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DIDS-40Z-ZMIO
Para 3-3/ 3-4·
(3)
The SOS signal is received from the control logic board.
The signal cuts off the video amplifier when the code of
000 000 is located in the video entry register.
(4)
. The CRT protection signal and the brightness level are con-
nected to the CRT grid.
(5)
The 'brightness control. located on the side of the chassis,
adjusts the brightness of the display.
(6)
The amplified video is sent to the CRT cathode.
3-4 DlSPLAY TERMINAL RASTER GENERATION
The raster is generated by horizontal and vertical deflection amplifier A2
and minor vertical sweep driverA3.
The horizontal deflection amplifier receives a horizontal drive trigger
which starts a sawtooth generator.
The resulting sawtooth is amplified and
applied to the horizont.al deflection coil as a linearly increasing current.
Thus,
the beam moves horizontally across the {ace of the CRT (figure 3-5).
If there were no vertical deflection, the sweep would be perfectly hori-
zontal.
As soon as the vertical deflection begins to push the sweep downward.
however, the sweeps are no longer horizontal but decline as they move across
the face of the CRT (figure 3-5B).
To correct this unwanted movement. a
sawtooth is taken from the horizontal deflection amplifier and applied to the
vertical deflection amplifier as a corrective' skew-out' voltage.
As the beam
sweeps across the. tube, the effect of this voltage is to maintain a constant
vertical deflection current until the end of the sweep.
Thus. the sweeps remain
pe rfectly horizontal (figure 3-5C).
The CRT protect circuit detects the negative
peak of the horizontal sweep.
When the horizontal sweep is not present, the
CRT grid is grounded.
This cuts off the CRT, which would.normallybe painting
one extremely intense vertical line, and thereby prevents damage to the CRT
phosphor.
The vertical deflection amplifier receives a vertical drive trigger which
starts a sawtooth generator.
The resulting sawtooth is combined with the
'skew' voltage. amplified, and applied to the vertical deflection coil.
Thus,
the sawtooth which drives the sweeps downward has a horizontal step for each
horizontal line during the time that the horizontalsweeps occur (figure 3-5C).
The CRT protection circuit in the horizontal deflection amplifier operates in
the same way, except that it has a longer time constant.
If the vertical sweep
is not present, the CRT grid is grounded.
This cuts off the CRT. which would
otherwise be painting one extremely intense horizontal line, and thereby pre-
vents damage to the CRT phosphor.
The minor vertical sweep voltage consists ()fa sine wave from monoscope
deflection amplifier A3 (figures 3-1 and 3-5E).
When·the controls are properly
adjusted. only the characters are visible.
If
the minor vertical sweep signal is
absent and the brightness potentiometer on the control panel is turned up, nar-
3-10
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