Radio today (Apr-Dec 1939)

<X> ■ ■ VIDEO a AUDIO R.F ft I.F CIRCUITS Block diagram showing the high-frequency circuits in a video and audio receiver. Dynatesting of video sets same as audio in basic principles. Quality and magnitude of signals in television sets equally as important in audio receivers. By Vinton K. Ulkich, Managing Editor Basically there is no difference between dynatesting a sound receiver and a sight receiver. In both cases the serviceman is confronted with the problem: "Is the voltage output of a stage great enough to meet the requirements of the following stage or device — and is the quality (waveform) suitable for the desired purposes?" From some viewpoints television dynamic servicing is simpler than corresponding tests in a sound set. This is because of the presence of the cathode-ray picture tube, and because the eye is more critical and has a better memory than the ear. CHECKING OPERATION BY EYE "When listening to a sound receiver, it is extremely difficult to determine what sort or distortion is present, if any; and where it might occur without staging a series of tests. With video, on the other hand, it is often possible just by viewing the picture to tell what sort of trouble is present and where it occurs. In future issues of Radio Today, it is planned to publish a series of television images with the standard test patterns to illustrate various difficulties that may occur. Two block diagrams of television sets are shown on these pages. One diagram is for the radio frequency and intermediate frequency circuits and includes all circuit elements up to the video and audio second detectors. The other is the video section comprised of the video voltage amplifier and the sweep circuits. The two high-voltage power supplies have been omitted. TEST FROM TUBE TO ANTENNA As in audio dynamic testing, the writer prefers to work backward from the cathode-ray tube in video testing. First all, by adjustment of the brightness control, it should be possible to illuminate the screen without any signal being fed into the set. If this is possible, it indicates that the high voltage is present as well as sweep frequencies. If a single vertical trace is obtainable, it means the horizontal sweep is not being applied to the tube. Likewise if a horizontal trace is the only image, the vertical sweep is absent. If only a white spot near the center of the tube is present, no sweep voltages are available. Incidentally, these tests for sweep voltages are identical to those used with any cathode-ray oscilloscope. If a perfect image is obtainable on the screen when tuned to a station, the set can be pronounced perfect, without further tests. Should the image slide around on the screen, then something is wrong with the synchronizing circuits (or even the transmitted signals). SWEEP OSCILLATORS Referring to the block diagram of the deflection chassis, one sees that the video signal is fed into the sync amplifiers. After being amplified, the sync pulses are separated into the 60 (vertical) and 13,230 (horizonal) channels by means of filter circuits. The synchronizing signal is then applied to the respective sweep oscillators to hold them in step with the oscillators at the transmitter. So if difficulty is had in keeping the picture synchronized, the stages between the video input and the sweep oscillator are checked. "When the picture is completely absent, the two most obvious troubles are absence of proper voltages on the tube or a defective tube. A quick method of checking the video amplifier, when no picture can be obtained, is to tune in the audio signal on the audio circuits, and then tie the two second detectors together. If the video amplifier is operating the sound signals will be fed into the picture tube and form some sort of a rapidly moving pattern. If no image results, this is a sure sign that the video amplifier is inoperative. CHECKING VIDEO AMPLIFIER "With a dead video amplifier, the dynamic testing procedure is similar to that used in the audio amplifier and described in previous issues of Radio 30 RADIO TODAY