Theory of Operation-2445A/2455A Service
output levels from the Focus Amplifiers; that,
in
turn,
changes the convergence characteristics of the quadrapole
lenses.
Initially, at the time of adjustment, the
FOCUS
and
ASTIG potentiometers
are
set for optimum focus of the crt
beam
at low intensity. After that initial adjustment, the
ASTIG pot normally remains
as
set,
and
the
FOCUS
con-
trol is positioned
by
the user
as
required when viewing the
displays.
When
using the
FOCUS
control, transistor
01852
is
controlled as described above; however,
an
addi-
tional current is also supplied to the base node of 01851
from the
FOCUS
pot through R1855. This additional
current varies the base-drive current to 01851
and
pro-
vides tracking between the two lenses
as
the
FOCUS
con-
trol
is
adjusted during use of the instrument.
Z-AXIS
OUTPUT
(VZ
OUT)
+BV
TO
+73V
+BV
AC
DRIVE
C1950
FROM
PIN
7
OF
T1970
ov
-150V
CR1990
+15V
+42V
6RID
BIAS
+42V
The
convergence strengths of the quadrapole lenses
also dynamically track changes
in
the display intensity.
The
VO
OUT signal, applied to the crt at pins 5 and
6,
is
exponentially related to the
VZ
OUT (intensity) signal driv-
ing
the crt control grid
and
increases the strength of the
lenses more at higher crt
beam
currents.
(A
higher beam
current requires a stronger lens to cause
an
equal conver -
gence of the
beam.)
DC
Restorer
The
DC
Restorer provides crt control-grid bias
and
cou-
ples both the
de
and
the low-frequency components of the
Z-Axis
drive signal to the crt control grid. This circuit
allows the
Z-Axis
Amplifier to control the display intensity
by
coupling the low-voltage
Z-Axis
drive signal (VZ OUT)
- - - -1900V
PARTIAL
U1830
-1985V
D
CONTROL
GRID
3
DSBO
CATHODE
DS91
2
6019-11
Figure
3-9.
De
restorer
circuit.
3-36
To Next Page
