Swann CUBIC ASTRO 102 BX Service Manual page 12

OCUBIC
COMMUNICATIONB
5 . .
11", DIVIBION
0104
R14'
CI74
RI"'", CI73 Rl(
r--.......
.JVvv..,
1--0
OUT
J13.
R14.
CRI24
R
136
RI34
0105
....- - -.....---,CI• •
+IO.5V
RECEIVE PREAMPLIFER
RI33
I1
LIO'
.---tt~~.p
TO 0102
RI31
.:I:.
CI.4
?
CRI22 CRI23 ':" R14.
+IO.5V
R' " FROM
ALC
;>-"V'oAr-t-.
The secondary of Tl04 is resonated by the C120-C122 series combination,
and they act as an impedance transformation voltage divider. Tl05 and C121
form a series resonant circuit, and its output is coupled to C124-C125, wh-
ich again transforms the impedance of the signal path, and also resonates
with output transformer TI06. The combination of the two parallel rêsonant
circuits and one series resonant circuit forms a bandpass filter of excel-
lent characteristics. Output switching is provided by Rl18, in conjunct ion
with CRII), in a manner identical to the input switching scheme.
The output of the preselector filters is coupled to the preamplifier
stage, QlOl, via autotransformer Tl19 and coupling
c~pacitor
C163. R127
provides a
De
path to ground for the switching diode, and provides reverse
bias for the "off" diodes in the other filters, while C159 effectively gr-
ounds the bottom of Tl19 for the signal •
. Bias for QlOl is pro-
vided both by the source
circuitry and the R128-R-
129 combination. So far
as gate two is concerqed,
the bias is determined by
the source network as weIl
as whatever is applied to
it via the ALC line. CR122
and CR123,. biased in the
forward direction by the
voltage applied by R150,
provide a fixed drop of
approximately 1.4 volts,
while additional bias is
supplied by the drop ac-
ross source resistor R132.
C160 and C161 are both by-
passes.
Signal from the drain
of QlOl is taken via C165,
after being developed ac-
ross load LIO). R131 and c164 form a decoupling network for the supply
line.
In the receive condition, the "R" line is high, causing Ql06 to satur-
ate, thus lowering the voltage on the base of Ql05, causing it to cease con-
duction. Diode switch CR124 is then reverse biased by the voltage drop ac-
ross R145 , allowing Ql04 to operate in anormal manner. Bias for Ql04 is
derived by R134 and Rl)6, while R149 serves to prevent instability. Output
of the preselector is taken from the emitter of Ql04 via Rl44 and C173 and
coupled to the receiver input of the exciter board.
EXCITER BOARD--FIRST MIXER
After passing through the preselector and being amplified by the pre-
amplifier, the received signal is connected to mixer ZlOl via CI02, a block-
ing capacitor. Local oscillator signal, brought to the exciter board thr-
ough JI05, is coupled via blocking capacitor C164 to the base of amplifier
Ql14. . Bias for Ql14 is developed in a conventional manner via the
R167~R168
voltage divider. The emitter resistor of Ql14 is divided into two Parts,