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Intercom Input
Since U 107A 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.
U 106A and U 109 form a peak limiting am plifier
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
U 106A 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 1 and 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
U 106B 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, U118 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. U11 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
Q 108 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. U 112 and U 113 are also supplied by
U1l7.
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