Table of Contents
Advertisement
TYPE 1391-B PULSE, SWEEP, AND TIME-DELAY GENERATOR
taneous triggering of both sides of the sweep gate, the sweep
fails to reset, this loop will come to an equilibrium condition,
where the sweep voltage is high and the sweep amplitude com-
parator
is
stable and "locked out". Under these conditions, an
artificial reset trigger must be given the sweep control gate
multivibrator. T h i s i s accomplished by the momentary grounding
of the cathodes of V301 and V302 through the RESET switch
S202, which causes the sweep gate to reverse i t s state.
In c a s e of a "lock out" or a failure of V303, V304 will drive
the grid of V308A to a high positive value, and the cathode re-
sistors of V308 will be damaged. If V303 fails withV309 conduct-
ing, a high current could be drawn through V304 and V309 in
series, damaging these tubes. Because of these possibilities,
the cathodevoltage of V304 must be prevented from remaining in
a highly positive condition for a very long time. Protection is
afforded by a system in which a network consisting of R319,
C337, and R320 establishes a grid voltage normally negative on
V310A. If the sweep circuit fails to reset, C337 charges to
a
voltage sufficiently positive to cause V310 to conduct. When
V310 conducts it decreases the grid voltage of V304 and lowers
the cathode voltage sufficiently to protect a l l tubes and com-
ponent
s.
4.6 PULSE-TIMING CIRCUITS.
The pulse is normally timed by
start and stop triggers produced by amplitude comparators V401
and V402 (see Figure 5.4). These comparison circuits are pro-
vided with d-c reference voltages derived from bleeders in the
30Q-volt regulated supply. A calibrated voltage along R409 sup-
plies V401, the start comparator, while an equivalent voltage
from R410 supplies the stop comparator, V402. The rising sweep
voltage from V304 is connected through precision attenuators to
the left-hand grids of V401 and V402, and these stages trigger
in turn a s the sweep voltage rises to become equal to the ref-
erence voltages. Since the sweep is linear, i t is necessary that
the amplitude comparators draw a constant current from the ref-
erence voltage potentiometer. Hence, both the plate and cath-
ode of the comparator circuit are connected to ganged potentiom-
eters, maintaining the supply potential for the comparators con-
stant regardless of the potentiometer setting. The amplitude com-
parison take-off voltages are calibrated a t both the minimum and
maximum voltages for both the PULSE DELAY and DURATION
controls by R407, R408, R411, and R412. A small amount of un-
regulated power-supply voltage is fed across the amplitude com-
parison voltage pickoff network through R428 and R429. This
connection causes a minute variation i n the d-c amplitude com-
parison potentials with line voltage, and s e r J e s to compensate
for small shifts of pulse position due to line-voltage (and there-
fore heater-potential) changes. As an example of the operation
of two amplitude comparators ( s e e Figure 1.6b), suppose the in-
put sweep voltage is 120 volts in amplitude and that the sweep
duration i s 3 psec. Also, suppose it is desired to start the out-
put pulse with 1 p s e c of delay from the leading edge of the sweep
and with a 1-psec duration. Under these circumstances, the
PULSE DELAY controlwill be s e t a t a potential of 40 volts and
the PULSE DURATION control 40 volts above the PULSE DELAY
control, or a t a potential of 80 volts. The start amplitude com-
parator would then trigger one-third of the way along the sweep
a t 1-psec, and the stop amplitude comparator two-thirds of the
way along the sweep a t 2 p s e c , resulting in a time between trig-
gers produced by these two circuits of 1 p s e c , the desired pulse
duration.
V401 and V402 are Schmitt circuit amplitude comparators
similar in form to the equivalent comparators of the delay and
sweep circuits. The left-hand section of each tube i s normally
off, the right-hand section on. When the rising sweep voltage
reaches the critical level a t which the left-hand side will con-
duct, the circuit regenerates, turning the right-hand side off and
producing a fast-rising positive trigger. This positive trigger is
coupledin the start channel via C403 and R421 to the start-chan-
nelhalf of S401, and applied to the grid of buffer tube V403. The
resulting negative pulse a t the plate of V403 I s inverted by T401
and further amplified and shaped by V405, the start amplifier, t o
a f a s t negative pulse. This pulse flips V501 over to initiate the
pulse. The function of S401 h a s been described in Section 1.
Note that the positive trigger pulse marking the beginning of the
pulse i s produced across R427 by the plate current of V405, and
connected via S401 to the START binding post. The circuit ac-
tion and connections of V404 and V406, the stop-channel buffer
and amplifier, are identical with those of the start channel.
4.7 PULSE-GENERATING CIRCUITS.
4.7.1 GENERAL. The pulse-generating circuits consist of a
bistable multivibrator, identical to those in the sweep and delay
circuits. This multivibrator i s started and stopped by the trig-
gers derived from the pulse-timing circuits or by externally gen-
erated triggers. The bistable multivibrator controls the state of
a bistable push-pull pulse amplifier consisting of two power am-
plifier pentodes, which in turn are directly coupled t o a pair of
driver amplifiers. These amplifiers drive the output-pulse power
output stage. Since the system is both push-pull and direct
coupled throughout, the entire circuit is bistable, and the current
drawn from a l l power supplies is constant. The fact that the out-
put-power amplifier is itself direct coupled to the panel termi-
nals necessitates the use of a number of different power-supply
voltages. These voltages are a l l unregulated, s o there would
normally be a variation in pulse amplitude with line voltage;
however, V511 regulates the screen-to-cathode potential of the
output tubes to decrease the effects of line-voltage variation.
4.7.2 The right-hand side of V501 is normally in conduction s o
that its plate voltage is about
f
140 volts with respect to ground.
This voltage is translated negative 100 volts by the current flow-
ing in V502, s o V501-left is in plate-current cutoff. Since
V501-left is off, the grid of driver V503 is positive and V503 is
on.
When on, V503 draws about 60 ma, producing a drop of 36
volts across R507 and R545. The platevoltage of V503 is trans-
lated negative by the current flowing in V509-left, and driver
V505 is heldin cutoff. Since amplifierV504 is off, the translated
plate voltage of this stage due to plate current in V509-right
causes V506 to conduct. The plate current of V506 throughR528
and R530 holds the gridof output tubeV508 about 25 volts nega-
tive with respect to i t s cathode, and this tube is off. Since V505 is
off, V507 will be a t zero bias and conducting. These are the qui-
escent conditions before a start pulse is fed via D501 to turn
V501-left on. When this happens, a l l tubes on both s i d e s reverse
their conduction states. Thus V507 will be turned off and V508
Advertisement
Table of Contents
