving disc or its motor, which are transmitted to the amplifier or especially the detector tube, however, will introduce a periodic noise that will cause a black streak across the field of the picture. Any periodic interference, such as a 60-cycle hum, that may get into the signal will also cause streaks across the picture, but these will not remain stationary, but will move upward or downward across the field of the picture. THE OUTPUT CIRCUIT THE output circuit of the amplifier is arranged so that the neon or Kino-Lamp is always illuminated, and. when a station is received, the brilliancy of illumination merely varies in accordance with the signal. A good background will be obtained if the d.c. current through the neon tube is limited to 10 or 20 milliamperes. More current will cause the lamp to glow brighter and brighter but there is no advantage in this so far as the picture is concerned and it only serves to shorten the life of the lamp. Accordingly, care should be taken to adjust the current to the minimum satisfactory value. A Clarostat has been found excellent for such use, and it may be mounted conveniently on the front of the frame supporting the scanning apparatus, as shown in Figs. 1 and 2. For illuminating the Kino-Lamp either a standard high-grade B socket-power unit or heavy-duty B batteries may be used. Several different concerns are manufacturing scanning discs suitable for use in receiving the signals now on the air. The better grade discs are well made mechanically, so as to run true and require little power. The holes in such discs are also punched to the degree of accuracy necessary if the received image is to be free from black lines and streaks. The National disc uses radially-shaped holes, rather than round holes, for with this design the "lines" across the image are much less obvious. In driving the scanning disc successful result; have been obtained with a number of different types of small motors. However, the motoi which the writer is using at present is the |-horsepower type-YIV variable-speed condenser-type Baldor which is intended for operation on 110volt, single-phase, 60-cycle a.c. line. This is a ballbearing motor that operates very smoothly and FIG. 2. REAR VIEW OF SCANNING DISC portant to keep the receiver and amplifier on a support separate from that for the disc. The experimenter will find that the following convention has been adopted by the Raytheon Company in regard to neon tube mountings. The tube is fitted with a standard ux base. The plates {Continued from page 36) dicular as the correct motor speed is approached, finally forming the image of the televised object. Final tuning should now be effected on the receiver. Also, the bias to the last tube should be varied, by means of the high-range potentiometer Re, for best results. MECHANICAL BRAKE NEEDED IT IS desirable to use some simple form of mechanical brake in conjunction with the rheostats to control the motor. The device shown in Fig. 4 was designed by the writer for this purpose. An iron cross piece was fastened to the box housing the revolving apparatus so that it crossed in front of the motor shaft. A hole was drilled to the exact center of the shaft, and the nut from a i-inch iron bolt was soldered to the cross arm. A brass strip was bent as shown in the illustration, and bolted to the cross piece. The head of the j-inch bolt was sawed off and a knob mounted on the end. By screwing in the bolt, the brass strip is pressed against the end of the shaft, giving a very delicate braking action. The brass strip should be taped where it touches the shaft. RECEIVER AMPLIFIER TELEVISOR FIG. 3. COMPLETE SCHEMATIC DIAGRAM OF JAMES MILLEN's TELEVISION RECEIVER