Circuit Description - Roland SH-1 Service Notes

CIRCUIT DESCRIPTION
JUL
20 1978 SH-1
CIRCUIT DESCRIPTION
1.
KCV BOARD ASSEMBLY
(CVH-3)
l.'l. The current from the constant current source
is fed to the resistor chain
of 31
resistors connected in series.
When
a
key
is
depressed, the key volt-
age developes at the corresponding resistor. (When tvo keys are depressed,
lower key voltage
is taken.) The voltage
is
held by
the capacitor in the
S
& H
circuit after the key
is
released.
The voltage
is
sent to the portamento circuit. Portamento effect
is
added
by
the CR circuit (portamento control pot and the
above-mentioned capacitor).
The portamento time
is
varied by changing
the time constant with
the porta-
mento control.
1. 2.
When
a lower
key
is
depressed
or released while
one or more higher keys are held
down, the change in the key voltages
is
detected and differentiated
to
generate
a pulse, which triggers
the envelope generator,
during legato keying.
1.
3.
Bender circuit output voltage can be varied by moving the bender
lever, smoothly
changing pitch and tone
color. The
control voltages that are distributed
to VCO
and
VCF
can be varied by
Sensitivity
sliders.
2.
VCO BOARD ASSEMBLY
(VCOH-3)
2. 1.
The VCO
is a
highly stable
and precise oscillator whose frequency
is
controlled
by KCV
or
EXT
CV.
Linear voltage at
KCV
or
EXT CV
is
converted by
the exponen-
tial convertor
to
exponential
current, which generates sawtooth
wave.
The sawtooth wave
is
changed
to
square waves,
too,
by waveform convertor.
One
is
the
50fo
square wave and the other
is
the square wave whose pulse
width
is
modulated
by
LFO,
ENV-1
or
MANUAL.
To produce sub-oscillator
sound, the square wave frequency
is
divided
into l/2
or
1/4
by dividers. The
two resulting square waves are combined
to produce the third
v^ave. Thus three waves are obtained and one wave selected
is
applied
to
Mixer
as
one of the sound sources.
2. 2.
The LFO
is a
modulating signal oscillator which generates
triangular, square and
sine waves
of low frequencies. Besides, random note
is
generated by sampling
and
holding noise signal with LFO frequency. The sine wave only
is
affected by Delay
Time control.
2.
3.
Noise
is
generated from reversely biased base-emitter junction
of a transistor.
The noise
is
amplified
to
the required level and passed through
a filter where
it
is
converted
to
white
or
pink noise.
The resulting noise
is
used
to obtain
various effects
sounds.
2. 4.
The Autobend
is a
voltage generator which
is
triggered with GATE+TRIG pulse
and
has DEPTH and DELAY
TIMJ^
controls. A resulting voltage envelope controls
the
VCO.
3
Roland