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Tektronix 585 - Page 43

Tektronix 585
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Circuit
Description
—
Type
585
cuit.
(See
Fig.
4-11.)
This
circuit
is
a
difference
amplifier
which
compares
the
voltage
level
of
the
input
sawtooth
against
a
fixed
voltage
obtained
from
the
DELAY
TIME
MULTIPLIER
control.
When
the
voltage
level
of
the
input
sawtooth
waveform
reaches
a
certain
level,
as
determined
by
the
setting
of
the
DELAY
TIME
MULTIPLIER,
the
output
of
the
comparator
circuit
triggers
the
Delayed
Trigger
Multi
vibrator.
The
Delayed
Trigger
Multivibrator
then
initiates
the
delayed
trigger
which
is
applied
through
a
differentiating
circuit
to
the
delayed
trigger
cathode
follower.
Outputs
from
the
delayed
trigger
cathode
follower
are
applied
to
the
DEL'D
TRIG,
connector
on
the
front
panel
and
to
the
HORIZONTAL
DISPLAY
switch.
Delayed
trigger
pulses
are
connected
to
the
'A'
sweep
circuit
when
the
HORIZONTAL
DISPLAY
switch
is
in
the
positions
B'
INTENSIFIED
BY
'A',
and
'A'
DEL'D
BY
'B'.
Difference
Amplifier
The
Difference
Amplifier
V414
and
V424
receives
the
sweep
sawtooth
from
the
Time
Base
indicated
by
the
neon
lamps
directly
above
the
DELAY-TIME
MULTIPLIER
dial.
The
Difference
Amplifier
permits
a
time
selection
of
any
rate
sawtooth
voltage.
(The
accuracy
decreases
above
2
sec/cm).
The
time
selection
is
based
upon
the
position
of
the
DELAY-TIME
MULTIPLIER
dial,
setting
the
voltage
at
the
grid
of
V424.
Thus
a
voltage
comparison
becomes
a
time
selection
because
the
sawtooth
voltage
is
changing
at
a
definite
rate.
Assume
that
the
sawtooth
input
to
the
com
parator
difference
amplifier
was
moving
at
a
rate
of
15
volts
per
millisecond
(TIME/CM
=
1
MILLISEC/CM).
Then
if
the
DELAY-TIME
MULTIPLIER
dial
was
set
to
2.0
(2.0
is
equal
to
30
volts
of
a
150
volt
sawtooth)
the
delay
pickoff
would
generate
a
delayed
trigger
2
milliseconds
after
the
sawtooth
started.
To
permit
the
Difference
Amplifier
to
have
the
dynamic
range
of
150
volt
grid
signals
it
is
necessary
to
establish
a
constant
cathode
current.
A
second
reason
for
a
very
constant
cathode
current
is
that
the
Difference
Amplifier
output
voltage
must
always
go
between
the
same
limits
regardless
of
which
portion
of
the
sawtooth
is
amplified.
Constant
Current
Tube
The
normal
long-tailed
cathode
resistor
found
in
many
Tektronix
amplifier
circuits
provides
a
relatively
constant
current
for
a
tube
with
limited
grid
voltage
swing.
Thus
the
constant
current
system
of
the
Difference
Amplifier
must
be
improved
since
the
grid
signals
have
a
150
volt
amplitude.
Triode
V428A
grid
rests
at
—
100
volts
and
its
cathode
is
then
long-tailed
to
the
—
150
volt
supply.
R428
assures
that
the
cathode
current
of
V428A
will
remain
essentially
at
5
milliamps.
Thus
the
plate
can
move
over
a
wide
volt
age
range
without
appreciable
current
change
in
the
system.
The
triode
plate
resistance
changes
as
the
plate-
to-cathode
voltage
changes,
thus
assuring
a
constant-current
cathode
circuit
for
the
Difference
Amplifier.
Delayed
Trigger
Multivibrator
The
Delayed
Trigger
Multivibrator
receives
a
de
coupled
signal
from
the
Difference
Amplifier.
With
the
sawtooth
at
a
voltage
below
the
time-selection
point,
the
grid
voltage
of
V445A
rests
at
about
-f-
190
volts.
Shortly
after
the
saw
tooth
has
passed
the
time-selection
point,
the
grid
rests
at
the
+225
volt
supply.
It
is
within
this
30
volt
range
that
the
4
volt
hysterisis
limits
of
the
Delayed
Trigger
Multi
vibrator
exist.
The
multivibrator
flips
as
the
input
voltage
rises
to
about
+210
volts.
V445B
is
held
in
conduction
dur
ing
the
quiescent
state
of
the
Delayed
Trigger
Multivibrator.
As
the
grid
of
V445A
is
carried
positive,
causing
it
to
con
duct,
the
plate
of
V445B
rises
sharply
from
about
+330
volts
to
+350
volts.
The
output
square
wave
is
then
dif
ferentiated
by
C454
and
the
combined
resistance
of
R454
and
R455.
Delayed
Trigger
C.F.
The
grid
of
V428B
rests
at
—
4.5
volts,
holding
the
tube
at
cut-off.
As
the
output
of
the
Delayed
Trigger
Multivi
brator
switches
positive,
the
differentiated
pulse
takes
V428B
out
of
cut-off,
and
delivers
about
a
5
to
7
volt
pulse
to
the
DEL
’
D
TRIG
terminal
and
the
HORIZONTAL
DISPLAY
switch.
A
waveform
ladder
diagram
is
presented
in
Figure
4-11
showing
the
sequence
of
operation
throughout
one
complete
sawtooth.
LOW-VOLTAGE
POWER
SUPPLIES
General
The
low-voltage
power
supplies
produce
all
operating
voltages
for
the
oscilloscope
with
the
exception
of
parts
of
the
c
rt
circuit.
These
power
supplies
produce
regulated
voltages
of
—
150,
+100,
+225,
+350,
and
+500
volts
and
two
unregulated
outputs
of
+180
and
+340
volts.
In
addition
a
separate
transistorized
power
supply
provides
regulated
+12.6
volts
for
filaments
of
tubes
in
either
the
P80
Probe,
or
in
the
Type
81
Plug-In
Adapter,
and
other
80
series
plug-in
units.
Each
of
the
power
supplies
operates
in
a
similar
manner.
A
sensing
circuit
compares
a
sample
of
the
output
voltage
against
a
fixed
reference
voltage.
Any
error
in
the
output
voltage
produces
an
error
signal
which
is
amplified
and
applied
to
the
series
regulator
tube(s),
causing
the
series
regulators
to
compensate
for
the
error
and
return
the
voltage
to
normal.
Reference
voltage
for
the
—
150-volt
supply
is
obtained
from
a
gas
filled
voltage
regulator
tube.
Reference
voltages
for
the
other
regulated
power
supplies,
except
the
filament
supply,
are
obtained
from
the
output
of
the
—
150-volt
supply.
Consequently,
operation
of
the
regulated
power
supplies
is
dependent
on
operation
of
the
—
150-volt
supply.
The
output
voltages
of
all
the
regulated
power
supplies
can
be
changed
slightly
by
adjusting
the
output
of
the
—
150-volt
supply.
Power
Transformer
Plate
and
filament
power
for
the
tubes
in
the
Type
585
is
furnished
by
a
single
power
transformer,
T601.
The
primary
has
two
equal
windings
which
may
be
connected
in
parallel
for
117-volt
operation,
or
in
series
for
234-volt
4-13

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