Tektronix 7834 - Page 83

when this level is LO, Channel 1 can
be displayed . The
Horizontal Chopped
Blanking signal at pin 4 is connected
through LR4338 to the
Horizontal Binary stage U4358,
and to the L-Axis Logic
stage U4494 by way of 04336 .
When this signal is HI, the
crt is unblanked to display the
selected signal
. When
it
is LO, the crt is blanked to allow
switching between the horizontal
units .
A logic diagram of
the
Chop
Counter stage is shown in Fig-
ure 3-20 . Details of operation
for the flip-flops (FF) are
shown in Table 3-1 at
the front of this section . Use the
waveforms given
in Figure 3-19B along with this diagram .
Vertical Mode Logic
The Vertical
Mode Logic stage is made up of discrete com-
ponents CR4323-CR4322, CR4369-CR4368 and
buffer
04382-04392 . These components develop the Main
Frame
Channel Switch signal (Display Right), which is connected
to
the Main Interface, Vertical Interface, and Trigger
Select-
or circuits to indicate which vertical unit is to be
displayed .
When this output level is HI, the
RIGHT VERT unit is
displayed ; when it is LO, the
LEFT VERT unit is displayed .
The VERTICAL MODE switch shown on diagram 2 pro-
vides control levels to this stage . This switch
provides a H I
level on only one of five output lines to
indicate the selected
vertical mode ; the remaining lines are LO . Notice
that only
four of the lines from the VERTICAL
MODE switch are
connected to the Logic circuit
. Operation of this stage is as
follows :
Figure 3-20 .
Logic diagram of Chop Counter stage .
Theory of
Operation-7834
(1195-20) 1988-45
When the VERTICAL MODE switch is
set to RIGHT, a HI
level is connected to the base of 04382
through R4321 .
This
forward biases Q4382, and the positive-going
level at
its
emitter is connected to the emitter of
04392 . The col-
lector of 04392 goes HI to indicate that
the RIGHT VERT
unit is to be displayed . For the CHOP
position of the
VERTICAL MODE switch, a HI level is
applied to the
anodes
of CR4323-CR4322 through R4322
.
Both
diodes
are forward biased so that the Vertical Chopping
Signal
from
pin 1 of U4340 can pass to the base of
04382 . This
signal switches between the HI and LO levels at a
one-
megahertz rate and produces a corresponding
Display Right
output at
the collector of 04392 . When the output
is HI,
the RIGHT VERT unit is displayed and when it
switches to
LO, the LEFT" VERT unit is displayed .
In the ALT
Position
of the VERTICAL
MODE switch, a HI
level is applied to the anodes of
CR4369-CR4368 through
R4369 . These diodes are forward
biased so the Vertical
Alternate Command from pin 6 of
the Vertical Binary stage
can pass to the base of 04382 to
determine the Display
Right level . The Vertical Alternate
Command switches
between its HI and LO
levels
at
a rate determined by the
Vertical Binary stage .
The control levels in the LEFT and ADD positions
of the
VERTICAL MODE switch are not
connected to this stage .
However, since only the
line corresponding to the selected
vertical mode can be HI, the
RIGHT, CHOP, and ALT lines
must remain at their LO level
when either LEFT or ADD
are
selected . Therefore, the base of 04382 remains LO to
produce a LO Display Right output level at the collector of
04392 .
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