EasyManua.ls Logo

Tektronix 502A

Tektronix 502A
110 pages
To Next Page IconTo Next Page
To Next Page IconTo Next Page
To Previous Page IconTo Previous Page
To Previous Page IconTo Previous Page
Loading...
Circuit
Description
—
Type
502A
(or
C440
for
Input
B)
plus
the
tube
and
stray
capacitance.
C410
is
then
adjusted
so
that
the
total
input
capacitance
is
47
pf.
When
the
X
1
00
attenuator
is
connected
into
the
circuit,
C406L
(or
C407L
for
Input
B)
shunts
the
47
pf
capacitance.
This
value
of
capacitance
is
then
reduced
to
a
very
small
value
by
series
capacitor
C406J
(or
C407J).
The
capacitor
at
the
input
to
the
attenuator
then
shunts
this
small
capa
citance,
and
is
adjusted
to
make
the
total
capacitance
47
pf.
The
X10
attenuator
is
designed
in
the
same
manner,
except
that
stray
capacity
forms
the
lower
branch
of
the
divider.
Since
the
attenuator
networks
are
frequency-compensated
voltage
dividers,
a
constant
attenuation
ratio
is
maintained
from
DC
to
the
upper-frequency
limits
of
the
Vertical
Ampli
fiers.
The
Input
Amplifier
When
the
Input
Selector
switch
is
in
the
A
position
(either
AC-
or
DC-coupled)
the
grid
of
V414
is
connected
to
the
input
circuit
and
the
grid
of
V444
is
returned
to
ground
either
through
the
switch
or
through
one
of
the
series
at
tenuator
networks.
When
in
position
B
(AC
or
DC),
V444
is
the
input
tube
and
V414
is
the
grounded-grid
tube.
With
either
of
these
configurations,
the
Input
Amplifier
is
a
cathode
coupled,
paraphase
amplifier;
it
converts
a
single-
ended
input
signal
to
a
push-pull
output
signal.
The
cathode
resistor
R408
plays
an
important
role
in
determining
the
amount
of
negative
feedback
applied
to
the
Input
Amplifier
stage.
As
mentioned
previously,
this
feedback
voltage
comes
from
the
cathodes
of
the
Driver
C.F.
stage
and
the
Output
Amplifier
stage.
The
feedback
from
the
Driver
C.F.
stage
is
applied
through
a
divider
consisting
of
R470
on
one
side
and
R408,
and
R480
on
the
other
side
of
R408.
To
this
compensated
feedback
network
is
added
a
small
amount
of
feedback
from
the
uncompen
sated
network
of
R465
and
R467.
This
feedback
from
the
Output
Amplifier
stage
helps
in
positioning
the
trace.
The
smaller
the
value
of
R408
the
greater
the
drop
across
the
series
resistor
(R470,
R480,
R465,
and
R467)
and
the
less
the
negative
feedback
applied
to
the
Input
Amplifier
stage.
Conversely,
the
greater
the
value
of
R408
the
greater
the
drop
across
it
and
the
greater
the
negative
feedback.
Thus,
for
very
small
input
voltages,
when
the
SENSITIVITY
switch
is
set
so
that
the
resistance
of
R408
is
quite
small,
there
is
very
little
negative
feedback
and
the
Input
Amplifier
stage
operates
with
high
gain.
When
the
SENSITIVITY
switch
is
set
to
accommodate
larger
input
voltages
the
resistance
of
R408
is
increased.
This
means
that
there
is
a
greater
amount
of
negative
feedback
and
the
gain
of
the
Input
Amplifier
stage
is
decreased.
The
switch
diagram
shows
the
makeup
of
R408.
R408A
is
in
the
circuit
for
all
positions
of
the
SENSITIVITY
switch
In
the
.2,
2,
and
20
VOLTS/CM
positions
of
the
switch,
R408A
alone
makes
up
the
resistance
of
R408.
In
all
other
positions
of
the
switch
R408A
is
shunted
by
at
least
one
other
resistor.
In
the
.1
mVOLTS/CM
position
R408A
is
shunted
by
both
R408L
and
C409L;
in
the
.5
mVOLTS/CM
through
.1
mVOLTS/
CM
positions,
and
in
the
.5,
1,
5
and
10
VOLTS/CM
positions,
it
is
shunted
by
both
a
resistor
and
an
R-C
network.
The
total
value
of
R408
is
determined
by
the
degree
to
which
R408A
is
shunted.
The
greater
the
shunting
of
R408A
the
smaller
the
value
of
R408
and
the
greater
the
gain
of
the
Input
Amplifier
stage.
Conversely,
the
less
the
shunting
of
R408A
the
greater
the
values
of
R408
and
the
smaller
the
gain
of
the
stage.
The
cathodes
of
the
Input
Amplifier
stage
are
"long-tailed"
to
the
—
150
volt
supply
through
82.
5K
resistors
and
R429
(VAR
DC
BAL).
With
the
grids
of
the
Input
Amplifier
at
ground
potential,
the
cathodes
will
operate
very
close
to
ground
(actually,
a
couple
of
volts
positive
to
bias
the
stage).
The
approx
imately
150
volt
drop
across
the
82.
5K
cathode
resistors
(R427
and
R428)
and
R429
provides
a
constant
supply
of
cathode
current
to
stabilize
the
performance
of
the
6AU6
Input
Amplifier
tubes.
In
order
for
the
Input
Amplifier
stage
to
remain
in
a
state
of
DC
balance,
there
must
be
no
d
c
voltage
drop
across
R408
when
there
is
no
input
signal.
This
means
that
the
difference
in
potential
between
the
two
cathodes
must
always
be
zero,
regardless
of
the
value
of
R408.
In
order
to
provide
for
equal
cathode
voltages
under
this
condi
tion,
the
screen
voltage
of
the
two
tubes
can
be
varied
with
respect
to
each
other
with
the
DC
BAL
Control.
Any
change
in
the
voltage
at
the
screens
will
be
reflected
to
the
cathode
by
a
factor
of
1/
.
(the
screen
grid
)
and
thus
the
cathode
voltages
can
be
equalized.
Along
with
the
DC
BAL
Control,
the
DC
unbalance
in
the
Input
Amplifier
tubes
is
also
removed
by
R429.
Adjust
ment
of
this
control
will
remove
any
vdltage
change
which
might
be
seen
as
a
positioning
change
by
the
Output
Amplifier.
Vertical
positioning
of
the
crt
beam
is
accomplished
through
the
action
of
the
POSITION
control.
This
is
a
dual
control,
connected
between
—
150
volts
and
+100
volts.
It
is
wired
so
that
as
the
voltage
between
—
150
volts
and
the
movable
arm
in
one
side
increases,
the
voltage
between
—
150
volts
and
the
movable
arm
in
the
other
decreases.
A
change
in
the
setting
of
the
POSITION
Control
will
produce
a
large
change
in
the
cathode
voltage
of
the
Output
Amplifier
stage
and
at
the
same
time
produce
a
small
voltage
change
at
the
cathodes
of
the
Input
Amplifier
stage.
However,
the
compensated
negative
feedback
circuit
of
the
Driver
C.F.
stage
reacts
quickly
to
prevent
any
change
in
the
cathode
voltage
of
the
Input
Amplifier
stage,
and
as
a
result
a
change
in
the
voltage
at
the
cathodes
of
the
Driver
C.F.
stage
is
produced.
This
change
in
voltage
at
the
cathodes
of
the
Driver
C.F.
stage
along
with
the
voltage
change
at
the
Output
Amplifier
cathode,
is
amplified
by
the
Output
Amplifier
and
appears
as
a
change
in
the
positioning
voltage
at
the
vertical-deflection
plates.
When
the
Input
Selector
switch
is
set
to
the
A-B
DIFF
position
both
grids
of
the
Input
Amplifier
stage
are
connect
ed
to
the
Input
circuit.
With
this
configuration
the
Input
Amplifier
stage
is
connected
for
differential
operation.
Two
input
voltages
are
required,
and
the
push-pull
output
voltage
is
proportional
to
the
difference
between
the
two
input
voltages.
The
.2
MV/CM
DIFF
BAL
Control
R435
adjusts
the
plates
of
the
Input
Amplifier
stage
for
equal
voltages
when
common
mode
signals
are
applied
to
the
grids.
This
control
is
equally
effective
in
all
positions
of
the
SENSITIVITY
switch,
but
is
primarily
adjusted
in
the
high-sensitivity
position
(.2
mVOLTS/CM).
The
.2V/CM
DIFF
BAL
control
3-3

Related product manuals