Preselector - Swann CUBIC ASTRO 102 BX Service Manual

OCUBIC.
COMMUNICATIONS
S.,A_
DIVISION
as it is in the transmit condition, Ql02 is cut off, and its collector vol-
tage is high. This biases QlOl on, providing a path to ground for any tran-
smit signal coupled across the open contacts of KIOl due to stray capacity•.
In this manner, the input of the receiver is protected during transmission
periods.
SlO), a part of the BAND switch, is used to provide a voltage to each
of ten control lines, depending upon its position. There are separate con-
trol lines for the 160, 80,
40,
20,15, 10A, lOB, IOC and 10D positions, and
an additional line, which, through CRI02 through CRI05, is high for any of
the 10 meter positions of the BAND switch. These control lines are used in
other sections of the transceiver.
PRESELECTOR
TO Itll
lAND
PItlAIiPS
It,.
PRESELECTOR
BOARD
INPUT SWITCHIN.
RIl4
CliO
RII3
v
0
•
10 M lAND !fILTER
~C10'
TO ALl:
Itll7
0--"'--1>
TO 0101
OT"
o - - " ' - - I > T O Til P1tE:AIIP
OR"
ClOII
Til IN
0---4'-.......
...1
J13~
Ril IN CI07
JI32
0---41-..-......,
1:119
~
In the receive condition, the transmit ("T") line is low. Thus, CRI09
in the input circuitry of the preselector is reverse biased, and the TX IN
jack is effectively open circuited.
LI02 is an
RF
choke to prevent sig-
nal from being bypassed to ground
through CI09 on the "T" line.
On the other hand, the "R" line
is high in the receive condition,
and is connected through
RF
choke
LIOl to CRI08, which it biases to
an "on" condition. RIl) provides
a ground return and the voltage
drop across it reverse biases the
"off" diode. ClIO is a DG blocking
capacitor, and passes the signal on
to the input of the preselector
band filters. Rl14 acts as a DG
return for the switching currents
for the filter selection circuits,
and provides reverse bias to the
"off" diodes.
There are six band filters in the preselector (only one is used for all
four segments ofthe 10 meter land). These filters are selected by the BAND
switch, utilizing the voltages on the control lines from SlO), as mentioned
earlier. For discussion purposes, only the 80 meter filter will be consid-
ered.
The voltage from the 80 meter control line is coupled through isolating
resistor RIl?, the primary of TI04, and CRl12 to the output of the input sw-
itching circuit previously dis-
cussed, where it is provided
with a ground ret'l.Q3.. path th-
rough Rl14. Cl19 frovides a
~.
ground for the signal at one
end of the J?rimary of T1.04. The
signal is then J?assed through
FitOM I:ltll2
diode sw!tch CRl12 to T1.04, Wh_CliO
ile the voltage drop across R-
114 reverse biases the corres-
ponding diode switc;:hes in the
other filters.
-2
•