Tektronix 507, Instruction Manual

Page: 16 / 60

When the RESET button is depressed, C48 dis­
charges and the resulting negative pulse at the
plate of V49 extinguishes the thyratron. The re­
sulting rise in voltage at the plate of the thyra­
tron then pulls up the screen of V34 and permits
this tube to conduct. It also pulls up the cathode
of V63B and ignites the READY light. This in­
dicates that the trigger circuit is now armed and
the next trigger to arrive at the grid of V34 will
produce a sweep.
As the multivibrator switches to its unstable
state, and then reverts back to its stable state,
a negative pulse is produced at the plate of
V I05. This pulse is differentiated in the grid cir­
cuit of V49 and the resulting positive pulse fires
the thyratron; this action locks out the trigger
circuit again and prevents the sweep from being
started from the next trigger. Depressing the
RESET button will then arm the trigger circuit
again and permit one sweep to be produced up­
on reception of a trigger.
M anual Trigger
The sweep may be triggered manually, if
desired, by depressing the MANUAL TRIGGER
button. C25 is charged to about + 20 volts from
the divider R25-R26. When the MANUAL TRIG­
GER switch is depressed C25 discharges into C22,
creating a negative pulse at the top of C25. The
negative pulse is coupled through the diode V22
and C26 to the grid of V24 where it activates
the trigger circuitry to initiate a sweep.
Sweep Generator Clamp Circuit
In the quiescent state, the parallel clamp tubes
V I64 and V I74 conduct heavily and their plates
are down. When the multivibrator is triggered,
the resulting negative pulse at the plate of V I05
is coupled to the grids of the clamp tubes and
interrupts the flow of plate current very rapidly.
The plate voltage of the clamp tubes then begins
to rise at a rate determined by the charging
rate of 0 7 7 . This charging rate is determined
by the value of 0 7 7 and R176, both of which
are selected by the MICROSECONDS/CM timing
switch. The small choke LI 62 in the grid circuit
of the clamp tubes provides a 10-millimicrosec­
ond delay to enable the unblanking circuit to
reach full voltage before the sweep starts.
Bootstrap Circuit
For 0 7 7 to charge linearly rather than ex­
ponentially the voltage across the timing resistor
R176, and hence the charging current, must re­
main constant. This action is accomplished by
the sweep cathode-follower V I73 and the boot­
strap tubes V183-V193. The rise in voltage at the
cathodes of V I73, as 0 7 7 charges, pulls up the
cathodes of the bootstrap tubes. This rise in
voltage is coupled to the top of R176A and
keeps the voltage across the timing resistor more
nearly constant.
Decoupling Diode
A decoupling diode V I72, in series with the
-f-475-volt supply to the plates of the clamp
tubes, offers low resistance to the quiescent
plate current of the clamp tubes but disconnects
the upper end of the timing resistor from the
+475-volt supply when the bootstrap action
raises the cathode of the diode above +475
volts.
Sweep Cathode Follower
The sweep cathode-follower V I73 provides the
positive-going sweep sawtooth voltage for the
right-hand deflection plate in the cathode-ray
tube. This stage also drives the grid of the
sweep phase inverter to provide the negative­
going sweep sawtooth voltage for the left-hand
deflection plate.
Sweep Inverter
The phase-inverter V324 (Horizontal Amplifier
diagram) operates as a unity-gain amplifier to
supply the negative-going sawtooth sweep volt­
age to the left-hand deflection plate of the cath­
ode ray tube. The gain of this stage is kept low
by virtue of the frequency-compensated feed­
back network between plate and grid. V313A
and V313B provide a low-impedance bias and
screen voltage, respectively, for the phase-
inverter tube V324.
DC Restoration
Th diodes V332A and V332B remove the accu­
mulated charge from the sweep-coupling capac­
itors C324 and C325, permitting the sweep to
start at the same position on the cathode-ray
tube regardless of the repetition rate of the
sweep.
Unblanking Amplifier
During the waiting period, between sweeps,
the bias on the cathode-ray tube is such that the
2-2
CIRCUIT DESCRIPTION — TYPE 5 0 7