Tektronix 585 - Page 39

Circuit Description — Type 585
circuits to reach a steady-state condition after the completion
of a sweep.
During the trace portion of the sweep sawtooth, the Hold-
off Capacitor C 1 80 charges through V183A, as a result of
the rise in voltage at the cathode of V183A. At the same
time the grid of V135A is being pulled up by V133B, until
V135A comes out of cutoff and starts conducting, initiating
the retrace. At the start of the retrace interval C 1 80 starts
discharging through the Holdoff Resistor R181. The time
constant of this circuit is long enough, however, so that
during the retrace interval (and for a short period of time
after the completion of the retrace) C 1 80 holds the grid of
V135A high enough so that it cannot be triggered. How 
ever, when C180 discharges to the point where V133B is
cut off, it loses control over the grid of V135A which returns
to the level established by the STABILITY control somewhere
between about — 58 and — 70 volts. The holdoff time re 
quired is determined by the size of the Timing Capacitor.
For this reason the TIME/CM switch changes the time con 
stant of the Holdoff circuit simultaneously with the change of
Timing Capacitors. (In the  SEC positions of the TIME/CM
switch R181 is shunted by either R180A or R180B, shown
on the Timing Switch diagram.)
Stability Control
The operational mode of the Time-Base Generator is
determined by the setting of the STABILITY control R110.
By means of this control the sweep can be turned off, free
run, or adjusted for triggered operation. The STABILITY
control, through cathode follower V 1 25, regulates the grid
level of V135A. (V133A is inoperative during A SWEEP
operation.)
For triggered operation, the STABILITY control is adjusted
so that the grid of V135A is at about — 64 volts, just nega 
tive enough to prevent the Sweep-Gating Multivibrator
from free running. Adjusted in this manner a sweep can
only be produced when an incoming negative trigger pulse
drives the grid of V135A below cutoff, about — 66 volts.
Moving the arm of the STABILITY control toward ground,
(ccw rotation), but not so far as to actuate the PRESET
switch, will raise the grid level of V135A to about — 58
volts and prevent the Sweep-Gating Multivibrator from be 
ing triggered. This action turns off the sweep. Moving the
arm toward — 150 volts (cw) drops the grid of V135A to the
point where the discharge of the Holdoff Capacitor C180
can switch the multivibrator. Adjusted in this manner, the
Sweep-Gating Multivibrator will free-run and produce a
recurrent sweep.
When the STABILITY control is turned full ccw to the PRE 
SET position, R110 is switched out of the circuit and R 111
is switched in. This control, a front-panel screwdriver ad 
justment labeled PRESET ADJUST, provides a fixed d c volt 
age for the grid of V135A. When properly adjusted, PRESET
operation can be used for most triggering applications.
When triggering may be difficult, however, the manual
STABILITY control R110 should be used.
Lockout Multivibrator Modes of Operation
The Lockout Multivibrator is placed into operation by only
three positions of the seven position HORIZONTAL DISPLAY
switch. These modes of operation are: 1. A' SINGLE
SWEEP, 2. 'A' DEL'D BY B ’ , and 3. B ’ INTENSIFIED BY A'.
The detailed operation of the Lockout multivibrator is ex 
plained for the 'A' SINGLE SWEEP mode, with the other
two modes presented as variations of the first.
1. ‘ A ’ Single Sweep
When the HORIZONTAL DISPLAY switch is in the A'
SINGLE SWEEP position, plate voltage is applied to V133A
which then operates in conjunction with V 1 25 as a bistable
Lockout Multivibrator.
In the first stable state that exists after the completion
of a sweep, V 1 25 is cut off and V133A is conducting. In this
state, the divider between the plate of V125 and the grid
of V133A sets the common bus cathode voltage of the
Lockout Multivibrator and consequently the grid voltage of
V135A. The Lockout Level control R125 is adjusted to set the
grid of V135A a little less negative than — 55 volts so that
the Sweep-Gating Multivibrator cannot be triggered; this
"locks out" the sweep.
The front panel push-button labeled RESET permits the
Lockout Multivibrator to switch, unlocking the sweep gen 
erator for a single sweep. Depressing the RESET button
applies a positive voltage pulse to the grid of V 1 14. V 1 14
inverts and amplifies the pulse and applies it to the grid of
V133A placing V125 in conduction. The STABILITY control
again has control of the grid of V135A, and should a nega 
tive trigger pulse come from the triggering circuitry, a sweep
can occur. The RESET switch functions as follows: when
open, Cl 03 is charged to about — 20 volts. The drop across
R104 is essentially zero volts. As the RESET button is de 
pressed, grounding the switch side of C 1 03, the opposite
side rises to about +8 volts. The plate of VI 14 drops about
— 17 volts from ground (clamped there by D122), cutting off
V135A and resetting the Lockout Multivibrator until a single
sweep occurs.
Depending on the adjustment of the STABILITY control,
a sweep can now be produced in one of two ways. If the
STABILITY control is turned clockwise, the grid of V135A can
be pulled down below about — 70 volts causing the Sweep-
Gating Multivibrator to switch to its other state and initiate
a sweep. If the STABILITY control is adjusted for triggered
operation, the sweep will be initiated by the first negative
trigger pulse to arrive at the grid of V135A via T44.
As the sweep begins, the rising sawtooth voltage pulls up
the cathode of V133B by the holdoff action previously de 
scribed. As the cathodes of the Lockout Multivibrator follow
the cathode of V133B up, V125 cuts off and V133A conducts;
immediately raising the common cathode bus potential from
about — 64 to about — 53 volts. As the sawtooth voltage
continues to rise, the cathode of V133B catches the cathode
of V133A and carries the common cathode bus voltage up to
about — 48 volts causing V135A to conduct to revert the
Sweep-Gating Multivibrator, and terminate the sweep. (See
Figure 4-8).
As the Holdoff Capacitor C 1 80 discharges, the cathodes
of the Lockout Multivibrator start to fall. The grid level of
V133A is such that this tube comes out of cutoff first, hold 
ing the common cathode bus at the locked out voltage
level of about — 53 volts. As V133A conducts, its plate
drops, extinguishing the READY light. A new sweep cannot
be initiated until the RESET button is again depressed.
4-9