ARP Omni-2 - Lower Voicing Board; Circuit Operations

highest octave. (square waves) of the instrument
(06-07).
3.1.2 DIVIDER AND WAVE SHAPING CIRCUITS
(Refer хо Upper Voicing Schematic, sheet 1 of 2.)
‘The square wave outputs from Z2 are supplied to the
clock inputs of CD4520BE divider chips (23-27)
which produce square waves for each key, The
square wave outputs of the CD4520BEs are buffered
by inverters 29-216.
TYPICAL WAVESHAPE CIRCUIT
The square wave from inverters 29-213 and from Z2
are differentiated by capacitors C17-C49 (values are
selected for each frequency). CRI through СААТ
clip the negative portion of the differentiated square
wave resulting in sawtooth shaped waveforms. By
altering the DC bias (PG-7, Waveform Control Bus)
some of the negative portion of the differentiated
waveform is permitted to pass through diodes CR1-41
when the Waveform Enhancement is selected resul-
ting in a “hollow” type sound.
3.1.3 KEYING CIRCUITS
(Refer to Upper Voicing Schematic, sheet 2 of 2.)
Each key contact (P7-5, P7-4, P7-3, etc.) is connected
to а 22uf electrolytic capacitor through а 4700hm
resistor. The capacitor is normally charged to O volts
(The negative side of the capacitor is at -15 volts.).
When a key is depressed, the capacitor is discharged
to -15 volts. The time the capacitor takes to recharge
to O volts sets the release time of each note and can
be varied by the release slider (R31) on the String
Control Board, The key voltages produced for each
key (indicated by letter KV) are supplied to two gate
circuits; one for 8", one for 4"
3.1.4 GATE CIRCUITS
(Refer to Upper Voicing Schematic, sheet 2 of 2.)
There are a total of ten gate transistor arrays (TDA
470) in the OMNI -2 which gate signals from the
tone generator sections to the output section, Each
Gate has ten transistors (on a common substrate)
with common collectors and bases. The emitters
serve as the inputs to the devices. Five of the arrays
are used for 4" pitches and five for 8" pitches. There:
fore, one key controls two gate chips at a time
7 (4' and 8").
ошту
EE o
3s
«o
Я:
fet
key T
Шет эм
atv
з,
a
E77
₪ TRANSISTOR
wt Saar 1 bd
SATO
4 "ור󰾯
stame
177 Nd
שח | ה󰾯
TRIGGER
TYPICAL KEYING & GATING CIRCUIT
The Upper Voicing Board contains three 4" gate
arrays and three 8' arrays; the remaining arrays
are on the Lower Voicing Board. All of the arrays
have the base ріг (7) grounded. Each of the saw.
tooth
waveforms
are
supplied
to
an
emitter
through
а 100Kohm resistor (eg. Tone G 4 is supplied
through resistor pack 230 pins 5 and 6 to pin 12 of
231.). Keying voltages (denoted by KV) are supplied
to the emitters also through 150Kohm resistors (e.g.
KV21). As long as the keying voltage is at or near O
volts, the transistor remains off. When a key is de-
pressed, the keying voltage drops to -15 volts, which
turns on the transistor in the array and permits the
signal to.pass to the collector of the array and out
to the mixing circuitry (e.g. from pin 12 of 231 to
pin 14)
3.2 Lower Voicing Board
GENERAL: The Lower Voicing Board contains
the
Bass
Low
Note
Priority
circuitry,
4"
and
8"
Mixing
circuitry, Gate Sensing circuitry, Suppression
circuitry,
8°
and
16"
Bass
circuitry,
Keying
circuits
for
keys 1 through 20, Gate circuits for tones C2 through