Theory of
Operation-2445A/2455A
Service
The
potential difference between the control grid
and
the cathode controls the
beam
current
and
thus the
display intensity. With
no
Z-Axis signal applied
(INTENSITY control off), capacitor D will
be
charged to its
maximum negative value,
since
the difference between the
two clamped voltage levels
is
at its maximum
value.
This
is the minimum intensity condition
and
reflects the setting
of the
Grid
Bias potentiometer. During calibration,
the
Grid
Bias pot
is
adjusted so that the difference between the
upper clamping
level
(set by the Grid Bias pot)
and
the "no
signal"
level
of the Z-Axis drive signal
(VZ
OUT)
produces
a control grid bias that barely shuts off the crt electron
beam.
As the INTENSITY control
is
advanced, the amplitude
of the square-wave
Z-Axis
signal increases accordingly.
This increased signal amplitude decreases the difference
between the upper
and
lower clamped levels of the ac
waveform,
and
less charge
is
added
to capacitor
D.
The
decreased voltage across capacitor D decreases the
potential difference between the control grid
and
the
cathode,
and
more crt
beam
current
is
allowed to flow.
Increased
beam
current increases the crt display intensity.
During the periods that capacitor C is charging and
discharging, the control-grid voltage
is
held
stable by the
long-time-constant discharge path of capacitor D through
resistor F. Any charge removed from capacitor D during
the positive transitions of the ac waveform will
be
replaced
on
the negative transitions.
The fast-rise
and
fast-fall transitions of the
Z-Axis
sig-
nal
are coupled to the crt control grid through capacitor
D.
This ac-coupled fast-path signal quickly sends the crt elec-
tron
beam
to the new intensity
level,
then the slower
DC
Restorer path "catches up· to handle the
de
and
low-
frequency components of the
Z-Axis
drive signal.
Neon
lamps
DS90
and
DS91
prevent arcing inside the
crt should the control grid potential or cathode potential
be
lost for any reason.
CRT Control Circuits
The CRT Control circuits provide the various potentials
and
signal attenuation factors that set
up
the electrical
ele-
ments of the crt.
The
control circuitry
is
divided into two
separate categories:
(1)
level
setting
and
(2)
signal han-
dling.
The
level
setting circuitry produces voltages
and
current
level
necessary for the crt to operate, while the
signal-handling portion
is
associated with changing crt sig-
nal
levels.
LEVEL-SETTING CIRCUITRY. Operational amplifier
U1890B, transistor 01980,
and
associated components
3-38
form
an
edge-focus circuit that sets the voltages
on
the
elements of the third quadrapole lens.
The
positive
lens
element
is
set to its operating potential by
Edge
Focus
adjustment pot R1864 (via R1897). This voltage is also
divided
by
R1893
and
R1982
and
applied to the non-
inverting input of U1890B to control the voltage
on
the
other element of the lens.
The operational amplifier
and
transistor
are
configured
as
a feedback amplifier, with
R1891
and
R1990 setting the
stage gain.
Gain
of the amplifier
is
equal to the attenuation
factor of divider network
R1893
and R1892, so total
overall gain of the stage from the wiper of R1864 to the
collector of 01890
is
unity.
The
offset voltage between
lens elements
is
set
by
the ratio of
R1891
and
R1990
and
the + 10 V reference applied to R1990. This configuration
causes the two voltages applied to the third quadrapole
lens to track
each
other over the entire range of
Edge
Focus adjustment pot R1864.
Other adjustable level-setting circuits include Y -Axis
Alignment pot R1848, used to rotate the
beam
alignment
after vertical deflection. This adjustment controls the
amount of current through the Y -Axis Alignment coil
around the neck of the crt
and
is
set to produce precise
perpendicular alignment between x-
and
y-axis deflections.
The
TRACE ROTATION adjustment
R975
is
a front-panel
screwdriver-adjustable control. The effect of the adjust-
ment is similar to the Y-Axis Alignment pot, but when
adjusted, it rotates both the x-axis
and
the y-axis
deflections
of
the trace
on
the face
of
the crt. A final ad-
justable level-setting control
is
the Geometry pot R1870,
adjusted to optimize display geometry. The potential at
pin
8 for the vertical shield internal to the crt is produced
by
zener diode
VR1891
and
associated components.
SIGNAL-HANDLING CIRCUITRY. The crt termination
adjustment
R1501
is
set to match the loading
characteristics of the crt's vertical deflection structure to
the Vertical Output Amplifier.
LOW VOLTAGE POWER SUPPLY
The
low voltages required
by
the instrument
are
pro-
duced by a high-efficiency, switching power supply. This
type of supply directly rectifies
and
stores charge from the
ac line supply; then the stored charge is switched through
a special transformer at a high rate, generating the various
supply voltages.
Line Rectifier
Ac line voltages of either
115
V or 230 V may provide
the primary power for the instrument, depending
on
the
setting of LINE VOLTAGE SELECTOR switch S90
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