Tektronix 543B Instruction Manual page 18

and R168 form a network connected around B167 to im­
prove the risetime.
Bootstrap capacitor C l65 is connected between a tap
on the Miller tube plate load and the cathode of V173.
This bootstrap capacitor increases the charging rate of
the stray capacitances in the Miller-tube plate circuit. Its
action is most important in the generation of fast sweep
rates.
The cathode of V I73 follows the action of the grids
closely. This results in a linear rise in the voltage at the
upper end of timing capacitor Cl 60. Since the charge
on the capacitor cannot change instantaneously, this volt­
age is coupled to the grid of the Miller tube in a direction
to correct for the attempt of the Miller-tube grid to go
negative.
Current to charge the timing capacitor is supplied through
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 V I73 is used
as the time-base sawtooth. This voltage rise continues until
a positive step from the sweep gating multivibrator raises
the plate voltage on the disconnect diodes to the point
where they begin to conduct.
The posiiive-going voltage at the cathode of V I73 is
coupled back to the input of the sweep gating multivi­
brator and causes that circuit to revert to its other state.
It is kept from acting on further trigger pulses by the ac­
tion 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.
The holdoff circuit keeps the grid of V135A above cut­
off 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 multivi­
brator to return to its quiescent state is determined by the
adjustment of R176 (SWEEP LENGTH).
The sawtooth present at the cathode of the runup cathode
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 also on
Cl 80, thus determining when the sweep ends.
The positive-going pulse from the grid of V183A is coupled
to the cathode of V183A and the grid of V133B. The ac­
tion of Cl 80 retards the voltage at the grid of V133B.
The value of Cl 80 is chosen so that its 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.
Unblanking Circuit. In the quiescent state of the time-base
generator, the ert beam is cut off. To allow the ert beam
to be seen, the potential at the control grid of the ert must
be raised. The voltage rise appearing at the cathode of
Circuit Description — Type 543B/RMS43B
V135B in the time-base generator is used to drive cathode
follower V183B. The signal on the cathode of V183B un­
blanks 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 ert is blanked during the retrace
portion of the sweep and during quiescent period of the
time-base generator.
Output Waveforms. The time-base sawtooth from the
cathode of V I73 is fed through cathode-follower V193B
and is available at the SAWTOOTH OUT front-panel con­
nector.
The same pulse that is fed to the grid of V183B for un­
blanking purposes is also fed to cathode-follower V193A
which makes the pulse available at the H-GATE OUT front-
panel connector.
Single Sweep Circuit. When the SINGLE SWEEP switch
is in the center position, plate voltage is applied to V133A
and this tube operates in conjunction with V I25 as a bi­
stable multivibrator.
In the first stable state after the completion of a sweep,
V125 is cut off and V133A is conducting.
the divider between the plate of V I25 and the grid of
V133A sets the cathode voltage of the lockout multivi­
brator and consequently the grid voltage of V135A. LOCK­
OUT LEVEL ADJ R125 is adjusted to set the grid of V135A
high enough so that the sweep gating multivibrator cannot
be triggered; this locks out the sweep.
Placing the SINGLE SWEEP switch to the RESET posi­
tion grounds Cl 22. This causes a sharp drop in the plate
voltage of V125. The drop in the plate is fed through the
divider between the plate of V I25 and the grid of V133A
and forces the lockout multivibraior into its other stable
state with V I25 conducting and V133A cut off. With V I33A
cut off, its plate voltage rises and ignites the READY light.
With V I25 conducting, the STABILITY control regains con­
trol over the grid level of V135A.
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 fully clockwise, the grid
of V135A will be pulled down and cause the sweep gating
multivibrator to switch to its other state and initiate a
sweep. Or, if the STABILITY control is adjusted for trig­
gered operation, the sweep will be initiated by the first
negative trigger pulse to arrive at the grid of V135A.
As the sweep begins, the rising sawtooth voltage pulls
up the cathode of V133B by the holdoff action previously
described. As the cathode of the lockout multivibrator
follows the cathode of V133B up, V I25 cuts off and V133A
conducts. As the cathodes continue to rise (following the
rise in the sawtooth sweep voltage) V133A cuts off again.
Both tubes are then held cut off for the remainder of the
sweep and the READY light stays on. When the grid of
V135A rises to the point at which the sweep gating multi­
vibrator reverts, the sweep is terminated.
As hold-off capacitor Cl 80 discharges, the cathodes of
the lockout multivibrator start to fall.
V133A is such that this tube comes out of cutoff first, thus
V133A conducts and V125 remains in cutoff.
In this state,
The grid level of
As V133A
3-7