Swann CUBIC ASTRO 102 BX Service Manual page 25

OCUBIC
COMMUNICATIONS
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TIO. TI06
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0102
Rite
Lloa
RI31
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11 0
80 .. BAND FILTER
RI!!I
RII7
FROM
ALC
>-IV\/\r-~.....,
The voltage from the SO
meter control line is coupled
through isolating resistor R-
117, the primary of Tl04, and
CRl12 to the output of the
switching circuitry discussed
in the previous section, wh-
ere i t is provided with a
ground return path through R-
114. Cl19 provides a ground
for the signal at one end of
the primary of TI04. The
signal is then passed through
diode switch CRl12 to TI04,
while the voltage drop across
Rl14 reverse biases the cor-
responding diode switches in
the other filters.
The secondary of Tl04 is resonated
by
the C120-C122 series combination,
and they also act as an impedance transformation voltage divider. TI05 and
C121 form a series resonant circuit, and its output is coupled to C124-C125,
which again transfonns the impedance of the signal line, and also resonates
with output transfonner Tl06. The combination ofthe two parallel resonant
circuits and one series resonant circuit forms a bandpass filter of excel-
lent characteristics. Output switching is provided by RllS, in conjunction
with CRIIJ, in a manner identical to the input switching scheme.
The output of the preselector
filters is coupled to the preampli-
fier stage, QlOl, via autotransfor-
mer Tl19 and coupling capacitor C16J.
Rl 27 provides a
De
path to ground
for the switching diode, while C159
effectively grounds the bottom of
Tl19 for the signa1.
Bias for Q101 is provided
by
both the source circuitry and the
R128-R129
combination. So far as
gate two is concerned, the bias is
determined
by
the ~ource network and
what ALC is applied to i t from the
ALC circuitry, to be discussed later.
CR122 and CR12J, biased.in the for-
ward direction
by
the voltage appl-
ied to R150, provide a fixed drop of
approximately 1.4 volts, while ad-
di
tional bias is supplied
by
the
drop across source resistor RIJ2.
C160 and C161 are both bypasses.
Output, via c165, is taken from QlOl and applied to transmit preampli-
fier QI02, which is a conventionally biased emitter follower stage. This in
turn is coupled via C170 to frequency-equalizing network RIJS-C171, and then
to the base of feedback amplifier QlOJ. Gain of this stage is determined
by
4-5