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The
loop
filter has one
pole
and two zeros which
are adjusted
by
RV
1 and
RV 2
.
RV 1
adjusts
the
high frequency
zero which
effects the lock
characteristics
at
low motor speeds
,
RV 2
adjusts
the low
frequency
zero which effects lock
characteristics at
high
motor speeds
.
This seems backwards
,
but
consider that the kinetic
energy
stored
i n
the
rotating
motor increases with the
square
of the motor
speed
.
Therefore
,
it takes more high
frequency energy to phase
lock
the motor at low
speeds
than it does at
high speeds
.
IC 7
is
a
buffer for the
loop
filter
.
Its
output
can be
monitored at
TP
8
.
This drives IC 7 which is connected as
a
unity gain
differential
amplifier
.
Its function is to offset
the
loop
filter
voltage
,
which is
nominally 2.5
volts
DC
,
to
near 0
volts DC
.
This is
required
because the
phase
comparator output
is limited between
0 and 5 volts
DC
.
As the
loop
filter
voltage
deviates from 2.5 volts DC
,
the
phase
comparator
act ion becomes non
-
symmetrical
.
The motor drive
stage
,
IC 8 and the
associated transistors form
a high
g a i n
trans
-
conductance
amplifier
.
Therefore
,
as the motor speed
is
changed
from 7.5 to 15 to 30 inches
per
second
,
the
output
of the
phase comparator loop
filter monitored
at
TP
8 changes
very
little
.
Thus the
phase comparator operates at near
optimum
conditions for all motor
speeds
.
This offset
stage
,
IC 7
,
is
a major
factor
contributing
to the
performance
of the
CSL over a wide
range
of motor
speeds
.
IC 7 is connected
as a
band
pass
filter which
compensates
the
phase
lock
loop
for
the characteristics of the motor itself
.
The
output
of IC 7
can be monitored at
TP
9 which is
the
signal driving
the
output
trans
-
conductance
amplifier
.
JU
-
2 is not installed in
APR
-
5000
and
APR
-
24
transports
.
When
installed ,
this
jumper
allows
a
CTL
error signal generated on the
CTL board ( of
a
Sony
PCM — 3102 /
3202
)
to drive
the
capstan
motor
directly
.
In
this
case
,
the
phase comparator
IC 6 and the filter
circuits
which follow
it are
unused
.
o
The
output
stage
consists of IC 8 and the transistors Q 3
through Q 6
.
Since IC 8
is
limited to
a
maximum of 30 volts
across
its
power
supply pins
7 and 4
,
it
is driven from
+
24
and
-
5 volts
.
This is
regulated by
Zener diodes D
8 and D 9
.
This allows the
operational amp
to drive the
output stage
over
its
full
range
without exceeding the manufacturers
specifications
for the
power
supply
voltage of the chip
.
Transistors Q 3 and
Q 4 supply current to
increase the
speed
of
the
capstan
motor
.
Transistors Q 5 and Q 6 act to decrease the
current through the
capstan
motor
.
The motor current flows
through
R 54
,
a
.
39 ohm resistor
,
to ground
.
The
voltage
developed
across this resistor is fed
back
to IC 8 forming
a
stable
negative
feedback
loop
with a known trans
-
conductance
function
.
t
J
4
-
26
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