Service manual

IV.
TECHNICAL
INFORMATION
IV-A.
Circuit
Description
There
are
two
inputs
to
the
main
mic
switched
to
the
listen
buss
directly
by
preamp,
one
for
each
headset
connector.
contacts
on
the
Intercom
Select
switch
The
input
from
the
upper
connector
is
(S2).
Signals
from
the
selected
chan-
switched,
and
is
also
used
for
the
opt-
nels
are
combined
In
the
summing
ampli-
ional
gooseneck
mic.
The
two
inputs
are
fier
(lC
4
a),
which
also
mixes
in
side-
routed
to
the
summing
input
of
the
mic
tone
via
P6,
feeding
the
combined
output
preamp
(IC2
pin
2)
via
J3
pins
5
&
6.
A
to
the
headset
driver
via
P8
and
the
compressor
circuit
(Ql)
reduces
the
gain
speaker
amp
via
P7.
(nominally
+51
dB
with
a
-55
dBv
input)
as
necessary
to
maintain
an
output
level
A
signal
on
the
program
or
auxiliary
near
0
dB.
The
talk
signal
branches
to
input
is
amplified
by
IC4b,
configured
three
sections:
the
talk
buss,
which
as
a
differential
or
balanced-input
amp.
feeds
the
station
operator's
voice
to
(For
single-ended,
unbalanced
use,
the
the
intercom
channels;
the
S/A
(stage
or
signal
is
applied
to
one
of
the
input
set
announce)
circuit;
and
the
side
terminals
and
the
other
one
is
groun-
path,
which
bypasses
the
intercom
chan-
ded.)
This
output
branches
to
the
pro-
nels
to
feed
the
operator's
voice
di-
gram
buss
for
feeding
to
the
intercom
rectly
to
the
station's
headset/speaker
lines,
as
set
by
each
channels'
trimpot
output
(sidetone).
(P101-104)
on
the
adjustment
module,
and
directly
to
the
headset
and
speaker
amps
The
feed
from
the
talk
buss
to
the
in-
via
PS.
An
extra
headset
driver
(IOb)
tercom
channels
is
controlled
by
a
four-
permits
program
and
intercom
in
separate
section
analog
switch
(ICs).
Each
chan-
sides
of
a
binaural
headset,
yet
program
nel's
talk
access
may
be
turned
on
by
and
intercom
are
always
combined
in
the
engaging
that
channel's
Intercom
Select
speaker
output.
button
(if
its
talk
pre-set
is
on)
or
by
using
the
talk
toggle
associated
with
The
visual
signalling
circuits
utilize
that
channel.
Following
the
signal
path
a
DC
voltage
(momentarily)
impressed
on
trough
Channel
A's
line
circuit,
the
the
audio
channel.
The
PNP
darlington
station
operator's
voice
is
fed
to
the
(Q9
in
channel
A's
line
circuit)
is
intercom
line
through
the
line
driver
operated
as
a
current
source
(to
prevent
(IC6a),
while
IC6b
maintains
a
high
line
loading)
when
its
base
circuit
is
impedance
(to
prevent
line
loading)
and
pulled
low.
The
resulting
voltage
on
amplifies
signals
on
the
line
to
feed
the
intercom
line
is
sensed
by
the
NPN
the
listen
buss.
A
portion
of
the
talk
darlington,
Q5
(and
its
corresponding
signal
is
fed
directly
to
the
line
buf-
element
in
all
other
line
circuits
con-
fer
through
Pi
to
allow
the
talk
signal
nected
to
this
channel).
Collector
to
be
nulled
out
at
the
listen
side.
current
through
the
forward-biased
NPN
The
output
from
the
line
buffer
is
transistor
causes
the
lamp
to
light.
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