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Theory
4-14.
After the
erase cycle,
previously written in-
formation has been removed
and the storage
surface
is
ready to store
new information. Write gun
electrons
(with
much higher than
crossover energy) charge
the
storage surface
in
a
positive direction
only
in the
areas
where they
strike
the storage surface. Flood
electrons
pass
through
these
areas and
are pulled
to
the phos-
phor viewing
screen
by the high potential on the
post
accelerator.
4-15.
VARIABLE PERSISTENCE.
4-16.
It
is possible to disable the
storage elements
in the
CRT so
that the write gun beam writes directly
on
the phosphor viewing
surface.
In
such a
mode,
persistence would depend only on the type phosphor.
Model 1331A does not use this mode of operation;
it uses
a
method that permits the persistence to be
varied.
Variable persistence
CRTs
require some
method of removing
stored
information; otherwise,
all
information written on the storage
surface
remains
there
until the ERASE pushbutton is pressed or until
the natural
storage
time
(in
excess
of
15 minutes)
elapses.
4-17.
Figure
4-4 graphically
shows how
variable
persistence is
obtained.
The left
side
of the
graph
shows
that
the storage
mesh is
fixed at +4 volts.
The shaded area shows the
potential
of
both
the
unwritten
and written areas of the storage
surface.
In
the
write mode, short
erase
pulses
are applied to
the storage
mesh
and the storage surface
is
capaci-
tively
increased
10 volts for
the duration of the
pulse.
While
at
this increased potential,
the written areas
of the storage surface attract and ciwture
flood
gun
electrons.
This tends to lower the potential
of
the
written
areas
because the capacitance between the
storage
mesh and the
storage surface charges
to-
ward 0
volt.
At the
end
of the pulse, the
storage
mesh
returns
to its normal level and the
storage
sur-
face
drops 10
volts.
The unwritten
areas of
the storage
surface
return to -10
volts
and the written
areas
return to
a
potential
slightly
more negative than their
initial potential. This decrease in potential reduces
the ability of
the post
accelerator
to reach through
and capture
flood gun
electrons, and
trace brightness
is slightly reduced.
4-18. A train
of erase
pulses gradually
erases
the
written trace
as
shown
by the shaded areas in figure
4-4. The persistence of the written trace is varied
by varying the repetition rate of the pulses. A side
effect, noticible under some conditions, is
a
light
background glow on
the
CRT. While the storage
mesh is pulsed positive,
flood
electrons pass through
all
areas
of the mesh to the phosphor viewing
screen,
evenly
exciting the entire surface.
4-19.
STORAGE.
4-20.
In
store mode, erase pulses are not
applied
to the storage mesh, and persistence (store time) is
4-2
Model
1331A
iimited primarily by
an effect called fade positive.
This occurs on the storage surface and
is
caused by
ionization
of
residual
gas molecules by flood gun
electrons.
The
storage surface gradually charges more
positive and allows flood electrons to penetrate to the
phosphor viewing area where they
increase back-
ground
illumination until
the display is obscured. Fade
positive is reduced
(and store time
increased)
in
store
mode
by keeping
the flood gun turned off except
for brief
periods
to refresh the display.
4-21.
STORAGE PRECAUTIONS.
4-22.
When
write gun electrons strike the storage
surface,
they
cause a very
rapid increase in
po-
tential.
The
energy from this sudden change is con-
verted
to heat
and, when
it
exceeds the
heat
dissi-
pation rating
of the storage surface, burns the
di-
electric
material. The burning
action
is
visible as
trace
blooming.
The
intensity
level of the
write
gun
should always
be
set
below
the point where
bloom-
ing
occurs.
4-23. BLOCK
DIAGRAM
DISCUSSION.
4-24. This discussion illustrates the relationships of
the circuit groups
(blocks)
to each other.
It
is based
on
the
overall
block
diagram, figure 4-6.
4-25. LOW VOLTAGE POWER SUPPLIES.
4-26.
This block
provides power to all the other
blocks, including the
high voltage
power supply.
Out-
puts
at four de voltages are supplied: +50V,
-50V,
+12.5V, and +158V.
The -50V
supply
is
adjustable
and all
the
other
low
voltage supplies are
referenced
to
it.
The
+
12.6V
supply
is
also adjustable.
4-27. HIGH
VOLTAGE POWER
SUPPLY.
4-28.
The high
voltage power supply
provides
the
high
voltage potentials to operate
the
CRT: -2915
volts
for:
the
write
gun cathode, approximately -2955
volts for
the
write gun
grid,
and +7.5 kilovolts for the
post accelerator. The
write gun
intensity limit
adjust-
ment, hv adj,
and the
FOCUS
control are part of the
high voltage power
supply.
The
gate amplifier con-
trols the
intensity
and
blanking functions
of
the
CRT
by
controlling
the
voltage to the
write
gun grid.
4-29.
X- AND Y-AXIS AMPLIFIERS.
4-30.
The x- and
y-axis amplifiers are nearly
identical
and serve
to drive
the CRT
horizontal
and vertical
deflection plates. POSITION
and
BEAM FIND
con-
trols
are
included
in these
blocks
as
well
as the gain
and
phase shift
adjustment.
)
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