Service manual
works
with
the
Control
Logic
cir-
17
diB)
and
to
the
line
buffer
amp
cuitry
in
order
to
operate
the
(IC1-2).
The
line
buffer
feeds
analog
switches.
For
this
to
hap-
part
of
the
signal
back
to
the
pen,
the
dip
switches
S3-2
and
53-4
bridging
circuit,
raising
the
line
should
be
closed.
This
means
the
impedance
to
l5k
ohms.
When
the
output
of
IC3-2
is
low.
mic
is
off,
the
mic
preamp
gain
is
reduced
to
unity,
reducing
any
When
the
P-T-T
switch
is
activated,
noise
in
the
input
circuitry
by
30
the
output
of
IC3-2
goes
high.
dB.
When
dip
switch
S3-1
goes
from
This
causes
the
output
of
IC3-3
to
open.
to
closed,
the
gain
adjustment
go
high,
turning
on
the
"mis-on"
of
the
mic
input
is
increased
by
7
LED.
The
high
voltage
goes
to
the
dB.
input
of
R8,
which
turns
on
FET
Q2,
shutting
off
amplifier
IC2.
The
The
VISUAL
CALL
SIGNAL
is
accom-
same
high
level
also
turns
on
YET
plished
by
impressing
DC
voltage
on
Q3
and
supplies
power
to
the
elec-
the
audio
line.
Pressing
the
Call
tret
miC,
completing
the
feedback
button
(S2)
turns
on
transistor
Q6,
loop
of
IC1-1,
thus
turning
on
the
applying
about
11
volts
to
the
mic
pre-amp.
intercom
line.
A
receive-call
sig-
nal
entering
a
KB-112
is
detected
By
changing
the
settings
of
the
by
transistor
Q5,
which
in
turn
is
Control
Logic
dip
switches
(S3-2
to
detected
by
a
darlington
amp
which
S3-4),
we
change
the
control
vol-
turns
on
the
Call
light
(11;
during
tages
going
to
the
digital
ampli-
alternate
operating
modes
when
dip
fiers,
making
different
operating
switch
S3-3
is
closed,
the
same
modes
possible.
control
voltage
is
also
used
to
activate
the
mic
and
speaker
amp
on/off
functions).
The
call-receive
Communication
Circuitry
circuit
requires
only
4
volts
(at
In
the
"Talk"
circuit,
signals
from
100
ma)
to
turn
on
the
light.
The
the
mic
are
amplified
44
dB
by
a
7-volt
difference
between
the
send
low-level
preamp
(ICl-1).
Preampli-
and
receive
voltages
assures
posi-
fied
signals
are
sent
to
the
audio
tive
signalling,
even
on
very
long
line
(where
they
are
attenuated
by
lines.
KB-112
BLOCK
DIAGRAM
REFVE
qSE
11CCPiART
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To
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CALL"
LICHT
CALL
INTERC
tOM
V.
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-
LED
E~~RT x.
PROOC.
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ON
R
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TERMINAL
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CURRENT
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