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Tektronix 502A - Page 40

Tektronix 502A
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Maintenance
—
Type
502A
the
short
is
removed,
the
vertical
amplifier
is
in
a
state
of
d
c
unbalance.
To
determine
the
cause
of
this
condition,
short
the
plates
of
the
output
stage
(V474B
and
V484B)
together.
If
the
trace
does
not
appear,
one
side
of
the
circuit,
between
the
output
stage
and
the
crt,
is
open.
A
continuity
check
with
an
ohmmeter
is
perhaps
the
best
way
to
determine
which
side
is
open.
On
the
upper
beam
vertical
amplifier,
check
particularly
the
connections
at
the
HORIZ
DEF
PLATE
SELECTOR
switch,
SW489.
Fig.
4-10.
Checking
for
unbalance
in
the
upper
beam
vertical
am
plifier
by
shorting
the
vertical
deflection
plates
together.
If
the
trace
does
appear
when
the
plates
of
the
output
stage
are
shorted
together,
the
circuit
between
this
point
and
the
crt
pin
connections
is
normal.
The
trouble
then
lies
somewhere
in
the
vertical
amplifier
ahead
of
the
plate
circuit
of
the
output
stage.
To
locate
the
defective
stage,
move
the
shorting
strap
back,
point
by
point,
between
corres
ponding
points
on
opposite
sides
of
the
circuit.
As
you
short
between
the
points,
in
turn,
the
spot
should
return
on
the
screen
as
each
connection
is
made.
You
may
have
to
readjust
the
DC
BAL
controls
when
shorting
between
grids
of
the
input
stage.
When
you
reach
a
point
where
the
spot
does
not
return
to
the
screen,
the
stage
immediately
following
this
point
is
at
fault.
The
trouble
may
be
caused
by
a
de
fective
tube,
resistor,
or
capacitor.
For
insufficient
or
no
vertical
deflection.
Insufficient
vertical
deflection
indicates
a
change
in
the
gain
characteristics
of
the
Vertical
Amplifier.
If
the
change
is
small,
the
Vertical
Amplifier
can
usually
be
recalibrated
for
gain.
Refer
to
the
Calibration
Procedure
for
this.
If
the
change
in
gain
is
more
pronounced,
or
if
there
is
no
vertical
deflection
at
all,
the
tubes
should
be
checked
first.
Then
check
for
components
which
will
affect
the
gain
of
both
sides
of
the
amplifier
without
unbalancing
the
am
plifier.
Such
parts
are
common
cathode
resistors
in
the
amplifier
stages,
or
plate
dropping
resistors
which
are
com
mon
to
both
sides
of
the
amplifier.
Insufficient
vertical
deflection
will
also
be
caused
if
the
upper
or
lower
frequency
limit
of
the
amplifier
is
exceeded.
For
waveform
distortion
Waveform
distortion
can
be
divided
into
two
cate
gories
.
.
.
low
frequency
and
high
frequency.
If
a
square
wave
is
applied
to
an
input
of
the
oscilloscope,
the
type
of
distortion
can
be
determined
by
the
shape
of
the
displayed
waveform.
High
frequency
distortion
will
primarily
affect
the
leading
edge
and
trailing
edge
of
the
applied
square
wave
while
low
frequency
distortion
will
primarily
affect
the
mid-portion
of
the
waveform.
Waveforms
showing
low
frequency
distortion
and
two
types
of
high
frequency
distortion
are
shown
in
figure
4-11.
The
shape
of
these
waveforms
will
vary
widely
however,
with
the
cause
of
the
distortion
and
the
frequency
of
the
applied
wave.
A
nominal
amount
of
low
frequency
distortion
is
nor
mal
for
very
low
input
frequencies
when
ac
coupling
is
used,
and
a
nominal
amount
of
high
frequency
distortion
is
normal
at
the
upper
frequency
limits
of
the
instrument.
It
is
only
when
this
distortion
is
excessive
in
the
normal
frequency
range
of
the
instrument
that
it
constitutes
a
trouble.
Fig.
4-11.
Frequency
distortion,
(
a)
Low
frequency
distortion
of
a
20
cycle
square
wave
due
to
attenuation
of
the
low
frequency
components
of
the
waveform,
(b)
High
frequency
distortion
of
a
1200
cycle
square
wave
due
to
excessive
boost
of
the
high
frequency
components
of
the
waveform,
(c)
High
frequency
distortion
of
a
1200
cycle
square
wave
due
to
attenuation
of
the
high
frequency
components
of
the
wave-
form.
4-6

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