Circuit
Description
—
Type
561
A
Cathode-Ray
Tube
A
Tektronix
T561 ceramic-envelope
cathode-ray tube
is
used
in the
Type 561
A.
The
accelerating
potential
is
ap
proximately
3500 volts,
developed by
about
—3300
voits
at
the
cathode
and an average deflection
plate
voltage
of
about
+200
volts.
With
this
accelerating
potential,
the
nominal vertical and
horizontal
deflection
factors are
19.5
and
18.4
volts
per
centimeter
respectively.
Deflection
blanking
of
the
crt
beam
is
employed
in
the
Type
561
A.
The
crt
contains
a
special
set
of
defle-tion
plates,
pins
6
and
7,
for
this purpose. Both
plates
are
connected
to
+125
volts;
however
pin
6
is
also
driven
by
the
right-hand
plug-in unit.
During
sweep
time,
or
if
no
plug-in unit is installed, both
plates
rest
at
+125
volts
and
permit
the
beam
to
pas.
on
to
the
screen.
During
sweep retrace,
however,
pin
6
is
driven
considerably
away
from
+125
volts.
This
scatters
the
beam
and
prevents
it from reaching
the
screen.
High-Voltage
Supplies
Energy
for
both
high-voltage
supplies
is furnished
by
T801.
V800,
the
primary
of
T801,
and
the circuit
capacitance
(shown
by
the
dotted
capacitor
symbol
on
the
schem
itic)
form
a
Hartley oscillator which
operates
at
about
45
kc.
One
secondary
winding
of
T801
provides
voltage
for
the
crt
cathode and
focus
element.
This
voltage,
rectified
by
V822, is
about
—3300
volts
at
the
cathode,
and
between
about
—
2000
and
—
3000
volts
at
the
focusing element,
de
pending
on
the
setting
of
the
FOCUS
control. (The
6.3-/olt
crt
heater
is
also
elevated
to
the
cathode potential.)
The
output
of
the
other
secondary
winding of
T80'
is
rectified
by V832
for
the
control
grid.
The
grid
voltage
ranges from —
3100
to
—
3400
volts,
depending
on
the
set
ting
of
the
INTENSITY
control.
The
reference
to
ground
for
this
supply
is
determined
by
the
voltage
at
the
junctioi
of
diodes D838
and
D839.
The
voltage
at
this
junction,
plus
the
setting
of
the INTENSITY
control,
determines
the
crt
bias
and
therefore
the
intensity
of
the
display.
Regulation
of
the
high-voltage
supplies is
accomplished
through
feedback
from
the arm of R841. If,
due
to loading
or
a
change
in
input
voltage,
the
output
of
the high-voltage
supply
changes,
a proportionate
change
occurs
at
the
arm
of
R841.
This
change
is
amplified
by
V814
and
is
coupled
to
the
screen
grid
of
V800. A change at the
screen
of
V300
will
increase
or
decrease
the
amplitude
of
oscillation;
in
V800.
Thus
the
output
voltage
of
T801
changes
in
the
direc
tion
needed
to
return
it
to
the
desired level.
The
HIGH
VOLTAGE
control,
R841, controls
the
output
voltage
by
,et-
ting
the
bias
on
V814B.
Deflection
Signals
Signals
for
the
deflection
plates
are
equal
in
amplitude
but
opposite
in
polarity
(push-pull)
and
appear
at
pins
17
and
21
of
the
plug-in
connector. The
effective
capa- ity
these
signals
see
at
the
connector
affects the
bandpass
and
phase
shift
through the plug-in.
C760
and
C761
(Plug-In
Connectors
diagram)
are
set
at
the
factory
for
an
effective
deflection-plate
capacity of
16
picofarads
for
each
plug-in.
Intensifying
Signals
Two
signals
may
be
used
to
modulate
the
intensity
of
the
crt display.
First,
intensifying
signals
from
a
two-sweep
(delaying-sweep)
time-base
plug-in
unit
are applied
to
the
grid
supply
through
terminal 14
of the right-hand Inter
connecting
Socket.
When
the
overall
display
intensity
is
reduced
with
the
INTENSITY
control, positive
intensifying
pulses
from
a
two-sweep
time-base
unit
will
brighten
any
desired
portion
of
the
display.
Diodes D838
and
D839
provide
a
low-impedance
return
for
the
grid
supply
which
reduces
intensity
modulation
caused by
normal supply
ripple.
Other
external
intensifying
signals
can
be
fed
to
the
crt
cathode
from
the
EXT.
CRT.
CATHODE
binding
post.
Depending
on
the
setting
of
the
INTENSITY
control,
a
nega
tive
pulse
of
5
volts
or
more
will
turn
the crt
beam
on.
Crt
Controls
The
INTENSITY
control,
R835,
has
a range
of
about
300
volts
to
control
the
crt
bias
and
permit
changing
the
inten
sity
of
the display.
The
FOCUS
control,
R844,
adjusts the
focus
of
the
crt
by
varying
the
voltage
at the
focusing
anode
through
a
range
from
about
—
2000
to
—3000
volts.
The
ASTIGMATISM
control,
R864,
has
a
300-volt
adjust
ment
range.
The
GEOMETRY
control,
R865,
adjusts
the geometry
by
varying
the
voltage of
the
crt
isolation
shield
through
a
range
from +180
to
+246
volts.
CALIBRATOR
The
calibrator
for
the Type
561A
Oscilloscope
produces
a
line-frequency
amplitude-calibrated square wave.
The
6.3-volt
(approximately
18
volts peak-to-peak)
ac
heater
voltage
for
V884
is
applied through
C876
to
the
cathode
of
V884A,
driving
that
tube
into
and
out
of
cutoff
at
the
line-frequency
rate.
The
signal
at the
plate
of
V884A
is
then
coupled
to
the
grid
of
V884B
to
turn
that
tube
on
and
off.
Regenerative
feedback
from
the
plate
of
V884B
to
the grid of
V884A
speeds
up the switching
action
of
V884A.
The
voltage
present
at
the
cathode
of
V884B
during
the
time
that
V884B is
conducting
can
be
set
to
exactly
100
volts
with
the
CAL.
AMPL.
adjustment,
R871.
The
voltage
divider
in the
cathode circuit
of V884B
contains
precision
resistors
to
provide
an
output
accuracy
of
3%
or
better
at
the
various
settings
of
the
CALIBRATOR
control.
3-2
