Circuit Description-5110
Z-Axls
Amplifier
The
Z-Axis Amplifier is a current driven shunt-feedback
operational amplifier with
a voltage
output, and consists of
Q222, Q226,
and Q234. The feedback path is from the
collectors
of Q226 and Q234 through
R227-C227 to the
base
of
Q222. Q226
and Q234 are connected as a
collector-coupled
complementary amplifier to provide a
fast, linear
output
signal
while consuming minimum
quiescent
power. The output voltage provides the drive
signal
to control the crt intensity level through
the
Control-Grid
Supply.
The
output level of the Z-Axis Amplifier is established
by the
voltage
drop across R227 in
reference to virtual
ground
at
the base of
Q222
(the operational amplifier
summing point). The current through R227 is determined
by the input current
from any combination of several
sources,
such
as from the front-panel INTENSITY
control,
plug-in interface (blanking
intensification, etc.), and from
Q214.
Q214
is an operational amplifier with two inputs;
one
from
the
front-panel
EXT INTENSITY
INPUT
connec
tor
and the other from
the front-panel BEAM FINDER
switch.
It
sets those
input signals to a level suitable for
proper response by the Z-Axis Amplifier.
High-Voltage
Regulator
High-Voltage
Primary.
A
repetitive, non-sinusoidai
signal is produced by a phase-modulated switching
circuit
in
the
primary of T240
and induced into the secondaries.
Current
drive
for the primary winding is
furnished by Q252
in
its
conduction
state.
Q252 is turned on
by positive
going
feedback
applied
through C259
and L259 from the
feedback
winding, and then
turned off by switching action
from
Q262.
Asampleofthe output
de
voltage is modulated
by
the
ac voltage from another feedback winding at the
gate
of Q278 to
establish the conduction time of Q252and
thus
maintain the proper output level. Q252 delivers
energy
to T240 only once each cycle.
Assuming Q262andQ264are
off initally, R262 provides
base
drive for Q252, causing it
to deliver current to
T240
primary. As
Q252
conducts,
the increasing current
through
the
primary winding induces a voltage into the
secondaries.
The gate of Q278 is driven negative by the
voltage from
the feedback winding,
switching Q264
and
Q262
on.
With
conduction of Q262,
base drive for Q252 is
removed.
With
Q252 off, the transformer field collapses, rever
sing the polarity of the voltage induced
into the
secon
daries.
When the
gate of Q278 is driven sufficiently
positive to switch Q264and Q262 off, Q252 is switched on
again.
Q252
again
delivers
energy to the primary
winding
and
the
action is
repeated.
High-Voltage
Regulation.
Regulation
is
accomplished
as follows: Feedback from the
—3400-volt cathode supply
is summed
with low-voltage
levels through the
voltage
divider
consisting
of resistors
R272B-E, R275, and R276
to
establish
the
de level at the gate of Q278. The ac
component,
which
is
the
switching
signal, is derived from
the transformer
as described previously. If the output level
of
the cathode supply drops below the nominal
—3400
volts
(becomes
more positive), the level at the gate of Q278
rises.
A new
point
is selected on
the varying ac component to
cause switching
of
Q262-Q264 later and
hence increase
conduction time
of Q252. This allows more energy to
be
delivered to the primary winding
of T240, resulting in an
increase
of
voltage in the
secondaries. Conversely, if the
output level increases,
Q252 is allowed to conduct for a
shorter
length of time.
The de level at the gate of Q278 is
adjusted
by High Volts Adjust R275 to set the output at
exactly
-3400
volts.
High
Voltage Outputs
Transformer T240
has two high-voltage output win
dings which
provide the potentials required for the crt
cathode
and control grid supplies. The —3400-volt
accelerating potential
for the cathode is supplied by half
wave rectifier
CR247. The cathode
heater is elevated to the
cathode potential through
R273.
Half-wave
rectifier
CR241
provides about -3450 volts
to establish
bias
voltage
on
the crt control grid. This
voltage (and
hence
the crt beam current)
is dynamically
controlled by
the Z-Axis Amplifier, which contains the
INTENSITY
control,
blanking inputs, and intensification
inputs.
Intensity Range R245
provides afine adjustment
of
the
quiescent grid voltage
to bias the crt
just below cutoff
when
the Z-Axis Amplifier output is at its minimum
quiescent
level
(INTENSITY control
counterclockwise
and
no intensifying or blanking inputs).
Neon bulbs
DS271, DS272, and DS273 provide protec
tion
to the crt
if the voltage difference between the
control
grid and
the
cathode exceeds about 180 volts.
Crt
Control Circuits
In
addition
to the INTENSITY control discussed
previously, front-panel
FOCUS and internal Astigmatism
controls
have
been incorporated for arriving
at an op
timum
crt display. FOCUS
control R295 provides the
correct
voltage
for the second anode in the crt. Proper
voltage
for the third anode Is
obtained by adjusting Astig
control R286. In
order to obtain optimum spot size and
shape, both
the FOCUS and Astig controls
are
adjusted to
provide the proper
electrostatic
lens configuration in the
crt.
5-3
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