Circuit Description—434
Erase Generator
NOTE
The following description applies to both erase
generators; however, the circuit numbers used are
those o f the upper circuit.
In order to erase the stored display, a fade-positive pulse
is first applied to the storage target backplate. This
increases the potential difference between the flood gun
cathodes and target backplate, raising the operating level
above the upper w riting lim it and w riting the entire target
area w ith flood gun electrons. Next, the backplate voltage is
pulled negative, well below the retention theshold. Then as
the backplate is gradually returned, the target is charged to
the rest potential and returned to the ready-to-write state.
The following paragraphs describe how the erase waveform
is generated.
The Erase M ultivibrator is composed o f Q1210, Q1216,
and their associated circuitry. This is a monostable m u lti
vibrator w ith Q1210 quiescently saturated and Q1216
biased off. The collector of Q1216 is clamped slightly
above ground by the conduction of CR1219. C1214 is
charged to the voltage difference between the junction of
R1 214-R 1212 and the collector level of Q1216.
When the ERASE button is pushed, the contacts of
S90A are closed, grounding R1203. This produces a
negative-going step which turns Q1210 off and Q1216 on.
The collector of Q1216 moves down very close to —15
volts as the transistor saturates and conducts current
through R1 218 and R1239. The output of the feedback
am plifier steps positive pulling the target backplate w ith it.
This increases the operating level of the CRT and the entire
target area is written.
When Q1216 turns on, the negative-going step produced
at its collector is also coupled through C1214, which turns
CR1212 off, ensuring cutoff of Q1210. Cl 214 begins to
discharge through R1214, and after an RC-controlled
period of time the current through R1214 has diminished
sufficiently to allow the voltage at the anode of CR 1212 to
rise above the turn-on level. The base of Q1210 is also
raised to the turn-on level, and the m ultivibrator is switched
back to its quiescent state.
While Q1216 is conducting, the charge on C1219 is
removed. When G1216 turns off, its collector rises rapidly
and is clamped slightly above ground by CR1219. This
produces a positive-going step which is coupled through
C l219, reverse biasing CR1220. This positive movement is
applied to the input of the feedback amplifier, causing the
output to step sharply negative well below the rest poten
tial. As C1219 charges, the voltage at the junction
of R1220-R1222 decays at an RC-controlled rate until
CR1220 turns on and clamps it at about —15.5 volts. This
negative-going sawtooth voltage is applied to the feedback
am plifier, which produces a positive-going sawtooth at its
output to raise the backplate to the ready-to-write state.
When the CRT is shifted from the conventional mode to
the store mode, pushing the store button grounds C l200,
which produces a negative trigger to switch the Erase M u lti
vibrator to prepare the target for storage by applying an
erase waveform. Bootstrapping maintains operating voltage
fo r Q1232 and Q1235 during the fade-positive portion of
the erase waveform when the em itter of Q1235 is pulled
positive. The voltage drop across zener diode VR1236 sets
the base of Q1238 approximately 120 volts below the
em itter of Q1235. This voltage drop is kept constant under
dynamic conditions by the essentially constant current
established through R 1236, which is clamped by the Q1238
forward bias voltage. When the em itter of Q1235 is
suddenly stepped positive by the erase waveform, the base
of G1238 is stepped positive by the same amplitude. Q1238
em itter follows the base, and the positive-going step is
coupled through C l235 to raise the collector of Q1235
positive by essentially the same amplitude as that at its
em itter, thus maintaining a fairly constant collector-to-
em itter voltage. This action reverse biases CR1235, tempo
rarily disconnecting the -i-300-volt supply. When the fade
positive pulse is terminated and the em itter of Q1235 is
pulled negative, CR1237 turns o ff, disconnecting the boot
strap circuit and allowing the collector of Q1235 to return
to its +300-volt level.
Enhance Generator
W riting speed is prim arily a function of the w riting gun
beam current density and physical properties of the storage
tube. A t very fast sweep speeds, the w riting beam does not
charge the scanned portion of the target sufficiently to shift
it to the stored state, and the flood gun electrons discharge
the small deposited charge back down to the rest potential
before the next sweep.
Writing beyond the normal w riting speed o f the CRT is
attained through the process of enhancement or integra
tion. First to be discussed w ill be enhancement.
The enhance generator produces an approximate two-
millisecond negative-going pulse which is applied to the
feedback amplifier summing point, resulting in a positive
going pulse to the target backplate. This conditions the
target so that less w riting gun current is required to shift
the scanned section to the stored state.
Q1245, Q1246, and their associated circuitry form a
monostable multivibrator. Operation of this circuit is
3-28
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