Tektronix 545A Instruction Manual page 37

Circuit
Description
—
Type
545A
time,
V414
is
cut off.
Its
cathode
is
tied
to
the
cathode
of
V424 which
is
conducting,
and
therefore
determining
the
common-cathode
voltage.
The common-cathode voltage
is
adjustable by
means
of
R433, a 10-turn helical resistor,
labeled
DELAY-TIME MULTI-
PLIER 1-10 on
the front
panel.
V428A
is
a constant-current
triode supplying
cathode
current
to
the difference amplifiers
from
the
—
150
volt
supply.
This
arrangement
permits the
cathode
of
V424
to
follow
its
grid
over a wide range
with
very
little
variation of
cathode
voltage.
Plate current
through R424
and
L424 therefore also
re-
mains very nearly constant while
V424
is
conducting, no
matter
at
what
voltage
the grid
is
set
by
the
DELAY-TIME
MULTIPLIER
control,
R433.
This
is
important because
the
plate voltage of
V424
is
required
to
hold the grid voltage
of the
shaper
stage,
V445A,
near
the triggering point.
The positive-going
delaying-sweep
sawtooth
raises
the
grid of
non-conducting
V414 toward
its
cathode
voltage.
When
the
grid rises
past the cathode voltage
set
by
the
DELAY-TIME
MULTIPLIER
control,
V414
conducts
and V424
cuts
off.
Delayed
Trigger Multivibrator
When
V424
cuts off
because
of
conduction
in
V414,
its
plate
rises,
carrying the grid of
trigger
shaper
V445A
posi-
tive past
its
transition point.
The
trigger-shaper stage
is
regenerative
so
as
to
produce a
fast transition,
and
the
resulting positive step at the plate of
V445B
is
differentiated
through
C454 and
used
to
arm
or to trigger the
main-sweep
circuits.
The sharp
differentiated pulse
is
transmitted
to the
succeeding
circuits
through cathode
follower V428B.
Two
internal
screwdriver
controls
located on
the "B"
sweep
gate
at the
right side of
your
oscilloscope
and
labeled, respectively. Delay
Start
Adj.
and Delay
Stop
Adj.,
precisely adjust the upper
and
lower V424
grid
voltage
limits set
by
the
DELAY-TIME
MULTIPLIER
so
that centi-
meters
of
delay can be
read
within a
fraction of
one
percent
directly
from
the micrometer
dial of the
DELAY-TIME MULTI-
PLIER.
Single
Sweep
Operation
When
the
HORIZONTAL
DISPLAY
switch
is
in the
A
SINGLE SWEEP
position, plate
voltage
is
applied
to
V133A
and
this
tube operates
in
conjunction with VI
25 as a
bi-
stable multivibrator.
in
the
first
stable state that
exists after the completion
of a sweep, VI 25
is
cut off
and V133A
is
conducting.
In
this state the divider
between
the plate of VI 25
and
the
grid of
V133A
sets
the
cathode
voltage
of the
Lockout
Multivibrator
and
consequently
the grid voltage
of V135A.
The
Lockout
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.
Depressing
the
RESET
switch grounds Cl 02 and
R102
Th<
resulting positive
pulse at the grid of VI
25
forces the Lock-
out Multivibrator
into
its
other stable
state
with
V125
con-
ducing and V133A
cut
off.
With
V133A
cut
off,
its
plate
voltage
rises
and
ignites
the
READY
light.
With VI 25 con-
ducting, the
STABILITY
control
regains control
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
full
right (cw)
the grid of
V135A
will
be
pulled
down and
cause the
Sweep-Gating
Multi-
vibrator 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.
As
the
sweep
begins, the
rising
sawtooth voltage
pulls
up
the
cathode
of
V133B by
the holdoff action previously
described.
As
the cathodes
of
the Lockout Multivibrator
follow the cathode
of
V133B
up,
VI 25
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
cutoff 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
is
reverted, the
sweep
is
terminated.
As
the Hold-off
Capacitor Cl 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;
thus:
V133A
conducts
and
VI 25 remains
in
cutoff.
As
V133A
conducts
its
plate drops, extinguishing the
READY
light.
A
new
sweep
cannot be
initiated
until
the
RESET
switch
is
depressed
again.
Unblanking
The
positive rectangular
pulse at the
cathode
of
V135B,
in
the Sweep-Gating
Multivibrator
circuit,
is
coupled through
a cathode
follower
V183B
(shown on
the
CRT
circuit dia-
gram,
Fig. 4-10) to the grid supply
for the
crt.
This pulse,
whose
start
and
duration
are coincident
with the
rising
portion of the
sawtooth sweep
waveform,
pulls
up
the grid
of the
crt.
This unblanks
the
crt
during the trace portion
of the
sweep and
permits the trace
to
be
observed.
Output Waveforms
The
positive pulse coupled
to the
crt circuit for
unblank-
ing
is
also
coupled through
a cathode
follower
V193A
to
a
front-panel
binding
post
labeled
-f-GATE
A. This
positive
gate waveform
starts
at ground and
rises
to
+20
volts.
The sweep
sawtooth voltage
at the cathode
of VI 73
is
coupled through a cathode
follower
V193B
to
a
front-panel
binding
post labeled
SAWTOOTH
A.
This
waveform, which
starts at
about
ground, provides a
150-volt
linear
rise in
voltage.
Dual
Trace Sync
and
Blanking
Synchronizing
pulses
for dual-trace
plug-in
preamplifiers
are supplied
by V154A.
When
multi tube VI
45
cuts off a
sharply
differentiated
positive pulse
is
developed
at
its
screen.
This pulse, coupled
to
the grid of VJ54A, produces
a
negative
trigger at
the plate
of V154A.
This trigger
then
4-9