Tektronix 545A Instruction Manual page 44

Circuit
Description
—
Type
545A
CALIBRATOR
The
Calibrator
is
a square-wave
generator
whose
approxi-
mately
1-kc output
is
available
at
a front-panel connector
labeled CAL.
OUT.
It
consists of a multivibrator
V875 and
V885A
connected
so as
to
switch
the
cathode
follower
V885B between two
operating
states
.
.
.
cutoff
and
con-
duction.
During
the negative
portion of the Multivibrator
wave-
form
the grid of
V885B
is
driven well
below
cutoff
and
its
cathode
rests
at
ground
potential.
During
the
positive
por-
tion
of the
waveform V875
is
cut off
and
its
plate
rests
slightly
below +100
volts.
The
voltage at the
plate of
V875 when
this
tube
is
cut
off,
is
determined by
the
setting
of the Cal. Adj. control R879, part of the divider connected
between +100
volts
and
ground.
Cathode-follower
V885B
has a
precision,
tapped
divider
for
its
cathode
resistor.
When
the Cal. Adj. control
is
properly adjusted,
the
cathode
of
V885B
is
at
+100
volts
when V875
is
cut
off.
By
means
of the
tapped
divider
R885 through R893
and
a
second
1000/1 divider R896-
R897, output voltages from
.2 millivolts
to 100
volts,
in
steps,
are available. C885, connected between
the
cathode
of
V885B and
ground, corrects the output
waveform
for
a
slight
overshoot.
CRT
CIRCUITS
Cathode-Ray Tube
Control
Circuits
The
INTENSITY
control
R826
varies the
voltage
at
the
grid of the
crt
to control
the
beam
current.
The
FOCUS
control
R856
varies the voltage at the focusing
ring to
focus
the trace.
The
ASTIGMATISM
control
R864
varies the
volt-
age
at the astigmatism
anode
to
focus the spot
in
both
dimensions
simultaneously.
The
Geom
Adj. R861 varies
the
field
the
beam
encounters as
it
emerges from
the de-
flection
system
to
control the
linearity
at the extremes of
deflection.
The
CRT
CATHODE
SELECTOR
switch
SW848
connects
the
cathode
of the
crt
through
C848
to either
a rear-pane!
binding post labeled
EXTERNAL CRT
CATHODE
or
to
the
plate of
V154B
in
the
Time-Base Generator
circuit.
When
in
the
DUAL-TRACE CHOPPED BLANKING
position,
the cath-
ode
of the
crt
is
connected
to
receive positive pulses
from
the
Time-Base Generator
circuit
to
blank
the
crt
during
switching while operating a
dual-trace plug-in
unit in
the
chopped mode.
When SW848
is
in
the
EXTERNAL CRT
CATHODE
posi-
tion,
the
cathode
circuit
of the
crt
is
connected
to
the bind-
ing post
mentioned
previously.
A
bare bus bar normally
connects the binding post to
ground.
When
intensity
mod-
ulation of the
beam
is
desired, the bus bar can be
removed
so that the
modulating
signal
can be coupled
to
the
crt
cathode.
High-Voltage Supply
A
single 60-kc Hartley
oscillator furnishes
power
for
the three
power
supplies that provide accelerating
potentials
for the
crt.
The main components
in
the Oscillator
circuit
are the pentode
V800 and
the primary
of T801 tuned by
C808.
A
half-wave
rectifier
V862
provides
—1350
volts
for the
crt
cathode.
A
half-wave
voltage-tripler
circuit,
V832, V842, and
V852, provides
+8650
volts for the post-
anode
accelerator.
This
provides a
total accelerating
volt-
age
of 10,000
volts.
Both
supplies
are
tied to
the
+
100-
volt regulated supply through
the decoupling
filter
R801-
C801
.
A
floating half-wave
rectifier
V822
furnishes bias
volt-
age
(about
—
1450
volts) for
the
crt
grid. This floating grid
supply,
independent
of the cathode
supply,
is
required
in
order
to
provide dc-coupled unblanking
to the
crt
grid.
All three supplies
employ
capacitor-input
filters.
The -1350-volt cathode
supply
is
regulated by comparing
to
the -150-volt regulated supply
(the
cathode
of
V814B) a
voltage near
—
150
volts
obtained from a tap on
the
di-
vider connected between
the
decoupled
+
100-volt bus
and
the
—
1350-volt
bus.
The
total resistance of the divider,
and
hence
the voltage across
the divider,
is
determined by
the
setting of
R840
labeled H.V.
Adj.
When
this control
is
properly adjusted,
the
voltage
of the H.V. Adj. Test
Pt.
will
be
exactly
—
1350
volts.
If
variations
in
loading should tend
to
change
the
volt-
age
on the
—
1350-volt
bus,
an
error signal
will exist be-
tween
the grid
and
cathode
of
V814B. The
error signal
will
be
amplified
by V814B and V814A;
the output
of
V814A
varies the screen voltage
of the oscillator tube V800,
there-
by
controlling
its
output.
The
—
8650-volt supply
and
the
negative
bias supply are
regulated
indirectly,
as the output voltage
of
all
three sup-
plies
is
proportional
to the output of the Oscillator
circuit.
Unblanking
As mentioned
previously, dc-coupled unblanking
is
ac-
complished by employing separate
power
supplies for the
grid
and
cathode
of the
crt.
The unblanking
pulses
from
the Time-Base Generator are
transmitted
to the
crt
grid
through
the
cathode
follower,
V183B and
the floating grid
supply.
At
the faster
sweep
rates
the
stray
capacitance
in
the
circuit
makes
it
difficult
to
pull
up
the floating supply
fast
enough
to
unblank
the
crt
in
the required
time.
To
over-
come
this,
an
isolation
network
composed
of C827, R827,
and
R828
is
employed. By
this
arrangement
the
fast
lead-
ing
edge
of
the unblanking
pulse
is
coupled through
C827
to the grid of
crt.
For short-duration unblanking
pulses, at
the faster
sweep
rates, the
power
supply
itself
is
not appre-
ciably
moved.
The
longer unblanking
pulses, at the slower
sweep
rates,
charge
the stray capacitance
in
the
circuit
through
R827.
This
pulls
up
the floating
supply
and
holds the grid at the
unblanked
potential
for
the duration
of the
unblanking
pulse.
4-16
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