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the VCO
frequency,
to
increase. As the
phase-locked-
loop
comes
within
pull-in range
of
the
lock
point,
it
snaps
into
lock and
drives the beat frequency
to zero.
When
this
happens,
Ql,
which is
biased
Class C,
loses
its drive
signal
and shuts off, effectively
disconnecting
the acquisition circuit
from
the
loop
filter.
The
loop
then
begins
normal operation.
Loop-filter amplifier U3
drives
the
voltage-con-
trolled
oscillator,
Q2,
which
is a Colpitts-type
oscilla-
tor.
The VCO
t uning
curve is
controlled
by
the
length
of
L8. A resistive
pad
(R23,
R25,
and
R41)
and
isola-
tion amplifier U4
buffer
the VCO
output
from
the
module
output.
Part
of the VCO
output
is tapped by
a
resistor
pad
(R24,
R39,
R40)
and fed
back
to
the
divide-by-64
(Ul
)
input. The high-pass filter formed
by
C33,
C34, and
Ll2
prevents the
divider
from
con-
taminating
the VCO
output.
The board is
controlled
by
switching
off the
-
8V
supply
via
Q3.
This
is
done
by
raising
the reference
voltage of
U6 (the -8V
regulator)
to
+
1.2V,
thus
forcing
the
regulator output
to
zero
volts.
11.3 SYNTHESIZER MOTHERBOARD
(A9A7)
Figure 11-21
at the end of the section
shows
the
printed wiring
board
assembly
and
parts list
for the
Synthesizer
Motherboard.
11
-5
/(
11
-6
blanl£)
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Troubleshooting
