Radio today (Apr-Dec 1939)

Today. Of course, the video amplifier must be capable of handling signals whose frequencies range from a few cycles to several megacycles. More about the specific procedure in future issues. While three video amplifier stages are shown, it is likely that the small sets will use less. Likewise is true of the amplifiers in the synchronizing and sweep circuits. Naturally sets using large picture tubes, will have more amplifier tubes, since higher deflection voltages are required when higher accelerating voltages are applied to the tube. Consequently, the step from a 5-inch tube with 2,000 volts to a 12-inch tube with some 5,000 or more volts involves much more than just changing the tube and increasing the voltage on it. MAGNETIC AND ELECTROSTATIC DEFLECTION Both magnetic and electrostatic deflection are being used in the receivers that have been just announced. In the magnetic type, the deflecting voltages are applied to coils that have a magnetic effect on the electron beam, thereby causing deflection. The electrostatic sets have plates inside the tube, to which the deflection voltages are applied. Incidentally, electrostatic is the type of deflection used in all commercial test oscillographs. The deflection of a tube and deflecting yoke if used, can be easily checked by applying AC voltages of proper magnitude to the deflecting coils or plates of the picture tube. The LP. circuits can be checked by working backward from the detector as in the case of a sound receiver. Some variations in the procedure will be necessary since the LP. frequency for the video signal is on the order of 13 megacycles. The audio I.F. will be exactly 4 y2 megacycles away for singlesideband transmissions or 3*4 megacycles for double-sideband signals. If the high-frequency oscillator is higher in frequency than the signal, the audio I.F. will be lower in frequency than the video I.F. — and vice-versa. SINGLE H.F. OSCILLATOR FOR VIDEO AND AUDIO Working back to the first detector, one finds that the output divides into two channels. One for the video, the other the audio. A single oscillator is used to provide the two I.F. signals from the two transmitted carriers. Ahead of the first detector, both the video and audio carriers and modulation are amplified by the same circuits simultaneously. In some sets an R.F. amplifier stage will be used, so it is shown on the block diagram. Since the signal frequencies range from 44 megacycles to 108 megacycles (channels A to G), special signal generators will be needed. For overall set testing independent of transmitted signals, some sort of a special modulator is also essential. For a discussion of the types of interference that may create havoc with telecasts, the serviceman is referred to page 19 of this issue. For a proper understanding of television circuits and how they work, the reader is urged to study and master basic principles. With a thorough understanding of such concepts, television is not so hard — in fact, as the writer sees it, television is merely the same old principles expanded and applied in new ways. Television terms explained With the birth of commercial television, the radioman, either technical or sales-minded must learn the new lingo. Dozens of new terms or words are being added to the radioman's vocabulary— and many other words are being given new meanings by television. At present many manufacturers have their own exclusive names for certain developments. As a beginning on the list of television terms we present those compiled by Allen B. Dumont Labs. Audio — Pertaining to the sound section of the receiver. Amplitude — A term synonymous with gain or size. Axis — In television the horizontal plane is called the X Axis and the vertical the Y Axis. Cathode-Ray Tube — An evacuated glass tube comprised of a structure for producing and focusing a stream of electrons upon an internal screen. Coaxial Cable (or line) — A special cable for conveying television signals with as little loss as possible. Contrast Control — A control on the receiver adjusting the contrast between high lights and shadows in the picture. Deflection (Magnetic) — A system where the motion of the spot in producing the picture is controlled by magnetic fields. Deflection (Electrostatic) — A system where the motion of the spot in producing the picture is controlled by the static action of the deflection plates. Deflection (Plates)— These plates are located inside a cathode-ray tube and provide for electrostatic deflection of the beam. Deflection (Coils) — Coils mounted externally about the cathode-ray tube to produce magnetic deflection of the beam. Dipole — An aerial comprised of two separate rods. Double linage — Where two images appear separately on the screen, one of the sweep circuits is adjusted to half its correct speed. If the horizontal is at fault the images will appear side by side, conversely if the images are vertically displaced the vertical sweep is at fault. Field — In the RMA Television System there are two fields to each frame. In other words each picture is comprised of two fields scanning alternate lines. Frame — One complete picture, thirty of these a second are thrown on the screen. (Continued on page 56) The video low-frequency circuits are comprised of the video amplifiers, synchronizing amplifiers, sweep oscillators, and deflection voltage amplifiers. VIDEO SYNCHRONIZING ft DEFLECTION CIRCUITS