HP 181A Operating And Service Manual page 13

Section IV
Paragraphs 4-12 to 4-16
then decays to zero volts by action of the flood gun
electrons (surface below first crossover, brought to
flood gun cathode potential). After 100 milliseconds,
the storage mesh is raised to +13.3 volts and held
there for 200 milliseconds. The storage surface fol-
lows to +10 volts by capacitive coupling, but immedi-
ately starts decaying toward zero volts by capturing
flood gun electrons. Atthe end of the 200 milliseconds,
the storage mesh is brought back to +3.3 volts. The
storage surface is consequently. reduced from zero
volts to -10 volts by capacitive coupling.
4-12. Since the write gun electrons reach the storage
surface withenergy much higher than first crossover
energy, they charge the surface in a positive direc-
tion wherever they strike. This charge pattern on the
storage surface remains for a considerable length of
time since the storage material is a very good insula-
tor.
4-13. Those areas of the storage surface which are
charged to near zero volts allow the field created by
the high positive potential on the post accelerator to
"reach through" and capture flood gun electrons, ac-
celerating them to strike the phosphor viewing screen,
thereby causing the phosphor to emit light. Thusthe
pattern of charge on the storage surface is made vis-
ible.
: 4-14. The secondary electrons emitted by the storage
surface where the write gun electrons strike must
charge the surface from its erased potential to about
-9 volts before floodelectrons can be captured by the
RELEASE
ERASE
+ i56v
4+152e7V
*l42.7V
Model 181A/AR
post accelerator. Thus the writing speed of the CRT
could be enhanced by erasing the surface to just below
this "cutoff" level. Thisis whatthe MAX WRITE mode
does. The disadvantages of operating inthis mode are
reduced storage time and reducedcontrast ratio. The
"cutoff potentials of various areas of the storage sur-
face may not be exactly the same. Thus, the back-
. ground illumination may not be uniform when the storage
Surface is erased in the MAX WRITE mode.
4-15. VARIABLE PERSISTENCE.
4-16. Figure 4-4 represents the method of obtaining
variable persistence. The unwritten storage surface
after erasure is at approximately -10 volts. Those
areas of the storage surface which are struck by elec-
trons from the write gun become charged to near zero
volts. A +10 volt pulse applied to the storage mesh
moves the unwritten areas of the storage surface to
near zero voltsandthe writtenareastonear+10 volts.
While at this potential, the written areas of the stor-
age surface attract and capture flood gun electrons,
whichtends to lower the potential of these areas. When
the storage mesh returns toits normal level, the stor-
age surface drops 10 volts. The unwritien areas of
the storage surface return to a -10 volt potentialand
the written areas return to a slightly negative poten-
tial, somewhat lower (more negative) than their ini-
tial value. This decrease in potential reduces the
ability of the post accelerator potential to reach through
and capture flood electrons, thus reducing the trace
brightness slightly.
STORAGE MESH
POTENTIAL.
STORAGE SURFACE
P POTENTIAL
ESEE MEN
IBIA= B-S
Figure 4-3. Storage Mesh and Surface Potentials During Erasure.
4-2
02612-1