Theory of Operation-314 Service
through C530 to raise the collector of (2.528 positive by
essentially the same amplitude as that at its emitter, thus
maintaining a fairly constant collector-to-emitter voltage.
This action reverse biases
CR528, temporarily disconnec-
ting the +loo-volt supply. When the fade-positive pulse is
terminated and the emitter of Q528 is pulled negative,
CR529 turns off, disconnecting the bootstrap circuit,
allowing the collector of Q528 to return to its
+loo-volt
level.
Enhance Generator
Writing speed is primarily a function of the writing-gun
beam current density and physical properties of the
storage tube. At very fast sweep speeds, the writing beam
does not charge the scanned portion of the target
sufficiently to shift the target to the stored state, and the
flood-gun electrons discharge the small deposited charge
back down to the rest potential before the next sweep
occurs.
Writing beyond the normal writing speed of the crt is
attained through the process of enhancement or integra-
tion.
The Enhance Generator produces an approximate
10-
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 writing-gun current is required to shift
the scanned section to the stored state.
(253.5, Q537, and associated circutry form a monstable
multivibrator. Operation of this circuit is similar to that
described for the Erase multivibrator. When the
ENHANCE button is pushed in, Q535 has a conduction
path to ground through R535. (2535 saturates and the low
voltage level at (2535 collector keeps Q537 turned off. The
negative-going portion of the enhance trigger pulse from
the sweep circuit is coupled through C532 to switch the
Enhance
multivibrator. (2535 turns off and (2537 turns on.
The collector of (2537 steps down to about -12 volts,
producing a negative-going step that is coupled through
(2535, turning CR533 off. The time that the multivibrator
remains in this state, and thus the pulse width, is
determined by the values of R534 and C535. The setting of
ENHANCE LEVEL control R537A determines the
amplitude of the pulse that is applied to the feedback
amplifier summing point.
The viewing time can be varied from approximately 1
second to approximately 5 seconds.
Auto-Erase Generator
(2545-(2548 and associated circuitry form a monostable
multivibrator. (2545 is an N-channel FET that permits
conduction when its gate is grounded through
R542 and
R5378, VIEWING TIME. Q545 is quiescently conducting
and the low-voltage level at Q545 drain keeps Q548 turned
off.
When the sweep ends, the negative-going pulse from
the Sweep Generator is applied to Q545 gate through
(2540, R540, and C541. Q545 turns off and Q548 turns on.
Q548 collector steps to near ground level. This negative
step at Q548 collector is coupled through C548, ensuring
Q545 cutoff. C548 then begins to discharge through R542
and R537B. After the time determined by the RC value of
C548 and R5378, Q545 gate level recovers sufficiently to
permit
Q545 to turn on. At (2545 turn on, the negative-
going step at Q545 drain turns Q548 off, and the mul-
tivibrator switches back to its quiescent state.
The negative-going step produced at (2545 drain is also
coupled through C546 and CR550, initiating an erase
cycle in the Erase Generator circuit. The stored informa-
tion is therefore erased after the time period determined
by the VIEWING
TIME control.
The negative-going pulse at Q545 drain is also applied
to the Sweep Reset Delay Multivibrator circuit.
Sweep Reset Delay Multivibrator
When the instrument is operating in the Auto Erase
mode, the Sweep Reset Delay acts as a monostable
multivibrator. This delay multivibrator generates a sweep
reset after each erasure.
Q555, Q557, and associated circuitry form a
monostable multivibrator. When the AUTO ERASE button
is pushed in (in Single Sweep mode), Q577 is turned on
and
Q555 is turned off in the quiescent state. C555 is
charged to the voltage difference between Q555 collector
and R5.5843559 junction. The negative-going pulse from
the Auto Erase Generator is coupled through R551, C551,
and CR560 to switch the Sweeo Reset Multivibrator,
turning Q557 off and Q555 on.
Auto Erase
The Auto Erase circuit provides a repetitive trigger
Q555 collector steps down to about -12 volts. This
pulse to the Erase
multivibrator. When the AUTO ERASE negative-going step is coupled through C555, turning
button is pushed in, a trigger is generated after each
CR557 off. After a time determined by R559 and C555
sweep. A Reset Delay multivibrator is also triggered,
(C555 charges through R559), CR557 anode rises above
which resets the Time Base Single Sweep circuit.
its cathode, turning Q557 on and Q555 off. When Q555
3-1
8
REV.
A,
AUG.
1975
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