Intercom
Input
Since U107Ais acurrent source, signals arriving
from an external intercom appear unloaded at pin
1and 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 apeak limiting amplifier
similiar
to
that described for the microphone
amplifierabove, except that
CA10l
is used instead
of
aJFET to control the gain. The reason for this
is that the higher level signals being handled by
U106Awould cause high distortion
if
aJFETwere
used.
CA10l
is merely an LDR (light dependent
resistor) togetherwith an LED in the same package.
Morecurrent 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 externalspeaker
jack. On versions 1and 2this signal was derived
by mixing themicrophone signal with the intercom
output, via R175 and R177. On later versions the
intercom outputalone 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 transmitcompressor for proper
tracking. On versions 3and later, U
118
acts as a
four pole Chebyshev low pass filter with acutoff
frequency
of
5KHz. This is
to
prevent transmis-
sion
of
the 20 KHz tone which Telex intercomsuse
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 abalanced output.
Remote TransmitCircuit
When the transmitswitch on the rear panel is
in
the
remote position, the transmitter will only operate
when at least one receive squelch is open. On
versions 1through 4this is done with
Ul15
which
turns on Q109. Versions 3and 4have acircuit
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 3and 4
schematics show these inputs
as
they are un-
modified, that is, connected
to
the right side
of
the
resistors. On versions 5and above, U
115
is
replaced with
Ql12,
Ql13,
Ql14
and
Ql15
and
no
modifications are necessary.
PowerSupply
Q108 and U114 form a
10
volt regulated power
supply.
On
versions 1and
2,
ZD101 is used
as
the
5volt reference while on later versions this comes
from U
117.
U112 and U
113
are also supplied
by
U1l7.