Tektronix 1S1 Instruction Manual page 51

Circuit Description – Type 1S1
switch is set to SINGLE SWEEP, then when the START but-
ton is pressed, C205 is connected to ground, sending a
negative-going pulse through D205 to the base of Q225.
The sweep-gating multivibrator switches and permits a single
sweep of the staircase to occur. At the end of the sweep,
the multivibrator resets and is ready to be switched, but
with the unlock pulses disconnected, subsequent sweeps are
locked out until the START button is pressed again or until
the DISPLAY MODE switch is set to NORMAL.
Miller Staircase Generator
The negative-going output pulses from the staircase driver
circuit are applied to the grid of V263 and to the Miller
capacitor, C246. When the sweep-gating multivibrator is
holding off the sweep, the energy of the stepping pulses is
sent through D246 and is dissipated. As soon as the sweep-
gating multivibrator has switched and disconnected D246
(the discharge path), the pulse energy starts to charge up
the Miller capacitors. The voltage on the grid of V263
begins to drop and the voltage at the cathode follows. An
amplified positive-going signal is produced at the collector
of Q271 and connected to the output through emitter-fol-
lower Q283. This signal, coupled back to C246, keeps the
grid of V263 nearly stationary, so the energy pulses from
the staircase driver appear as a stepping increase of charge
on C246.
The staircase output voltage which runs from zero to 10.2
volts, is sent through the TIME/CM switch (shown on the
Timing Switch diagram) to the staircase inverter and is
connected to the front-panel HORIZ OUTPUT jack to be
applied to the oscilloscope horizontal amplifier. The output
dc level is adjusted by R270 (STAIRCASE DC LEVEL) so the
staircase starts at zero volts. Calibration of the Type 1S1
timing is adjusted to be correct when the horizontal deflec-
tion factor of the oscilloscope is set to produce 1 cm of
deflection for each volt of staircase amplitude.
When enough pulses have been received to raise the
charge on the Miller capacitor to the required staircase
amplitude, the sweep-gating multivibrator is reset, causing
the disconnect diodes to discharge the Miller capacitor, and
the staircase recovers to produce another sweep.
When the DISPLAY MODE switch is set to EXT HORIZ or
MAN position, the staircase is disabled by disconnecting
the output of Q271 from the base of Q283. With the switch
at EXT HORIZ, an external scanning voltage applied through
the front-panel EXT HORIZ INPUT jack is sent through R293
to the attenuator made up of R292 and R280 in the base
circuit of Q283. Diode D292 is turned on by current through
the attenuator. The external signal is connected through
R293, D292 and emitter-follower Q283 to the output of the
circuit. This output is then applied to the oscilloscope hori-
zontal amplifier and to the staircase inverter in the same
manner as the normal staircase voltage. The EXT HORIZ
ATTEN (MANUAL SCAN) control, R293, adjusts the input
signal attenuation to produce horizontal deflection factors of
from 1 volt/cm to more than 16 volts/cm of display.
With the DISPLAY MODE switch at MAN position, the
operation is nearly the same as with the switch at EXT
HORIZ, except that the voltage applied to Q283 is a vari-
able dc voltage taken from divider R295-R293 and applied
to attenuator R292-R280. The dc voltage can be adjusted
3-14
from zero volts to more than +10 volts with R293 (MANUAL
SCAN) to scan the display manually.
In either the EXT HORIZ or MAN mode, as in NORMAL
mode, the feedback voltage to the staircase inverter is
proportional to the output voltage applied to the horizontal
amplifier. Thus the comparison pulse is generated at a time
that is directly related to the horizontal position of the dot
on the crt screen, and the display remains calibrated in
equivalent time.
Retrace Blanking Amplifier
Transistor Q244 is turned on each time the sweep-gating
multivibrator is reset for retrace and is turned off when the
multivibrator switches to start a sweep. The negative-going
gate pulse appearing at the collector of Q244 is applied
through D175 to the grid circuit of the vertical output cath-
ode follower (V183A) to deflect the crt beam off the screen
during retrace.
POWER SUPPLY
The power supply circuit provides regulated voltage
sources for the -19-volt and +19-volt supplies and a low-
impedance current source for the -139-volt supply. These
circuits assure that the operating voltages will remain stable
and that ample current will be available for the current
pulses required for the sampling process.
- 19-Volt Supply
Voltage reference for the -19-volt supply is zener diode
D645. Ac power received from the oscilloscope through pins
13 and 14 of the interconnecting plug provide 6.3 volts for
the operation of T630. The secondary of T630 supplies
power to the rectifier consisting of D630 and D632 which
produces an output of approximately 30 volts across C632.
The series regulator circuit consists of comparator transistor
Q634, emitter follower Q653 and series transistor Q657. A
precision divider, R656-R658-R657, provides a comparison
voltage of approximately -14.5 volts at the base of Q634
when the -19 V CAL adjustment (R658) is set for the correct
output voltage. Current through Q634, set by the base
voltage, adjusts its collector voltage to provide the correct
current drive through Q653 to the base of Q657.
To illustrate the regulator action, assume that the output
voltage tends to go slightly positive due to an increased load
on the -19-volt supply. The precision voltage divider will
increase the bias on Q634, decreasing its collector current.
The voltage at the base of Q653 and the base of Q657
then goes slightly negative, increasing current through Q657
and returning the voltage output (through the rectifier)
to -19-volts.
Diode D638, in the emitter circuit of Q634, provides a
reference voltage for the -19-volt regulator during the
warm up period before the oscilloscope -150-volt supply
turns on. Temperature compensation for Q634 is provided
by diode D636.
+ 19-Volt Supply
Current for the +19-volt supply comes primarily from
the oscilloscope +75-volt supply through pin 15 of the inter-