Swann CUBIC ASTRO 102 BX Service Manual page 16

- -
~~M~!~ATIONS
S . . . ,.,
DIVISION
t
--------,\
I.
~~
,
,"OllA
10 PIITO
P.II.
TUMIIII
+tO.!IV
+IV
PBT
OSCILLATOR
PBT SWITCHING
+IO.5V
R213
C207
PI
TUNING
LSII
LINE
and C205 form a parallel re sonant collector load. The frequency of the osc-
illator is adjusted by the application of a voltage from the PASSBAND tuning
control on the front panel
through isolation resistor
R2l3, to varicap CRllO. The
range of adjustment is set
by trimmer c206, while C20?
is a bypass.
. Because the
s~e
freq-
uency oscillator signal is
applied to bath PBT mixers,
any signal going into the
first PBT. mixer will leave
the second on precisely the
same frequency. This is why
the PASSBAND tuning control
only affects the relative
position of the passband and
has no effect on the freq-
uency of the signal tuned in.
The direction of tuning
of the passband oscillator
must be one way for USBjCW
operation, and the opposite direction for LSB operation for the effect at
audio to remain the same. When in the LSB mode, a positive voltage is ap-
plied through CR12l on the audio baard to the base of Ql08, through limiting
resistor R164. Simultaneously,
it is also used to turn sect-
ions Band C of switch UllO on.
Section B of U1l0 applies +10.5
volts to the upper end of the
PASSBAND tuning control Rl03A,
while section C grounds the
other end. Since QJ.08 is sat-
urated, its collector is low,
and this voltage, applied to
sections A and D of UllO, turns
them off. When in the USB or
CW modes, however, the LSB line
is low, and thus sections A and D of U1l0 are on, while sections B and C
are off. This action reverses the voltage applied to the tuning control.
Operation of the PASSBAND tuning indicator will be covered in the "Anc-
illary Circuits" section of this manual.
CWN FILTER
Signal at the second I.F. of 13.8 MHz, from the first passband tuning
mixer (QJ.08), is also coupled to the base of QJ.09. Bias is received from
the same network that supplies QJ.lO, the filter driver. QJ.09 acts as an ein-
itter follower driver for the CW narrow filter, and the required filter ter-
mination impedance is provided by R138 and R139, while C129 is a blocking
capacitor. The CW narrow filter is composed of YlOl and Yl02, with C13l and
C133 acting as coupling capacitors and C130 and C134 providing adjustments