Delayed Sweep Operation; Horizontal Preamp; Readout Block - Tektronix 7B70 Time Base Instruction Manual

Circuit Description-7B7O (SN B20500 and Up)
Delayed Sweep Operation
In this mode of operation the time-base in the left hori-
zontal (A) compartment provides the sweep inhibit current
to the emitter of 0410. The sweep inhibit circuit operates
as described under Alternate Operation. The A time-base
also provides a delay mode control voltage t o the anode of
CR462 in the sweep generator and CR357 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 a s
described under Single Sweep Operation,
During sweep inhibit time 0424 is turned off. This turns
off CR428, and with CR463 off the current from R428
turns on CR469. C469 now charges to about +1.5 volts,
turning on 0468. This does not reset the sweep which is
locked out by the conduction of 0416.
When sweep inhibit ends 0416 turns off, making the
base of 0432 more positive. 0424 turns on to open CR469.
C469 holds i t s charge long enough to allow 0468 to reset
the sweep at the end of inhibit time. This action allows
only one sweep after each time the sweep is inhibited.
The delay mode control voltage is also connected to the
sweep gate TD, VR358, through C5357 and R356. With
the A time-base B DELAY MODE in B TRIGGERABLE
AFTER DELAY, the control voltage is about +3 volts,
which does not cause CR357 to conduct. Thus, the sweep
will wait for the first trigger after the delay interval to run.
With the A time-base B DELAY MODE in B STARTS
AFTER DELAY the control voltage is about +4.5 volts.
This is sufficient to cause CR357 to conduct and increase
the current through R356 to the sweep gate TD, VR358.
Now the TD will change to its high-level state and start the
sweep a s soon a s it is enabled by the sweep reset multi-
vibrator at the end of inhibit current to 0410.
When the INDEPEIUDENT mode is selected by the A
Plug-in DELAY MODE switch, the sweep inhibit is held at
about the 0 mA level. The delay mode control voltage is
clamped to ground. The A time-base has no control over
the B time-base and each operates as an independent Time-
Base.
HORIZONTAL PREAMP
Operational Amplifier, U752, provides horizontal posi-
tioning control current to 0784, one side of paraphase
amplifier 0784-0794. Current is determined by settings of
horizontal POSITION controls R774A and R774B. A
positive-going sawtooth from 0534 drives the base of
0794, the other side of .the paraphase amplifier. This
produces a push-pull sawtooth signal a t terminals CF and
CE to drive .the oscilloscope main-frame. X I 0 magnification
is provided when relay K780 i s switched on, K780 shunts
0784-0794 emitter-to-emitter resistance with an adjustable
resistance (R785) which allows
a
calibrated gain of 10.
Additional RC networks in the MAG position correct for
thermal time constants. Magnifier Registration control
R795 balances the output to eliminate trace shift when
switching from X I to X I 0 magnification.
When the DISPLAY MODE selector
i s s e t
to
AMP L I F I ER, U752 and the paraphase amplifiers
0784-0794 act as an amplifier for external horizontal input
signals applied to J1.
READOUT BLOCK
The Readout Block consists of switching resistors and a
set of contacts operated by the X I 0 MAG switch. The
switching resistors, selected by the setting of the Readout
Switch (part of the TIMEIDIV switch), are used to signal
the Indicator Oscilloscope of the setting of the TIMEIDIV
switch.
Refer to Schematic Diagram 7, Timing Switch, at the
rear of the manual to find the resistors associated with a
particular setting of the TIMEIDIV switch. The number 1,
2 or 5
is selected by resistors R704-R705-R706. The num-
ber of zeros is selected by resistors R711-R712-R713-R715.
The TIME sub-unit (milli, micro, or nano) is selected by
resistors R721-R722-R723-R724. R728-R729 select the
symbol S (seconds). R701-R702 select the symbol
>
(greater than) when the VAR TIMEIDIV knob is in the out
(uncal) position. R715 is switched out by the X I 0 MAG to
increase the displayed readout by a factor of 10.
Scans by ArtekMedia O 2006