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3
Figure 3 - Schematic
and
Connection Diagram
Frequency Response
Settings
Por sound
sources
greater
than
three
feet
from the microphone
the
frequency
response
selector
can
be
used
in
the
M
position
for
any
of
the
directional
resl)Onse
patterns.
If
the
non
-
directional
characteristic
is
used,
no
low-frequency
attenuation
should
be
re-
quired
even
for
very
small
distances.
If
the
bi-directional
or
uni-directional
patterns
are
used, low-frequency
attenuation
will
be
required
when
the
sound
source
is
less
than 3
feet
from the microphone,
unless
special
effects
are
desired.
It
is
suggested
that
the
v1
position
be
used
for
distances
down
to
l
foot
and
the v2
position
for
still
shorter
distances.
Refer
to
figure
5
for
frequency-response
curves
of
the M, v1, and
v2
settings.
Hum
Hum
may
originate
in any
part
of
the audio
system. In
the
microphone
circuit,
it
may
re
sult
from ground loops
or
unbalance caused
by
improper
cable
connections
to
the
pre-
amplifier
terminal
board
or
microphone plug.
Hum
may
also
be
induced
into
the microphone
transformer
or
ribbon
by
magnetic
fields
emanating
from
J)OWer
transformers
or
electri-
cal
machinery. In the
Type
77-D
Microphone
the
design
of
the
ribbon
circuit
and
the
transformer,
and
the
thorough
shielding
of
the
transformer
have reduced
hum
pickup from
these
sources
to
a
minimum.
In the event
that
exceptionally
strong
fields
are
encountered,
the induced
hum
may
be
minimized ~Y
turning
or
tilting
the
microphone,
or
changing
its
location.
Turning
the
frequency
response
selector
to
the v1 or v2
positions
•ill
also
reduce the
hum
together
with the
overall
low-
frequency response.
MAINTENANCE
It
is
recommended
that
no
attempt
be
made
to
make
repairs
other
than
replacement
of
screens,
transformers,
mounting
parts
and
cables.
Por
microphone mechanism
repairs,
return
the
unit
to
the
factory
.
Secure
a
Repair Order and
Retunied
Apparatus Taf
from
the
RCA
Field
Office
or
write
to
RCA
Service
Company,
Returned Apparatus Control, Canden,
H.J.
Attach the tag,
properly
fiHed
out,
to
the
damaged
equipment and send the equipment
and
the
repair
order
to
the manufacturer.
CAllI'ION:
To
prevent permanent
dama.fe
to the ribbon
do
not
test
continuity
of
the microphone with a
circuit
checker
without
connect
inf
a
resistor
of
at
least
50,000
ohms
in
series
