Tektronix 585 - Page 38

Circuit Description — Type 585
Sweep-Gating Multivibrator
The Sweep-Gating Multivibrator operates as a bistable
circuit. In the quiescent state V135A is conducting and its
plate is down at about +35 volts. This cuts off V 1 45 through
cathode follower V135B and the divider R141-R143, by tak 
ing the grid of VI 45 about 12 volts below its cathode which
rests at about — 60 volts. With V 1 45 cutoff, its plate is held
at about — 3 volts with respect to ground by conduction of
Disconnect Diodes V152A and B through R147 and R148. The
greater amount of diode current of about 5 ma flows through
V152A with smaller amounts of current flowing through
V152B. Conduction of the lower diode V152B through the
Timing Resistor R160 then clamps the control grid of the
Miller tube V 1 61 at about — 2.9 volts. Conduction through
diode V152A places the Time-Base Generator output volt 
age at about — 10 volts.
Miller Runup Circuit
The quiescent state of the Miller Runup circuit, including
V 1 61 and V 1 73, is determined by a d c network between
plate and grid of V 1 61. This network consists of the neon
glow tube B167, the Runup C.F. V173, the Disconnect Diodes
V152A and B, and R151. The purpose of this network is to
establish the voltage at the plate of V161 at a value such
that it will operate linearly during its runup period. The
quiescent plate voltage is about +31 volts.
Sweep Generation
If the STABILITY and TRIGGERING LEVEL controls are
adjusted for triggered operation, a negative trigger pulse
drives the grid of V135A below cutoff (below about — 70
volts) forcing the Sweep-Gating Multivibrator into its other
state in which V 1 45 is the conducting tube. Diode D 1 34 and
R133 hold the plate of V135A solidly at +100 volts pre 
venting triggered pulses from being amplified, coupled to
the c rt unblanking waveform, and intensity modulating the
display. As V 1 45 conducts, its plate drops to about — 7.5
volts, cutting off the Disconnect Diodes V152A and B. Any
spiking that may occur during this transition is attenuated by
the network C150-R150. Since R151 is in series with the
transition of the plate of V145 and diode V152A, C151 is
used as a speed-up capacitance to assure V152A being cut
off at the same time V152B is cut off.
With the Disconnect Diodes cut off, the grid of the Miller
Runup tube V161 and the cathodes of the Runup C.F. V173
are free to seek their own voltages. Current through the
Timing Resistor R160 then starts V 1 61 control grid toward
— 150 volts. The plate voltage of V161 begins to rise by an
amount equal to the drop in grid voltage times the gain
of the stage, carrying with it the grid and cathodes of
V 1 73. This raises the voltage at the top of Timing Capacitor
C 1 60, which in turn pulls up on the grid of V 1 61 and pre 
vents it from dropping more than about a volt during about
a 150 volt rise of the plate.
The Timing Capacitor charging current is then equal to
the current through the Timing Resistor. Since the voltage
at the grid of the Miller Runup tube remains essentially
constant, the voltage drop across the Timing Resistor also
remains essentially constant. This provides a constant source
of current for charging the Timing Capacitor C 1 60. By this
action C 1 60 charges linearly, and the voltage at the
cathodes of V173 rises linearly. Any departure from a linear
rise in voltage at this point will produce a change in voltage
at the grid of the Miller Runup tube in a direction to correct
for the error.
Timing Switch
The linear rise in voltage at the cathode of V 1 73 is used
as the sweep time-base. Timing Capacitor C 1 60 and Timing
Resistor R160 are selected by the TIME/CM switch SW160.
R160 determines the current that charges C 1 60. By means of
the TIME/CM switch both the size of the capacitor being
charged and the charging current can be selected to cover
a wide range of sawtooth slopes (sweep rates). For high
sweep rates bootstrap capacitor C 1 65 helps supply current
to charge the stray capacitance at the plate of the Miller
Runup tube. This permits the plate voltage to rise linearly
at the required rate.
If uncalibrated sweep rates are desired, the VARIABLE
TIME/CM (red knob) control may be turned away from
the CALIBRATED position. This control, R160Y, varies the
sweep rate over a 2 ½ to one range. Switch 160B is ganged
with the VARIABLE control in such a way that the UNCALI 
BRATED light comes on when the control is turned away
from the CALIBRATED position.
Sweep Length
The sweep rate, the rate at which the spot moves across
the face of the c rt, is determined by the timing circuit C160
and R160. The length of the sweep, the distance the spot
moves across the face of the c rt, however, is determined
by the setting of the SWP. LENGTH control R1 76. The sweep
sawtooth voltage increases the voltage at the grid and
cathode of V183A and at the grid and cathode of V133B.
As the voltage at the cathode of V133B rises, the voltage
at the grid of V135A will rise. When the voltage at this
point is sufficient to bring V135A out of cutoff (about — 50
volts), the Sweep-Gating Multivibrator circuit will revert
to its original state with V135A conducting and VI 45 cut
off. The voltage at the plate of VI 45 rises, carrying with it
the voltage at the diode plates of V152A and B. The diodes
then conduct and provide a discharge path for C 1 60
through R151, R1 47 and R148, and through the resistance
of the cathode circuit of VI 73. The plate voltage of the
Miller Runup tube now falls linearly under feedback condi 
tions essentially the same as when it generated the sweep
portion of the waveform except for a reversal of direction.
The resistance through which C 1 60 discharges is much less
than that of the Timing Resistor through which it charged.
The capacitor current for this period will therefore be much
larger than during the sweep portion, and the plate of the
Miller Runup tube will return rapidly to its quiescent voltage.
This produces the retrace portion of the sweep sawtooth dur 
ing which time the c rt beam returns rapidly to its starting
point. The unblanking voltage drops turning off the c rt
beam current during retrace.
Holdoff
The Holdoff circuit prevents the Time-Base Generator
from being triggered during the retrace interval. That is,
the Holdoff circuit allows a finite time for the Time-Base
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