to the elevated crt control-grid potential (about
-1
.9 kV).
The
DC
Restorer circuit (Figure 3-9) operates
by
impressing the crt grid bias setting
and
the Z-Axis drive
signal
on
an
ac
voltage waveform. The shaped
ac
waveform is then coupled to the crt control grid through a
coupling capacitor that restores the
de
components of the
signal.
GRID
BIAS
LEVEL.
An
ac
drive voltage of
approximately 300 V peak-to-peak
is
applied to the
DC
Restorer circuit from
pin
7 of transformer T1970. The
negative half cycle of the sinusoidal waveform
is
clipped
by
CR1953,
and
the positive half cycle
(150
V peak)
is
applied
to the Junction of CR1930, CR1950,
and
R1941
via
R1950
and
R1953. Transistor 01980, operational amplifier
U1890A,
and
associated components form a voltage
clamp circuit that limits the positive swing of the
ac
waveform at the junction.
Transistor 01980
is
configured
as
a shunt-feedback
amplifier, with
C1991
and
R1994
as
the feedback ele-
ments. The feedback current through R1994 develops a
voltage across the resistor that is positive with respect to
the
+42
.6 V on the base
of
the transistor. The value of
this additive voltage plus the diode drop across CR1950
sets the upper clamping threshold. Grid Bias potentiome-
ter R1878 sinks varying amounts
of
current away from the
base node
of
the transistor and thus sets the feedback
current through R1994. The adjustment range of the pot
can
set the nominal clamping level between +
71
V
and
+133
V.
When
the amplitude of the
ac
waveform
is
below the
clamping threshold, series diode CR1950 will
be
reverse
biased
and
the
ac
waveform
is
not clamped. During the
time the diode
is
reverse biased, transistor 01980
is
kept
biased
in
the active region
by
the charge retained on
C1971
from the previous cycle. As the amplitude
of
the
ac
waveform at the junction of CR1930
and
CR1950 exceeds
the voltage at the collector of 01980, diode CR1950
becomes forward biased,
and
the ac waveform
is
clamped
at that level. Any current greater than that required to
maintain the clamp voltage will
be
shunted to the
+42
V
supply
by
transistor 01980.
Operational amplifier U1890A sinks a time-dependent
variable current away from the base node of 01980 that
modifies the crt control-grid bias during the first few
minutes of instrument operation. The circuit compensates
for the changing drive characteristics of the crt
as
it
warms up.
At power-up, capacitor C1990 begins charging through
R1991
toward the + 15 V supply. The output of U1890A
Theory
of
Operation-2445A/2455A Service
follows the rising voltage
on
pin
3;
and
after about ten
minutes (for
all
practical purposes), it reaches +
15
V. As
the output voltage slowly increases, the charging current
through R1992 causes the Grid Bias voltage to gradually
lower about ten volts from its power-on level. The charge
on
C1990 dissipates slowly; therefore, if instrument power
is
turned off
and
then immediately back on again, the out-
put of U 1890A will still
be
near the +
15
V limit rather than
starting at zero volts
as
when the crt was cold.
Z-AXIS
DRIVE
LEVEL. The variable-level Z-Axis signal
(VZ OUT) establishes the lower clamping level of the ac
waveform applied to the High Voltage Module.
When
the
amplitude of the waveform drops below the Z-Axis signal,
CR
1930 becomes forward biased,
and
the ac waveform
is
clamped
to
the Z-Axis signal level. The VZ OUT
level
may
vary between
+8
V
and
+
75
V,
depending on the setting
of the front-panel INTENSITY
and
READOUT INTENSITY
controls.
The
ac
waveform, now carrying both the grid-bias infor-
mation
and
the Z-Axis drive information,
is
applied to a
DC
Restorer circuit
in
the High Voltage Module where it is
raised to the high-voltage levels of the crt control grid.
DC
RESTORATION. The
DC
Restorer circuit
in
the
High Voltage Module
is
referenced to the crt cathode
voltage via a connection within U1830. Capacitor C
(in
Figure 3-9), connected to pin
15
of
U1830, initially charges
to a level determined
by
the difference between the
Z-Axis
signal level and the crt cathode potential.
The
Z-Axis
signal sets the
level
on
the positive plate of capacitor C
through R1920, CR1930,
and
R1941
; the level
on
the
negative plate is set
by
the crt cathode voltage through
resistor E
and
diode A. Capacitor D
is
charged to a similar
de
level
through resistors
F,
R1922,
and
R1913.
When the ac waveform applied to pin 15 begins its
transition from the lower clamped level (set by the
Z-Axis
signal) towards the upper clamped
level
(set
by
the Grid
Bias potentiometer), the charge on capacitor C increases.
The additional charge
is
proportional to the voltage
difference between the two clamped voltage levels.
When
the
ac
waveform begins itsits transition from the
upper clamped
level
back to the lower clamped level, diode
A becomes reverse biased. Diode B becomes forward
biased,
and
an
additional charge proportional
to
the nega-
tive excursion of the ac waveform (difference between the
upper clamped
level
and
the lower clamped level)
is
added
to capacitor D through diode B
and
resistor
G.
The
amount of change added
to
capacitor D depends on the
setting of the front-panel INTENSITY control,
as
it sets the
lower clamping
level
of the ac waveform. This added
charge determines the potential of the control grid with
respect to the crt cathode.
3.37
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