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Theory
of
Operaton
-
494A/494Ap
Servicg
Vot.
1
U1016A
delays
the
marker
dot to
allow
retrace
white
gating
DOT
INV low
to set
up
th€ disptay.
DOT
tNV
afiects
th€
readout
deflection
outputs in
the fottowing
ways.
1.
The
horizontal
output
is connected
either
to
ground,
for a
center-scr€en
dot, or
MAX
DOT
CON-
TROL,
for
a max
span pointer.
MAX
DOT
CON-
TROL
is
proportional
to
the
center-frequency
readout
offset
from
the center of
the frequency
range.
2. The
U10258
output
goes
tow
during
the
dot
interval
to cause
0301
I
to insert an
offset
current
Into
th6
vertical
output,
to
shift
th€
dot
position
down
on
the
screEn.
3. R/O
OFF
is
gat€d
low
to
switch
the d€flection
amplifier
inputs
from
the Trace Mode
to
the
Readout
Mode,
using
the
marker
dot
horizontal
and
vertical
signals.
When
the r€trac€
one-shot
tim€s out
aft€r about
5.9
ps,
it's
Q
line
triggers
the
unblanking one-shot
u10168,
which sets
R/o
UNBLANK high for
5ps,
via
DISPLAY MKR
DOT through
U1G}7A. This
refreshes
the
dot.
DISPLAY
MKR DOT also holds DOT
INWL
low
through
CRl013,
until the dot
marker is
drawn. Rl015
and
G1011
slow
the
rise of
DOT INTVL
to
prevent
a
spurious
signal through the
'diode
AND'gate.
o
o
o
o
?
a
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
t
o
o
o
o
o
o
o
o
o
a
o
o
o
o
o
o
o
o
o
O
o
o
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