Swann CUBIC ASTRO 102 BX Service Manual page 27

_CUBIC
COMMUNICATIONS
SIIWAIfI
DIVISION
4-7
TC
'ltATTME rER
80.&"0
+14V
Ll03
I1
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QI04
Ll02
CRI02
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CIOT
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RII4
- 8 ....0 SWl1"CH - - - - - ,
B'.'
POWER
AMPLIFIERS
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CIO.
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IC 101 •
P.A
RI20
RI21
To PA
BOARD
STAGES
CI02 CI22
PA BOARD BIAS
I'l10111 TIR SWITCHING
The output from the power
amplifier is coupled to switch
SlOl on the bandswitch/1ow pass
filter board, and thence thr-
ough the appropriate low pass
filter for the band se1ected,
and out through 3102 to the
wattmeterjSWR board (eovered
later). Each of the low pass
filters on this board is a five
pole type, designed to greatly
attenuate out-of-band harmonies
and spurious outputs.
and Rl12-CIIO networks providè feedback for the stages. C111 he1ps to pro-
vide proper frequency response at the primary of the interstage transformer,
T102, while Cl12 and FBI02 provide required decoupling of the supply line.
Power amplifier stages Q104 and
QI05 both are also driven in push-
pull, from the secondary of trans-
former Tlü2. Bias is supp1ied in a
manner simi1ar to that of the driver
stages, and again, a network com-
posed of Cl15, Rl15, Rl16 , Rl17 and
Rl18 assure proper termination im-
pedance for the secondary of Tlü2.
Feedback networks are provided for
these stages a1so, and output ap-
pears across the secondary of TlO].
C118 and RFC Llü] provide supply de-
coup1ing.
The bias line for the PA board
is controlled by Ql06. Normally, the
base of this device is held at the
+14 volt supply potential through
R12ü, thus cutting it off. When ,
however, the transmit mode is acti-
vated, the T/R switching circuits
ground the lower end of R121, thus
10wering the voltage on the base of Q1ü6 and allowing
i t
to saturate. This
energizes the bias line and all circuits supp1ied from it.
Capacitors Clü2, C122 and Cl19 on
the bias line, and CI01, Cl14 and C121
onthe +14 volt supp1y 1ine assure good
bypassing at all frequencies from audio
to radio.