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Tektronix 502A

Tektronix 502A
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Circuit
Description
—
Type
502A
R424
provides
additional
differential
balance
for
the
low-
sensitivity
ranges,
and
is
most
effective
when
the
SENSI
TIVITY
switch
is
in
the
.2
VOLTS
PER
CM
position.
The
Second
Amplifier
and
Driver
C.F.
Stages
The
push-pull
output
from
the
Input
Amplifier
stage
is
amplified
in
the
Second
Amplifier
and
coupled
through
the
Driver
C.F.
stage
to
the
Output
Amplifier
stage.
As
men
tioned
previously,
the
positive
feedback
from
the
plate
circuit
on
one
side
of
the
amplifier
to
the
grid
circuit
on
the
other
makes
this
stage
an
infinite
gain
amplifier.
How
ever,
the
amplifier
does
not
oscillate
due
to
the
overall
negative
feedback.
The
.2
MV/CM
GAIN
Control
R456,
located
between
the
two
cathodes,
sets
the
small-signal
gain
of
the
Vertical
Amplifier.
This
control
is
most
effective
when
the
SENSI
TIVITY
switch
is
in
the
.2
mVOLTS
PER
CM
position.
The
FEEDBACK
BAL
control
R455
balances
the
negative
feedback
from
both
sides
of
the
Driver
C.F.
stage;
this
control
is
also
most
effective
when
the
SENSITIVITY
switch
is
in
the
.2
mVOLTS
PER
CM
position.
The
Driver
C.F.
stage
is
the
load
for
the
Second
Amplifier
stage.
Its
grid
circuit
provides
the
necessary
high
impedance
with
low
capacitance
to
maintain
the
gain
of
the
Second
Amplifier
stage,
its
cathode
circuit
provides
a
very
low
impedance
to
drive
the
input
capacitance
of
the
Output
Amplifier
stage.'The
low
output
impedance
of
the
cathode
circuit
also
provides
an
excellent
point
from
which
to
obtain
most
of
the
compensated
negative
feedback
for
the
Input
Amplifier
stage.
The
Output
Amplifier
The
Output
Amplifier
stage
is
the
stage
that
drives
the
vertical-deflection
plates
in
the
crt.
The
gain
of
this
stage
can
be
adjusted
by
means
of
the
.2
V/CM
GAIN
Control
R488.
This
control
is
most
effective
when
the
SENSITIVITY
switch
is
in
the
.2
VOLTS
PER
CM
position.
The
variable
capacitor
C478
adjusts
the
high-frequency
response
of
the
amplifier,
and
is
used
principally
to
provide
a
constant
output
capacitance.
The
FINDER
switch
reduces
the
swing
of
the
Output
Am
plifier,
enough
to
bring
the
trace
onto
the
crt.
By
means
of
the
HORIZ.
DEF.
PLATE
SELECTOR
switch
SW489
the
UPPER
BEAM
Vertical
Amplifier
can
be
con
nected
to
the
horizontal-deflection
plates
in
the
crt.
With
this
configuration
the
instrument
can
be
used
as
a
single
beam
X-Y
oscilloscope.
The
GAIN
ADJ
FOR
HORIZ
MODE
Control
R489
is
used
to
increase
the
gain
of
the
Amplifier
slightly
for
this
application.
Trigger
Pickoff
When
internal
triggering
of
the
Time-Base
Generator
is
desired
(TRIGGER
SELECTOR
in
either
of
the
UPPER
or
LOWER
positions)
a
sample
of
the
vertical-output
signal
is
used
to
develop
the
triggering
pulse.
This
sample,
obtained
from
the
plate
circuit
of
V484B,
is
coupled
through
a
frequency-compensated
voltage
divider
to
V493,
a
cathode
follower
which
drives
the
Time-Base
Trigger
circuitry.
The
DC
TRIG
Control
R495
is
adjusted
to
set
the
cathode
volt
age
to
zero
when
triggering
with
DC
coupling
and
with
the
beam
positioned
at
its
respective
zero-center
graticule
line.
TIME-BASE
TRIGGER
The
Time-Base
Trigger
circuit
consists
of
a
triggering
signal
amplifier
V24
and
a
multivibrator
(Schmitt
Trigger)
circuit
V45.
The
function
of
the
trigger
circuitry
is
to
pro
duce
a
negative-going
rectangular
pulse
at
the
plate
of
V45A
whose
repetition
rate
is
the
same
as
that
of
the
trig
gering
signal.
This
negative
step
is
then
differentiated
to
produce
a
very
sharp
negative
spike
(trigger)
to
trigger
the
Time-Base
Generator
in
the
proper
time
sequence.
A
posi
tive
spike
is
also
produced
by
the
differentiation
process,
but
this
spike
is
not
used.
The
signal
from
which
the
rectangular
output
is
produced
may
emanate
from
one
of
four
sources.
When
the
TRIGGER
SELECTOR
switch
is
in
the
LINE
position,
a
6.3-volt
signal
at
the
power
line
frequency
is
used
for
this
application.
When
the
switch
is
in
the
UPPER
or
LOWER
position
(AC
or
DC),
the
signal
is
obtained
from
the
UPPER
or
LOWER
Beam
Vertical
Amplifier,
respectively.
In
the
EXT
position
(AC
or
DC),
the
signal
is
obtained
from
an
external
source
through
a
front-panel
connector
(TRIGGER
INPUT).
In
any
of
the
DC
positions
of
the
TRIGGER
SELECTOR
switch
the
signal
is
coupled
directly
from
its
source
to
the
Slope
switch
SW20.
In
any
of
the
AC
positions,
the
signal
is
coupled
through
C
10
and
then
to
SW20.
Although
the
output
of
the
Trigger
Multivibrator
is
always
a
negative
rectangular
pulse,
the
start
of
the
pulse
may
be
initiated
by
either
the
rising
(positive-going)
or
falling
(negative-going)
portion
of
the
triggering
signal.
To
see
how
this
is
accomplished
the
operation
of
the
Trigger
Multi
vibrator
will
be
described
first.
In
the
quiescent
state,
that
is,
ready
to
receive
a
sig
nal,
V45B
is
conducting
and
its
plate
voltage
is
down.
This
holds
the
grid
of
V45A
below
cutoff,
since
the
two
circuits
are
dc-coupled.
With
V45A
in
a
state
of
cutoff
its
plate
voltage
is
up,
hence
no
output
is
being
produced.
A
negative-going
signal
is
required
at
the
grid
of
V45B
to
force
the
Trigger
Multivibrator
into
its
other
state
in
which
a
trigger
pulse
can
be
produced.
However,
since
the
signal
at
the
grid
of
V45B
is
an
amplification
of
the
trig
gering
signal,
it
contains
both
negative-
and
positive-going
portions.
The
negative-going
portion
of
the
signal
will
drive
the
grid
of
V45B
in
the
negative
direction,
and
the
cathodes
of
both
tubes
will
follow
the
grid
down.
At
the
same
time
the
plate
voltage
of
V45B
starts
to
rise,
which
causes
the
grid
voltage
of
V45A
to
rise.
With
the
grid
of
V45A
going
up
and
its
cathode
going
down,
V45A
starts
to
conduct.
As
V45A
starts
conducting
its
cathode
starts
going
up;
hence
the
cathode
of
V45B
starts
going
up.
With
the
grid
of
V45B
down
and
its
cathode
up,
V45B
cuts
off.
And
since
V45A
is
conducting
its
plate
voltage
drops,
creating
a
negative
step
in
the
output.
This
transition
occurs
very
rapidly,
regard
less
of
how
slowly
the
grid
signal
of
V45B
falls.
When
the
signal
at
the
grid
of
V45B
starts
in
the
posi
tive
direction,
just
the
opposite
chain
of
events
will
occur.
V45B
will
start
conducting
again,
which
in
turn
will
drive
the
grid
of
V45A
below
cutoff.
This
will
cause
the
voltage
3-5

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