Circuit Description—475
Delayed Latch circuit. The positive-going signal turns Q594
on which in turn turns on Q596. The positive-going signal
at the collector of Q596 is connected to pin 16 of U600 to
start the holdoff period and is also used in the Vertical
Switching Circuit to switch between channels in the ALT
mode of operation.
Z Axis Logic Multivibrator
Q688 and Q698 are basically a Schmitt Multivibrator
and form the Z Axis Logic Multivibrator. Quiescently, with
the CRT display blanked, Q688 is on and Q698 is off. The
signal to switch states of the multivibrator (and thereby
unblank the CRT) can come from the Main Gate Com
parator (via Q588 and CR681) or from the Delayed Gate
Comparator (via Q826 and C684). The negative going
signals turn off Q688 and cause Q698 to turn on. The
positive going signals that return the multivibrator to its
original state (thereby blanking the CRT display) come
from Q588 via CR682 and CR683, or from Q680 via
CR680 and CR683. Q680 is used primarily to ensure
proper early turn-off of the CRT display at faster sweep
rates. In the MIX or A INTEN positions of the HORIZ
DISPLAY switch, some additive unblanking is accom
plished to slightly increase the intensity of the B portion of
the display. In these positions of the switch, —8 volts is
connected to the cathode of either CR691 or CR692. Now,
when the Z Axis signal from the collector of Q826 steps
negative at the beginning of the B portion of the display,
CR693 is made to conduct less current, which makes a
slight amount of additional unblanking current available to
the Z Axis Amplifier (via CR694, R692, and either CR691
or CR692).
Delayed Sweep Latch Circuit
Q788, Q794, and Q796 form the Delayed Sweep Latch
Circuit. The circuit function is used during the three
delayed sweep modes of operation that the instrument is
capable of (M IX, A INTEN, and B DLY'D). Q938A, in the
Delay Pickoff Comparator of the Delaying Sweep Gen
erator, is the current source for the emitters of Q794 and
Q796. Prior to the end of the delay time selected by the A
TIM E/D IV switch and the DELAY TIME POSITION
control, Q938A is off, thereby causing both Q794 and
Q796 to be off also. The base level of Q794 sits at a more
negative level than does the base of Q796 so that when
Q938A turns on, at the delay pickoff point, C749 will turn
on and Q796 will remain off. The resultant positive
movement at the collector of Q794 is connected to the B
Trigger TD Reset Circuit.
The end-of-sweep pulse from the Non-Delaying Sweep
Generators connects to the emitter of Q788. When this
pulse steps positive at the end of a delayed sweep,
Q788 turns off and the current through R788 pulls the base
of Q794 positive. This turns off Q794 and causes Q796 to
turn on. The circuit remains in this state until the Delaying
Sweep Generator ends its sweep and resets the Delay
Pickoff Comparator. In the B ENDS A position of the A
TRIG HOLDOFF control +5 volts is connected to R798.
This enables the diode gate composed of CR799 and
CR592. Now, when the end-of-sweep pulse signals the end
of a delayed sweep, the positive movement at the collector
of Q796 is connected to the base of Q594. This starts the
beginning of holdoff at the end of the delayed sweep
instead of waiting until the end of the delaying sweep. This
mode of operation gives the fastest possible sweep repeti
tion rate when operating in the delayed sweep mode.
B Trigger TD Reset Circuit
Q790 and Q792 are connected as a voltage comparator
and form the B Trigger TD Reset Circuit. At all times other
than when the Non-Delaying Sweep Generators are running,
Q790 is on, which holds the trigger tunnel diodes in the B
Trigger Generator circuit in their LO states. At the end of
the delay time selected by the A TIM E/D IV switch and the
DELAY TIME POSITION control, the positive movement
at the collector of Q794 turns on Q792 and causes Q790 to
turn off. The tunnel diodes in the B Trigger Generator are
capable of being biased into their HI states. Also, if the B
Triggering SOURCE switch is in the STARTS AFTER
DELAY TIME position, the negative movement at the
collector of Q792 reverse biases CR811, letting the —8 volt
supply pull down on the base of Q804 in the Delayed Gate
Comparator. This turns on Q804 and causes Q802 to turn
off, which initiates a delayed sweep.
Delayed Sweep Override Amplifier
Q798 is the Delayed Sweep Override Amplifier. It is
possible, with the right combination of control settings, to
achieve a delayed sweep presentation where the delayed
sweep would normally want to continue running after the
end of the delaying sweep. For instance, if the TIM E/DIV
switches are set only one or two ranges apart and the
DELAY TIME POSITION control is set to a 8.50 setting.
Under these conditions the delaying sweep end-of-sweep
pulse occurs before the delayed sweep end-of-sweep pulse.
The positive movement on the collector of Q588 at the
time of the delaying sweep end-of-sweep pulse turns Q798
on. The negative movement on the collector of Q798 pulls
down on the collectors of Q794 and Q796, through CR793
and CR792 respectively, which in turn pulls down on the
base of Q792. This turns off Q792 and causes Q790 to turn
on, which resets the B Trigger Tunnel Diodes to their LO
states, thereby terminating the delayed sweep.
A +GATE And B +GATE Amplifiers
Q584 and Q824 are the A +GATE and B +GATE
amplifiers respectively. They provide the +GATE output
signals available at the instrument rear panel. These output
gate signals are positive-going rectangular waveforms, ap
proximately 5 volts in amplitude, coincident with their
respective sweep waveforms.
®
3-11
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