~----9_4_54_2_4_-9_1_0_1
______________________________________________
~
\OJ
1.,:)0~/0
RB
TO
DIFFERENCE
AMPLIFIER
Rll
Rl4
C4
RIS
151
Rl6
NOTE.
IN
EARLIER
MODELS
SI
IS
A
SWITCH
;
LATER
MODELS
HAVE
A
JUMPER
PLUG
INSTALLED.
t
R56
FROM
AUXILIARY
H-SYNC
r.ENER.0..TOR
I
F'ROivi
VC:RTiCAL
----~
SYNC
GENERATOR
Figure 1-26. Model 911 VDT Power/Logic
PWB
Dual-Gain Video Amplifier
+SVDC
VIDEO
TO
CRT
MONITOR
Transistor Q4 (clamping circuit) conducts during horizontal
or
vertical sync
to
pull the level
of
the video signal
to
ground. This prevents display
of
extraneous video components during retrace,
and
the
passing
of
horizontal and vertical synchronization pulses
by
the
dual-gain video amplifier.
1.4.2.8 Vertical Sync Generator. The vertical sync generator amplifies and passes only the
vertical component
of
the composite sync signal from the difference amplifier. Figure 1-27
is
a
schematic diagram
of
the vertical sync generator. The circuitry at the base
of
QS
acts
as
a
low-pass filter
to
pass only the 60-hertz (or SO-hertz) vertical synchronization component
of
the
composite sync signal. Trapsistor
QS
then amplifies and inverts the vertical synchronization
component
of
the composite sync signal. The
output
of
transistor
QS
drives two cascaded
Schmitt-Trigger inverters which form a pulse shaping and delay circuit. The low-active
output
of
this circuit
is
nominally low for 220 microseconds.
1.4.2.9 H-Drive Generator. Figure
1-28
is
a schematic diagram for the H-drive generator
that
passes the horizontal synchronization component
of
the composite sync signal during normal
operation, and produces an "artificial" horizontal synchronization when the terminal
is
not
connected
to
an operating controller. Horizontal synchronization signal (CRTCLK)
is
also used
to
clock the keyboard UART.
1-42
Digital
Systems Division
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