Wireless Intercom System Service Manual
Intercom
Input
Since U107A is a current source, signals arriving
from an external intercom appear unloaded at pin
1 and are coupled via C126 and R154
to
U106.
To
prevent intercom output signals from appearing at
the output
of
U106, these signals are nulled by the
branch
containing
C124,
VRlOl
and
R152.
U106A and U109 form a peak limiting amplifier
similiar
to
that described for the microphone
amplifier above, except that
CA10l
is used instead
of
a JFET to control the gain. The reason for this
is that the higher level signals being handled by
U106A would cause high distortion
if
a JFET were
used.
CA10l
is merely an LDR (light dependent
resistor) together with an LED in the same package.
More current through the LED reduces the value
of
resistance.
Headset
and
Speaker
Amplifiers
U
111
and
Ul19B
(U107B in versions 1and
2)
drive
the headset while U108 drives the external speaker
jack. On versions 1 and 2 this signal was derived
by mixing the microphone signal with the intercom
output, via R175 and R177. On later versions the
intercom output alone is used, being first amplified
by U119A. S112B maintains the headset/speaker
level constant when switching intercom output
levels.
Transmit
Mixer
and
Filter
U106B mixes the intercom input, auxiliary input,
microphone and receive board signals together
to
modulate the transmitter. VR102 is used
to
set the
level going into the transmit compressor for proper
tracking. On versions 3 and later, U
118
acts as a
four pole Chebyshev low pass filter with a cutoff
frequency
of
5 KHz. This is
to
prevent transmis-
sion
of
the 20 KHz tone which Telex intercoms use
as the call signal.
3-28
Auxiliary
Output
UllOA
mixes
together
intercom
input,
microphone, and receive board signals. U
11
OB
inverts the resultant
to
provide a balanced output.
Remote Transmit Circuit
When the transmit switch on the rear panel is
in
the
remote position, the transmitter will only operate
when at least one receive squelch is open. On
versions 1 through 4 this is done with
Ul15
which
turns on Q109. Versions 3 and 4 have a circuit
board error, however, which prevents this option
from working.
To
correct this move the inputs
of
Ul15
to
the left side (on the schematic)
of
R195,
R198,
R20l,
and R204. The version 3 and 4
schematics show these inputs
as
they are un-
modified, that is, connected
to
the right side
of
the
resistors. On versions 5 and above, U
115
is
replaced with
Ql12,
Ql13,
Ql14
and
Ql15
and
no
modifications are necessary.
Power Supply
Q108 and U114 form a
10
volt regulated power
supply.
On
versions 1 and
2,
ZD101 is used
as
the
5 volt reference while on later versions this comes
from U
117.
U112 and U
113
are also supplied
by
U1l7.