Single Sweep Operation; Alternate Operation - Tektronix 7B51 Instruction Manual

Circuit Description--Type 71351
During sweep time the voltage on the holdoff capacitor
reverse-biases CR579 and the current in 13573 switches to
CR572, turning off Q468 . VR573 determines the time
when the current switch takes place. During holdoff time,
holdoff disconnect diode CR571 controls the current avail-
able to the reset control amplifier, 0468 . The positive-going
edge of the holdoff gate turns CR571 on . This moves the
emitter of Q468 positive, turning it off . At the end of hold-
off time the negative-going edge of the holdoff gate opens
CR571, making the current through 13468 available to the
reset control amplifier.
Reset Control Amplifier. Q468 provides current to the
base of Q436 to switch the sweep reset multivibrator to its
reset state. The current through R468 into the emitter of
0468 is controlled by CR571 and CR572 as stated above,
or by CR468 as explained under single-sweep operation .
Sweep Reset Multivibrator. The sweep reset multivibra-
tor, 0432-Q436, resets the sweep gate TD at the end of
holdoff time . In the reset state, Q432 is off and 0436 is
conducting . The junction of 13437-13438 is pulled up to its
high level . The sweep is terminated by current from Q406
moving the base of 0436 positive to cutoff . This switches
the multivibrator to its other stable state; 0432 on and
Q436 off. The voltage at the junction of Q437-13438 drops
negative to its low-level . This negative step is applied to the
sweep gate stage to latch the sweep gate TD in its low-
voltage state. At the end of holdoff time 0468 is turned on,
driving the base of Q436 less positive . This turns Q436 on
and the multivibrator switches back to its reset state. The
voltage at the junction of 13437-13438 is pulled up to its
high level. This positive-going step is applied to the Sweep
Gate Stage to unlatch the Sweep Gate TD, and the sweep
circuits are ready for the next sweep to start.
Single Sweep Operation
General. Operation of the Sweep Generator Circuit in
the SINGLE
SWEEP
positiop of the TRIGGERING MODE
switch is similar to operation in the other modes. However,
after one sweep has been produced, the Sweep Reset Multi-
vibrator Stage does not reset. All succeeding trigger pulses
are locked out until the RESET button is pressed.
In the SINGLE SWEEP position of the MODE switch,
the --15 volts is removed from the top of 13464 . CR466 is
opened and CR468 becomes forward-biased, opening a cur-
rent path through 13468-CR468-13466 . Now, at the end of
holdoff, there is no current through 0468 to pull down the
base of 0436, and the Sweep Reset Multivibrator does not
reset. The circuit remains in this condition until reset by
the Single Sweep Reset Amplifier Stage.
Single Sweep Reset Amplifier . The Single Sweep Reset
Amplifier produces a negative pulse to reset the Sweep Re-
3- 1 0
set Multivibrator stage so another sweep can be produced in
the SINGLE SWEEP mode of operation.
Quiescently, Q454 is conducting hard and Q456 is held
cut off. When the RESET button is pressed, CR451. is
turned on . This clamps the junction of CR451-CR453 at
+0 .6 volt. CR453 and CR454 open and the base of Q454
drops to zero volts, turning it off. This turns Q456 on hard,
pulling down its collector voltage . This negative-going step
applied to C457-CR459 produces a negative pulse at the
base of Q436 which is sufficient to reset the sweep reset
multivibrator. This enables the sweep gate TD, and the next
trigger pulse will produce another sweep on the display .
C456 provides positive feedback with a long time-constant
to keep 0454 off long enough to prevent switching tran-
sients from producing multiple sweeps .
Lamp Driver . In the SINGLE SWEEP MODE, the base
of Q446 is connected through CR446, CR448 and CR449
to the collector of Q436 . When 0436 is cut off, during
holdoff time and until the RESET button is pressed, the
base of Q446 is biased to cutoff . When the RESET button
is pressed, 0436 is turned on and its collector voltage rises.
This positive-going voltage applied to the base of 0446
turns it on (C446 controls turn-on time) . 0.446 collector
voltage drops and the READY lamp turns on . The READY
lamp remains on until the end of the sweep when Q436 is
again turned off.
Alternate Operation
In alternate operation the indicator oscilloscope provides
a sweep inhibit current to the emitter of Q410 . Q410 pro-
vides a low impedance input and a voltage drive for the in-
hibit comparator, Q416 and 0424 . A sweep inhibit current
input of 1 mA or greater turns Q410 on . This raises the
base voltage of Q416, turning it on and turning 0424 off .
The collector current of Q416 pulls the base of Q432 in a
negative direction turning, Q432 on and Q436 off, to latch
the sweep gate TO as described previously .
Delayed Sweep Operation
The Type 71351 can be used in the right horizontal (B)
compartment of the indicator oscilloscope as a delayed
sweep time-base . In this position, the delay pickoff circuit
and the B DELAY MODE switch will be inoperative .
In this mode of operation, the time-base in the left hori-
zontal (A) compartment provides the sweep inhibit current
to the emitter of Q410 . The sweep inhibit circuit operates
as described under Alternate Operation . The A time-base
also provides a delay mode control voltage to the anodes of
CR462 in the sweep generator and CR301 in the trigger
generator .
The delay mode control voltage causes CR462 to con-
duct CR463 to turn off, CR464 to conduct and CR466 to
turn off. This puts the B sweep in single sweep mode as de-
scribed under Single Sweep Operation .