B&K 1076, Инструкция по эксплуатации, Страница: 44 / 64

venience, a block diagram of the complete instruments is shown in Fig. 26.
Many of the circuits in this instrument are quite familiar to the service
technician, because they are used in most television receivers. Some of these
circuits are new and greater emphasis will be placed upon the description of
these circuits.
The Scanning System
The first of these circuits pertains to the conversion of light energy to elec­
trical energy. Certain materials have the ability to emit electrons when they
are exposed to light, much the same as the cathode of a conventional electron
tube does when it is heated. These materials are known as photoemissive mate­
rials. When a photoemissive material is. used as the cathode of a diode, any
light striking the tube will cause the cathode to emit electrons. These electrons,
flowing through the diode load resistor, cause a voltage to be developed across
the resistor. Such a tube is shown in front of a transparent slide in Figure 27.
As the small spot of light from the cathode-ray tube sweeps across the slide
from left to right, light energy passes · through the transparent portions of the
slide. When the spot passes behind parts of the opaque "A" painted on the
slide, the pattern blocks the light energy. Therefore, during some parts of
the spot's sweep across the slide, light energy strikes the photoemissive cathode
of the photo tube, while at other times it does not. This energy develops a
varying voltage across the load resistor. Thus, part of the ".A" on the slide
has been changed to an electrical signal, as shown in Figure 27. As illus­
trated, when light energy is transmitted through the transparent portion of
the slide, a voltage is developed across the resistor, and when the "A" blocks
the light, no voltage is measured.
Figure 27-Changing Video Information
to an Electric Si
gn
al
TRANSPARENT
SLIDE
BEAM
The cathode-ray, or "scanner" tube used in the Generator is specially de­
signed to match the spectral response of the photomultiplier pickup tube.
As shown in Figure 27, the spot of light produced by the scanner starts at
the top of the faceplate, sweeps across to one side, is blanked while it returns
to the other side (retraces), sweeps to one side, is blanked while it returns
to the other side (retraces), etc., until it finally reaches the bottom of the
screen. It is then blanked and returned to the top of the screen. If the "A"
of Figures 27 and 28 is placed over the face of the scanner tube, the dot of
light and photomultiplier tube translates the entire pattern, spot by spot, into
a video signal.
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