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Model
339A
Section VIII
SECTION
VIII
SERVICE
8-1.
INTRODUCTION.
8-2.
This section contains theory of operation,
troubleshooting information,
safety considerations,
and
general
service
information
for
the
Model
339A
Distortion
Measurement
Set.
8-3.
SAFETY CONSIDERATIONS.
t
4.
Although
this
instrument
has
been designed
in
cordance
with
international
safety
standards,
this
manual
contains information, cautions,
and
warnings
which must
be followed
to
ensure
safe
operation
and
to
maintain
the
instrument
in
safe
operating condition.
Service
and
adjustments should
be
performed
only by
qualified service
personnel.
8-5.
Any
adjustment, maintenance,
and
repair of the
opened
instrument while
any power
or voltage
is
applied
should
be
avoided
as
much
as possible,
and,
when
inevitable,
should
be carried out only
by
a
skilled
person
who
is
aware
of the
hazard
involved.
WARNING
I
Any
interruption
of
the protective
grounding
conductor
(inside
or outside
the
instrument
)
or
disconnection
of
the protective
earth
terminal
is
likely to
make
the
instrument
dangerous.
Intentional interruption
of
the
protective
grounding conductor
is
strictly
prohibited.
8-6.
It is
possible for capacitors inside the
instrument
to
still
be
charged
even
if
the
instrument
has
been
disconnected
from
its
power
source.
8-7.
Be
certain that
only
fuses
with
the
required current
rating
and
of the
specified
type
(normal
blow, time
delay,
etc.)
are
used
for
replacement.
The
use of repaired
fuses
and
the short-circuiting of fuse
holders
must
be avoided.
WARNING
|
The
service
information presented
in
this
manual
is
normally used
with
the protective
covers
removed and power
applied
to
the
instrument.
Energy
available
at
many
points
may,
if
contacted,
result in
personal
injury.
8-8.
RECOMMENDED
TEST EQUIPMENT.
8-9.
Test
equipment
required
to
maintain
the Distortion
Measurement
Set
is
listed in
Table
1-3.
Equipment
other
than
that
listed
may
be
used
if
it
meets
the
critical
specifications.
THEORY
OF OPERATION
8-10.
GENERAL
DESCRIPTION.
8-11.
Figure
8-1
shows
a simplified block
diagram
of
the
Model 339A
Distortion
Measurement
Set.
The 339A
combines
an
automatic,
high
resolution
distortion
analyzer/ voltmeter
and
a
low
distortion oscillator to
provide a
drive signal to the device
under
test.
The
frequency of both
the oscillator
and
the
fundamental
rejection
circuit
(notch
filter)
of
the distortion
analyzer
are
tuned simultaneously
to simplify
operation.
8-12.
The Model 339A
features
an
AM
DETECTOR
input,
in
addition
to
the
normal
analyzer/ voltmeter
input,
which
permits the user
to
measure
the distortion of
a
modulating
signal
on an
RF
carrier.
Selection of the
AM
DETECTOR
input or
Distortion
ANalyzer
input
is
^iade
by
a front panel switch.
S-
1
3.
An
OSCILLATOR LEVEL
function
is
provided
to
allow
the
operator
to
monitor
the
oscillator
output
level
without connecting
external
cables.
8-14.
The
selected
input
signal
is
applied
to the
input
attenuator/ amplifier
which
provides
the
proper
amount
of attenuation or gain required
to
place the
signal
within
the input
range of
the
analyzer
circuits.
8-15.
The Fundamental
Rejection
Circuit consists of a
"bridged
T"
filter
network
in
conjunction with
a
"notch
amplifier"
and
feed-back
amplifier
which enhance
the
rejection characteristics.
The
"nulling" process of the
circuit
is
fully
automatic
to
simplify
operation
and
to
provide
maximum
accuracy.
If
the
fundamental
frequency
of
the
input
signal
is
not within
the "pull-in"
range
of the
rejection
circuit (in
cases
where an
external
signal
source
is
used), a front
panel
LED
indicator
is
lit
to
indicate
which
direction to
turn
the
FREQUENCY
controls
to
bring the
rejection circuit
within
range.
The
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