Service manual
2.9
FILTER
CONTOUR
GENERATOR
The
filter
contour
generator
contains
most
of the
features
of
the
amplitude
contour
generator.
Q15
of
the
filter
contour
generator
corresponds
to
Q35
of
the
amplitude
contour
generator,
Q13
to
Q34
and
Q12
to
Q36.
The
filter
decay
mode
control
voltage
from
matrix
column
11
through
R88
and
R91
determines
whether
the
filter
contour
will
rise
and
then
immediately
fall
or
fall
only
upon
release
of
all
keys.
Q9
and
Q10
of
the
filter
contour
generator
corresponds
to
Q26
and
Q27
of
the
amplitude
contour
generator.
The
current
from
Q10
determines
the
decay
time
of
the
contour.
Similarly,
IC14
corresponds
to
IC18,
Q18/Q19
corresponds
to
Q28/Q29
and
Q16
corresponds
to
Q30.
R95
and
R101
couple
the
key
board
voltage
to
the
attack
and decay
control
circuits.
The
voltage
applied
to
pin
B18
from
the
ATTACK
potentiometer
varies
the
attack
time
of
the
filter
contour.
The
voltage
applied
to
pin
813
from
the
DECAY
potentiometer
varies
the
decay
time
of
the
filter
contour.
Q22
and
Q23
are
routing
switches
and
only
one
is
on
at
a
time.
The
filter
contour
routing
control
voltage
from
matrix
column
5
determines
whether
Q17
is
open
or
saturated.
If
Q17
is
open,
then
Q21
is
also
open
and
Q24
is
saturated.
Thus,
Q22
is
biased
on
and
Q23
is
biased
off
and
the
contour
is
routed
to
the
low
pass
filter.
On
the
other
hand
if
Q17
is
saturated,
Q23
is
biased
on
and
the
contour
is
routed
to
the
center
frequency
control
input of the
band
pass
filter.
2.10
MODULATION
OSCILLATOR
The
modulation
oscillator
is
mounted
on
power
supply
board
No.
2
along
with
seven
slide
potentiom
eters
and
consists
of
Schmitt
trigger
IC1 and
integrator
IC2
with
their
associated
components.
The
output
of
IC2
is
a
triangular
waveform
and
the
output
of
IC1
is
a
square
wave.
The
current
supplying
integrator
IC2,
and
therefore
the
oscillator
frequency,
is
varied
over
the
frequency
range
of
1
to
50
Hz
by
RATE
control
RIO.
2.11
TOUCH
SENSOR
2.11.1
MECHANISM
The
touch
sensor
mechanism,
mounted
under
neath
the keys,
has
a
20
inch
long
anodized
aluminum
rod
on
which
a
key
bears
when
it
is
fully
down
(bottomed).
Excess
key
pressure
forces
the
rod
to
compress
its
foam
rubber
support
pad
causing
the
rod
to
come
into
more
intimate
contact
with
the
grounded
conductive
nylon
strip
glued
to
the
foam
rubber
pad.
The
assembly
functions
as
a
variable
capacitor
and
the
more
force
with
which
one
holds
a
key
down,
the
greater
the
capacitance.
The
touch
sensor
circuit
on
board
No.
4
senses
this
capacitance
increase
and
produces
a
dc
control
voltage
ranging
from
0
(no
excess
pressure)
to
.+6
volts
(maximum
pressure).
2.11.2
VARIABLE
FILTER
AND
AVERAGE
VALUE
DETECTOR
Multivibrator
IC401,
located
on
touch
sensor
board
No.
4,
produces
a
square
wave
at
a
nominal
frequency
of
100
KHz.
The
touch
sensor
element
is
a
variable
capacitance
(C410)
connected
across
pin
E
and
ground.
C410
and
R402
form
a
variable
low
pass
filter
wherein
the
peak-to-peak
voltage
at
pin
E
decreases
as
the
value
of
variable
capacitor
C410
increases.
C402
couples
the
waveform
to
clamp
CR401.
The
dc
component
of
the
signal
appearing
at
CR401
becomes
less
negative
as
the
touch
sensor's
capacitance
is
increased.
R403
and
C403
filter
out
the
ac
components
leaving
only
the
dc
component
of
the
signal
to
be
applied
to
emitter
follower
Q401.
R404
at
the
emitter
of
.Q401
and
C404
provide
additional
filtering
of
the
output
signal.
2.11.3
DC
RESTORER
The
keyboard
circuitry
generates a
trigger
voltage
which
is
applied
to
pin
A
of
board
No.
4
whenever
a
key
is
depressed.
With
no key
depressed,
this
voltage
is
zero,
Q403
conducts
and
IC402
turns
on.
The
voltage
at
the
junction
of
R404
and
R406
(pin
2
of
IC402)
is
kept
very
close
to
zero
when
the
input
trigger
is
zero
through
a
feedback
loop
consisting
of
Q402
and
R406.
Whenever
any
key
is
depressed,
the
trigger
voltage
at
pin
A
rises
to
+9
volts
and
Q403
shuts
off,
shutting
off
IC402.
C405
holds
the
voltage
that
existed
before
the
trigger
appeared
and
the
junction
of
R406
and
R404
remains
close
to
zero
until
the
touch
sensor
element
capacitance
increases.
When
the
element's
capacitance
begins
to
increase
22