Tektronix 535A, Instruction Manual

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Circuit Description— Type 535A
Tube plate voltage by the voltage drop across the neon
bulb, B167. C167 and R168 form a network connected
around B167 to improve the risetime. A bootstrap capacitor,
C 165, is connected between a tap on the Miller Tube plate
load and the cathode of V173. This bootstrap capacitor in­
creases the charging rate of the stray capacitances in the
Miller Tube plate circuit. Its action is most important in the
generation of fast time-bases.
The cathode of V173 follows the action of the grids
closely. This results in a linear rise in the voltage at the
upper end of the Timing Capacitor, C160. Since the charge
on the capacitor cannot change instantaneously this voltage
is coupled to the grid of the Miller Tube in a direction to
correct for the attempt of the Miller Tube grid to go nega­
tive.
Current to charge the Timing Capacitor is supplied
through the Timing Resistor, R160. Since the voltage across
the Timing Resistor is virtually constant a constant current
source is thus provided for charging the Timing Capacitor.
The linear voltage rise at the cathode of V173 is used
as the Time-Base sawtooth. This voltage rise continues
until a positive step from the Sweep-Gating Multivibrator
raises the plate voltage of the On-Off Diodes to the point
at which they begin to conduct.
The positive going voltage at the cathode of V173 is
coupled back to the. input of the Sweep-Gating Multivibrator
and causes that circuit to revert to its other state. It is kept
from acting on further trigger pulses by the action of the
Holdoff Circuit.
The waveform coupled to the Time-Base Generator from
the Time-Base Trigger circuit contains both positive- and
negative-going pulses. To prevent a negative-going pulse
from triggering the Sweep-Gating Multivibrator before the
action of the Time-Base Generator is completed the grid of
V135A must be held above cutoff.
Output Waveforms
Various waveforms available during the cycle of action
in the Time-Base Generator are made available through
cathode followers for other functions. Shown in Fig. 4-3 are
the unblanking pulse, the + Gate, and the Sawtooth from the
Time-Base. The same signal used to provide the -(-Gate is
used to provide the unblanking signal for the crt circuit.
Unblanking
In the quiescent state of the Time-Base Generator the crt
beam is cut off. To allow the crt beam to be seen the poten­
tial at the control grid of the crt must be raised. The voltage
rise appearing at the cathode of V135B in the Time-Base
Generator is used to drive a cathode-follower in the crt
circuit. This "unblanks" the beam during the time a sawtooth
is generated, permitting the left-to-right motion of the beam
to be seen.
The end of the unblanking pulse coincides with the end
of the time-base, and the crt is 'blanked" during the retrace
portion of the sweep, and during quiescent periods of the
Time-Base Generator.
Sweep “B” Circuits
In the Type 535A, the operator is offered a selection of
two sweep circuits of very similar configuration and operat­
ing principles. The Time-Base Trigger B and Time-Base
Generator B circuits are practially identical to those found in
Time-Base A. There are, however, some noticeable differ­
ences between the Time Base A and Time Base B Timing
switches, due to the difference in sweep rate selections be­
tween the two circuits.
Holdoff Circuit
The Holdoff Circuit keeps the grid of V135A above cutoff
until the capacitances in the Time-Base Generator have had
time to reach their quiescent state. The point at which the
Holdoff Circuit will allow the Sweep-Gating Multivibrator to
return to its quiescent state is determined by the adjust­
ment of R176.
The sawtooth present at the cathode of the Run-Up Cath­
ode-Follower is coupled to the grid of V183A through R176.
During calibration R176 is adjusted so that the time-base
terminates after it has passed the right-hand limit of the
graticule. R176 adjusts the voltage at the grid of V183A and
consequently at the cathode of V183A and on the capacitor
C 180.
The positive-going pulse from the cathode of V183A is
coupled to the bus connecting the cathode of V183A and
the grid of V133B. The action of the capacitor C180 re­
tards the voltage at the grid of V133B. The value of C180 is
chosen so that is capacitance will prevent the voltage at the
grid of V133B from falling until all capacitances in the
Time-Base Generator have returned to their quiescent level.
Delay Pickoff Circuit
Delayed triggers can be applied to Sweep Generator A
from the Delayed Trigger Amplifier, V114, through V133A
which acts as a coupling C.F. to apply delayed triggers to
the Sweep-Gating Multivibrator. Delayed Trigger Pulses are
applied to the grid of V114 from the cathode of V428B.
These pulses are shaped and amplified in the Delay Pick­
off Circuit, comprised of V414, V424, V445 and V428. The
first stage of this circuit, V414 and V424 combine to form a
difference amplifier which picks off a sample of the sawtooth
output from Sweep Generator A or B and converts it into
a positive pulse of step form. Before the pickoff time, V414
is cut off. Its cathode is tied to the cathode of V424 which
is conducting, and therefore determining the common-cath­
ode voltage.
The common-cathode voltage is adjustable by means of
R433, a 10-turn helical resistor, labeled DELAY-TIME MULTI-
PLIER 1-10 on the front panel. V428A is a constant-current
triode supplying cathode current to the difference amplifiers
from the —150-volt supply. This arrangement permits the
cathode of V424 to follow its grid over a wide range with
very little variation of cathode voltage.
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