Tektronix 570, Руководство пользователя

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todes have a common plate-load resistor . the
pulses from both pentodes appear at the grid
of the shaper cathode follower.
The STEP/SEC switch biases one of the
S h aper Amplifiers below cutoff in each of the
1 20 positions. This eliminates the corresponding
pulses and reduces the stepping rate from 240
to 1 20 steps per second.
The cathode of the Shaper Cathode Follower .
V 5 5 A , is held positive by divider R55 and R56.
The grid of the cathode followe r rests at a
point selected by the VOLTS/STEP ADJ Contro l ,
R 25 . and a divider consisting o f R26 and R27 ,
when the Shaper Amplifiers are cut off. When
the Shaper Amplifiers conduct , the grid of the
c athode follower is driven below plate-current
cut off. The amplitude of the pulse from the
c athode follower can therefore be closely con­
trolled by the VOLTS/STEP ADJ Control.
Cla m p and Coupling Diodes
The clamp and coupling diodes . V95A and B .
differentiate the pulse from the shaper cathode
follower. During the positive portion of the
pulse waveform the capacito r . C95 , is charged
through V 9 5 B . When the negative-going pulse
occurs , this charge is released through V95A
and adds to the charge in C 85.
Step Generator
B ecause of their interdependence , the multi­
vibrator. disconnect diodes . step generator and
associated circuitry will be considered at one
time.
To provide a starting place. single­
family operation will be considered first.
For the single-family type of presentation
the STEPS/F AMIL Y Control. R 91 . is turned
clockwise so that the arm is at the most neg­
ative end of its range. In this condition , the
voltage on the cathode of the Step-Control
C athode Follower. V55B . is sufficiently negative
to hold one half of the Multivibrator. V65 A .
cut off. The Multivibrator consists o f V65A
and V65B in a dc-coupled circuit. In the
quiescent state the grid of V65A is held at
about - 100 volts and the grid of V65B
is
at
about -65 volts.
V65B is conducting and its
plate rests at - 10 volts.
When the SINGLE
F
AMIL Y button is pushed ,
C 8 8 is discharged into C90 and the grid of
V 65 A is raised so that V65A begins to conduct.
The Multivibrator switches so that V65A is
conducting and V6SB is cut off. The grid
of V65A begins to go negative immediately
as C90 loses the charge it received and the
Multivibrator returns to its quiescent condition.
The result is a short positive pulse at the plate
of V 6 5 B .
In the quiescent condition . the Step Generator
tube. V86 . is cut off. Its grid is held negative
as a result of current flow through V95 . V76B
and cathode follower V75B. The grid of V75B
is held at -10 volts by the Multivibrator .
resulting in -8 volts on the grid of V 86. The
grid and cathode of V75A rest at about 200
volts as a result of the divider action of R81 .
B 80 . and R 82. C85 is charged to about 2 1 0
volts.
The positive pulse from the Multivibrator
passes through the cathode follower . V75B . and
diode V76B to raise the grid of V86 to ground
potential. V 8 6 conducts and its plate voltage
drops cutting off cathode followe r .
V7
SA. C85
discharges through R85 , R 91 and R 92 until
clamped by V76A.
When the Multivibrator
reverts , both diodes are cut off and the resistors
R 8 5 . R 9 l and R92 tend to pull the capacitor
and the grid of V 86 negative.
At this point
the plate of V86 resumes control and any
tendency of the grid to go negative is compensated
for by a ' rise in plate voltage. The Step
G enerator is now ready for its first step.
The time required for the preceding operation
after the SINGLE F AMILY button is pushed
is less than the duration of one step.
The step i s formed as follows : C95 has
ch arged through V95B. When the negative pulse
is applied to C95 it tends to pull the grid of
V 8 6 down with it. The rapid rise in the plate
voltage of V86 is coupled back through
V7
S A
t o C 85. This reduces t h e voltage change on
the grid to a very small step. The result
is a step in the voltage across C85 as the
ch arge from C9S is transferred to C8S. Be­
tween pulses no current reaches the grid
circuit and the output voltage does not change.
The steps are repeated for 1 2 or
13
steps at
which time the plate of V 86 loses control , the
grid of V 86 goes negative to plate -current
cut -off and the quiescent condition is reached.
If the recurrent mode of operation is used.
the arm of the STEPS/F AMIL Y Control is setto
a more positive position. The stepping wavefoIDl
is developed across a divider consisting
of R85,
3 - 2
C i rcuit Description - Type 570
AA