ARP 3301 User manual

ARPPIANO
16 VOICE ELECTRONIC PIANO
SERVICE MANUAL
Models 3301, 3302, & 3303
MUSIC DEALER SERVICE
A Div. of T.E.A.M.S. Inc.
4700 West Fullerton
/ Chicago, Illinois 60639
[phone]
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SECTION 1 GENERAL DESCRIPTION
The ARP 16-voice Electronic piano is the first ;/iano
to offer a wide variety of touch responsive percussive
sounds in one instrument. Among the sixteen sounds
are acoustic piano, vibes, bells, harpsichord, harp,
tunings, envelope follower effects and other voices
which are ideal for both solo and group performance
applications.
The instrument has a 73-note keyboard with sta. .dard
wood piano keys and a specially designed and weight
ed maple action which faithfully reproduces tho feel
and response of traditional grand pianos.
In addition, a pedal assembly is included with two
pedals, one for sustain and the other may be used as
a soft pedal or may be reassigned to become a vibrato
on/off pedal. Also a stereo phase shifter is included
which can be switched on or off with any of the
voices. Other features include vibrato, master tune
controls and detune controls for effects such as
honky-tonk piano or to "warm-up" the sound. A
stereo headphone jack is provided on the control
panel along with the volume and tone controls.
The instrument comes with four detachable legs,
a keyboard cover and the pedal assembly.
SECTION 2 SPECIFICATIONS & CONTROLS
■■55-
CONTROLS
1. STEREO HEADPHONE JACK: Accepts I ohm
stereo headphones.
2. VOLUME & TONE: Affects all 16 Voices.
3. PHASOR: Stereo phasor may be used on .II 16
voices. Speed & Resonance controls adjust the
effect to taste.
4. MASTER TUNE & DETUNE: Master Tune con
trol is used to tune the entire instrument, and the
Detune control is used to produce a "slightly out
of tune" character or flavor.
5. LEFT PEDAL SELECT SWITCH: Determines
whether the left pedal is to be used for the "soft"
effect, or for vibrato.
6. VIBRATO SWITCH: Used to provide continuous
vibrato on all 16 voices.
7. VIBRATO SPEED & DEPTH: Sets vibrato speed
& depth, and the Speed control is also used to
set the trill rate for voice 5.
8. VOICE SELECT SWITCHES: Used to select any
one of the 16 voices provided.
AUDIO OUTPUTS
LEFT: 8V PP, low impedance
RIGHT: 8V PP, low impedance
MONO: High, 8V PP; Low, 0.8V PP; low impedance
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SECTION 3 THEORY OF OPERATION
3.1 Tone Generation
The tone generation system in the ARP 16-voice
Electronic piano consists of a master high frequency
oscillator, a clock frequency shifter circuit, top
octave generator chips and 8 divider/keyer ICs. In
order to produce pitches for each key on the key
board, there are two divider chains in parallel. This
dual tone generation system is used for two main
reasons; 1) it permits the two pitches generated for
each key to be detuned relative to each other and,
2) a dual keying system permits a more complex
envelope decay characteristic.
To permit accurate detuning, a high frequency
clock which is derived from the high-frequency
oscillator drives one of the divider chains. The
other divider chain is driven from the same clock
signal after it has been processed, by a unique pulse
chopping circuit which effectively increases its fre
quency. This scheme, which is. described in more
detail in the power supply circuit description, permits
accurate detuning of the two divider chains and there
fore establishes accurate detuning of the pitches on
each key.
The final divisions in the divider chain are made in
special purpose MOS integrated circuits. These
devices contain all of the frequency dividers for each
note and the keying circuits. There are three audio
outputs on each chip (8', 4' and 2') which are
buffered and then sent to the voicing circuits.
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3.2 Keying Circuit
3.3 Voicing
Since the ARP Electronic Piano is touch-responsive,
envelopes must be generated which are proportional
to the speed of keys which are depressed. This is
accomplished by an RC Time-of-Flight muasure-
ment scheme. When a key is depressed, a capacitor
begins to discharge until the key reaches iu final
position, then the remaining charge on the capacitor
is used to charge two other capacitors (one for each
of the two tones per key) which decay at a much
slower rate. These two capacitors are directly con
nected to the divider/keyer chips, to gate out the
audio signals. Thus the harder (faster) a key is de
pressed, the larger the initial envelope voltage on the
two decay capacitors.
The architecture of the voicing circuit is defined by
the audio signals which are sent over from the
divider/keyer circuitry. In total, there are four
groups of three audio signals (all square waves)
arranged as follows:
8', 4', 2' Upper keyboard half, Short
8', 4', 2' Upper keyboard half, Long
8', 4', 2' Lower half keyboard, Short
8', 4', 2' Lower half keyboard. Long
VOICE BOARD SIGNAL FLOW
1. Key at rest.
2. Key being depressed.
(Capacitor begins to discharge.)
1
T
I
3. Key stops. (Regaining
voltage discharges
through decay circuit.)
Fast, hard strike.
Medium strike.
Soft, slow strike.
The decay characteristics of the struck and plucked
string share a common form. The initial decay is
more rapid (and frequently brighter) than the final
decay rate. It is for this reason there are two differ
ent decay rates on each key, one rapid and one
relatively long. These two decays are referred to as
the "Short" envelope (or signal) and the "Long"
envelope. The Short envelope decay rate can be
altered by a PMD (pulse modulated decay) line to
shorten its decay rate even further for piucked
sounds. These two audio signals keyed on each note
are combined in the voicing circuit, producing com
posite envelope on the output.
; t
j ;
The first voicing step is to mix and pref ilter the three
footages in each voice, according to the voice select
ed. Some voices use all three footages, others may
use only one or two. The outputs of the first mix
stages yield four pre-mixed signals: Upper Short,
Upper Long, Lower Short and Lower Long. These
signals are then processed through select fixed filters
which can be low-pass, band-pass, or high-pass,
depending upon the voice which is selected. The
outputs of the fixed filters are then combined from
four signals (US, UL, LS, LL) to two signals in a
4-to-2 mix circuit. Depending again on the voice
which is selected, two mixdown outputs are either
Upper Keyboard/Lower Keyboard or Short Signal/
Long Signal.
In the final voicing step, the two signals from the 4-
to-2 mix circuit are sent to a six-band resonator bank
which is similar to a graphic equalizer. Each voice has
its own frequency response established by a resistor
matrix before the resonator bank. The resonator
bank has one output which is sent to the phase shifter
circuit which harmonically derives a left and right
output signal.
All of the signal paths and many of the circuit values
are different for each voice. When a voice is selected,
the control panel circuitry sends out a digital code
which addresses a ROM (read only memory) which
in turn sets up the voicing circuit for the appropriate
signal paths and filter values.
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DISSASSEMBLY
Bezel/Top Cover
i
Ol
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SECTION 4 CIRCUIT DESCRIPTIONS
4.1 Power Supply
The power supply is in a central location from an
interconnection standpoint therefore all of the
support circuitry for the keyer boards is on this
circuit board. The support circuits are the High
Frequency VCO, the Interval Logic and Frequency
Shifter which determines the tuning or interval,
Beat Frequency VCO which sets the amount of de
tuning, Pulse Generator Circuit which is used to
modulate the decay rate of the Short envelopes on
the keyer boards, Line Drivers which drive the top
octave synthesizer on the keyer boards, and the
Keyer Bias Supply Circuit, which supplies the keyer/
divider chips on the keyer boards with the correct
reference voltages.
TROUBLE SHOOTING HINTS
The Interval Logic and Frequency Shifter
Circuit is running at a high frequency and it is
difficult to determine whether the circuit is
functioning properly or not
To quickly check out the circuit, connect the
trigger input of an oscilloscope to the D output
of Z8B (Pin 14,TP1). This will permit a com
plete cycle to be displayed exactly like the
timing diagram on page 7 (except that the
waveforms are somewhat rounded). Then
simply probe each gate output illustrated on
the timing diagram.
4.1.1 POWER
Almost all of the instrument is run +12 volts, which is
provided by two 3-terminal regulators (Z1 and Z2).
Z3, Q1 and Q2 provide +15 volts for the keyer/
divider ICs on the upper and lower keyer boards. A
power sense line is sent through P9-6 to the voice
board to activate a mute circuit when the instrument
is turned on or off.
4.1.2 HIGH FREQUENCY VCO
The High Frequency Oscillator (Z6 and Q5) oscillates
at about 3.4MHz. The 'M' tune line comes from the
Master Tune control on the control panel and also has
the vibrato signal from the control panel when it is
selected.
4.1.3 INTERVAL LOGIC AND FREQUENCY
SHIFTER
In total, there are three different pitch relationships
used in the instrument: Unison tuning (voices P, 3,
4, and 7 through 16), musical fifth tunings (voices
2, 5 and 6), and a high-pitched eleven to eight ratio
which is used to create a metallic "ting" (voices 7 and
8). (In the case of the fifth tuning relationship, the
clock signal for the short chain is actually lowered by
a musical fourth and the 8' signal is unused.) In
all cases, the clock which drives the Short channel is
the one which is altered. The Long divider chain is
not alterable (except, of course, by the master tune).
The different tuning relationships, including the
unison detuning, are created by chopping pulses to
create a derived signal.
The High Frequency VCO oscillates at approximately
3.4 MHz, which is the frequency required to produce
the metallic "ting" (voices 7 and 8). The remaining
pitch relationships are derived from this clock signal
in the Interval Logic Frequency Shifter Circuit. Long
Clock: Z13B, Z14A and B, Z15B and C, and Z16A
chop three pulses for every eleven of the High Fre
quency VCO (see timing diagram, TP Z13-11), which
then becomes the clock signal for the Long divider
chain (called the Upper Long clock or UL clock).
Short Clock: When the Flat control line is high
(Z15A, pin 1) then another two pulses are added to
the same derived clock signal via Z14C. This lowers
the pitch of the short clock signal, which will be
supplied to the Short chain by a musical fourth.
Since the 8' audio will not be used, it will come
out sounding like a musical fifth above the Long
chain.
The Beat Frequency VCO periodically sends a pulse
over to Z16B and Z17, to add in pulses occasionally
which slightly raises the frequency of the clock signal
which is supplied to the Short divider chain. If the
frequency of the Beat Frequency VCO is increased,
the rate of pulse adding also increases, which in turn
raises the effective frequency of the clock. The Beat
Frequency VCO ranges from about 450Hz to 60KHz.
At maximum frequency the Beat Frequency
VCO raises the Sharp clock approximately % of a
semitone. Z13, Pin 10 is the "Upper Short Clock."
The "Lower Long Clock" is picked off of counter
Z8B, Pin 14 and is three octaves lower than the
"Upper Long Clock." This clock is for the Lower
Keyer boards. The "Lower Short" is provided by
Z8A, which also divides the Upper Short clock in
half, three times, for the Lower keyer boards.
4.1.4 BEAT FREQUENCY VCO
The Beat Frequency VCO determines the exact
amount of detuning between the Short and Long
clock signals by supplying a pulse of constant width
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TIMING DIAGRAM
Z13A1MASTIR CLOCK
Z1«A2<FEEOBACKI
213 • U UL CLOCK. COUNTER INPUT
2» • II COUNTER OUTPUT A
ZB • 12 COUNTER OUTPUT B
28 • 13 COUNTER OUTPUT C
Z8 ■ 14 COUNTER OUTPUT O
TEST POINT 1
(Tf>Mtf ICOfM •( ttttl pOMtll
BEAT FREO VCO
ZI7B O
Z17B 0
Z13C
NORMAL TUNING
£«> , < FLATUNEH.GH
UPPER SHORT CLOCK ...
^ SHARP LINE HIGH
ru~L_n
L
_n n ri
L
1 I
J~L_TL
n n nn n
"in n nn n
TTLJ~U~U LJ L.i
ICLOCKS , |CLOCKS
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(4.5 microseconds) to the Interval Logic ai J Fre
quency Shifter Circuit. Its frequency ranges between
450Hz (minimum detune) and 60KHz (maximum
detune), depending upon the setting of the Detune
control on the front panel. The more often pulses
are supplied to the Frequency Shifter Circi it, the
more often are pulses added to the clock signa which
raises the frequency of the Short Clock. However,
these pulses are ignored on voices 7 and 8.
4.1.5 PULSE GENERATOR
The Pulse Generator derives from available clock
signals, one of four signals which are supplied to the
keyer board to set the decay rate of the Short
envelopes on the PMD (Pulse Modulated Decay) line.
A two-bit code supplied from the voice board deter
mines which decay rate is set by this circuit (P9, pins
5 and 12). The four signals are as follows:
Notice that Z10B, which drives the PMD line, is
powered from a voltage which is derived (and is
always less than) the keying bias voltage. This is to
insure that the modulating signal does not affect the
envelope voltage levels when the Short envelopes
decay near the keying bias voltage level.
4.1.6 KEYER BIAS SUPPLY
This circuit supplies the Keyer board with the follow
ing reference voltages: Pedal Control (PED), Keyer
Bias (KB), Timing Bias (TB), +7 Volts for the Keyer/
Divider chips, and the decay chopping power line
(PMD line).
Z4A sets the Keyer Bias level to approximately
+ 10.5 volts. This voltage is set to coincide exactly
with the pinch-off voltage of the keyers in the Keyer/
Divider chip. This voltage is typically two diode
drops (about 1.6 volts) below the Keyer/Divider
chip's supply voltage (+12 volts). In practice, the
best envelope decay characteristic is achieved by set
ting the Keyer Bias such that the Keyer/Divider
chip does not quite pinch-off. This prevents the
the decaying sound from abruptly stopping when the
envelope reaches the pinch-off value. To prevent
this truncation, a trimmer (R37) is set to allow a
very small signal through when the envelope has fully
decayed. The damper circuit on each key supplies
a voltage to pinch-off the signal when keys are not
being depressed, thus preventing unwanted bleed-
through.
Q4 supplies the damper buffers on the Keyer board
with a supply voltage of +12 volts when the pedal
is not depressed. When the Sustain Pedal is depres
sed, the PED line goes low, which removes the damp
ing function from all of the keys. When this happens,
the Keyer Bias voltage level is shifted up slightly via
R9 and R10 so that the keyers in the Keyer/Divider
chips pinch-off during the time the dampers are
removed.
Z4B and Q3 generate the Timing Bias reference for
the Keyer boards. The capacitors which establish the
time of flight are discharged to this reference level.
It is set to be about a half-volt lower than the Keyer
Bias level. Z5A supplies about 9.8 volts to the
pulse modulated decay drive (Z10). It is also de
rived from the Keyer Bias voltage so that the decay
rate of the Short envelope only affects the first part
of the decay.
4.2 Upper and Lower Keyer Boards
(Refer to the Upper Keyer Board
schematic.)
The Upper and Lower Keyer Boards are almost
identical in design. They both use the same printed
circuit layout but are assembled with slightly differ
ent component values. The Upper Keyer Board is
assembled to key only 36 notes and the envelope
decay resistors are scaled to produce short decay
times for the higher pitches. The Lower Keyer Board
is wired for 37 notes and has a longer decay time for
the bass notes. In all other respects, the two boards
are the same.
TROUBLE SHOOTING HINTS
Envelope Generators: Often problems assumed
to be associated with the keyswitches are actu
ally on the Keyer Board in the Envelope Gener
ator Circuits. These circuits cannot be probed
easily since they are very high impedance
circuits. Just touching a probe to most of the
envelope circuit nodes is usually enough to key
the pitch on.
When a problem occurs, use a small scribe or
screwdriver as a probe. Touch the damper bar
(ground) with one hand and probe with the
other (Stay away from the Power Supply!).
-(CONTINUED)-
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Just touching the various nodes in the envelope
circuit will be enough to key on the note and,
by comparing to adjacent keys, problems can
be quickly traced,
CAUTION: Avoid touching the board with
your fingers as this will leave salts which can
contaminate the surface. Then, in high humid
ity, notes may bleed through.
NOTE: The 73 keys are numbered 8 through
80 to correspond with the key numbers found
on a standard 88-note piano keyboard. Since
the keys on the keyboard are numbered this
way, the schematic references are to the key
numbers on the key. Note that "L" denotes
Long,
and "S" denotes Short. Therefore, the
symbol L37 denotes Long envelope, key num
ber 37 as stamped on the key, not the number
from the lowest key.
4.2.1 ENVELOPE GENERATORS
For each key there is an Envelope Generator Circuit
which produces two envelopes. All of the Envelope
Generators are identical except for the value of the
decay resistors. The envelopes are all negative going;
that is, they are initially at a high voltage (about +11
volts) and they drop to a negative voltage when a key
is struck and then they decay back up to the Keyer
Bias voltage of approximately +10.5 volts.
4.2.2 TIME OF FLIGHT MEASUREMENT
1) When the key is at rest, the key switch holds the
timing capacitor T110 at -12 volts through R112.
2) When a key is pressed down, the keyswitch breaks
contact with the -12 volts, and the timing capacitor
quickly begins to discharge through R111 and R110
to the timiag bias bus (about +10 volts). 3) When
the key nears the end of its travel, the key switch
makes contact with the B contact and the rest of the
charge on the timing capacitor, which is propor
tional to the time elapsed since it left the -12 volt
contact, is then deposited on the envelope capaci
tors C111 and C112, through diodes CR112 and
CR113. Rapidly struck keys will deposit a more
negative peak voltage on the two envelope capaci
tors and thus produce louder notes. Softly played
keys will produce a small negative peak voltage on
the envelope capacitors.
The envelopes which are generated by C111 and
C112 are directly connected to the envelope inputs
of the Keyer/Divider chips (Z1, 2, 3 and 4). These
inputs are very high impedance and have no effect
upon the discharge rate of the envelope capacitors.
The more negative the voltage on these inputs, the
larger the audio signal on the output of the Keyer/
Divider chips.
C111 is the Long envelope generator capacitor and
its decay path is through R114. In the bass region of
the Lower Keyer Board where very long decay times
are required, the value of this resistor is 33 megohms.
On the treble end of the Upper Keyer Board the
value is 3.9 megohms. C112 is the Short envelope
capacitor and its decay path is through R115. The
pulse modulated decay line (PMD) provides an
additional discharge path for C112 through diode
CR114 and R46. This line provides a high frequency
pulse to discharge the Short envelope capacitor more
quickly for some of the voices. The PMD line has one
of four values; 0 volts, 25% duty cycle pulse, 75%
duty cycle pulse or +10 volts. These signals are
generated on the Power Supply PC Board.
1. KEY AT REST
2. KEY IN MOTION
3. KEY DOWN ("STRIKE")
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Z100F and R113 provide the damping function for
the envelopes. The input of Z100 is connected to the
timing capacitor via R111 to monitor its voltage
level. R111 and R110 are scaled so that when the
timing capacitor is at -12V (indicating the key is at
rest), the output of Z100F is a logic 1 (about H2V).
When the output is high, the envelope generators are
"damped" or held to this voltage through CR110 and
CR111, however, as soon as the key is presse i down
and the timing capacitor begins to charge towards
+10 volts, the output of Z100F goes low, thus having
no effect on the decay rate of the envelope genera
tors. When the Sustain Pedal is depressed, the power
supplied to all of the damper buffers (Z100, itc.) is
removed to allow all of the notes to decay v ithout
any damping action.
4.2.3 KEYER/DIVIDER CIRCUIT
On each Keyer Board there are two pairs of Keyer/
Divider chips. Z1 and Z2 are for the Long divider
chain and Z3 and Z4 are for the Short divider chain.
Each of the Keyer/Divider chips have six audio inputs
for six of the twelve pitches in an octave. These
pitches are supplied by a top octave synthesizer chip
(either Z1 or Z11) which derives the 12 master fre
quencies in an octave from the upper Short clock
(USCLK) and the upper Long clock (ULCLK). Inside
the chip, these six pitches are divided down and con
nected to internal keyers. When a negative voltage is
supplied to the envelope inputs of the Keyer/Divider
chips, square waves are gated out on the 8', 4' and 2'
audio outputs.
The three audio outputs from the Keyer/Divider chip
are supplied to very low input impedance buffers to
limit intermodulation distortion between keys. The
outputs of the buffers (Z4, 5, and 14) are sent over
to the Voice Board.
4.3 Voice Board
TROUBLE SHOOTING HINTS
All of the audio signals on the Voice Board are
percussive, and most are polyphonic. Therefore,
it is very difficult to probe the audio paths and
be sure that what is displayed is in fact what it
seems to be.
A more effective technique for trouble shotting
problems associated with the audio paths on
the Voice Board is to construct an audio probe.
(See Audio Probe diagram)
Connect the probe to a power amp (one with
a fair amount of gain), and hold the mute
(CONTINUED)
switch while you connect the probe to the
circuit Remember that all of the audio on the
Voice Board is referenced to +6 volts and will
cause a loud "POP" if you forget to use the
mute switch.
To trouble shoot, start at the 8', 4' and 2' sig
nals from the Keyer Boards and work your way
through the test points. Refer to the voice flow
charts on page 21-36.
NOTE: THE FOLLOWING TEST
POINTS ARE SUMMING JUNC
TIONS AND ARE ONL Y USED
AT THE FACTORY. THESE
TEST POINTS SHOULD BE
DISREGARDED.
TP1, TP3, TP4, TP5, TP10, TP11, TP12
Usually, the presence of a signal on a test point
means the circuit is working properly. Compare
upper and lower keyboard test points if in
doubt (e.g., compare TP7, upper keyboard, and
TP6, lower keyboard, to see if they are similar).
AUDIO TEST PROBE
Mute Switch Connection
100K
To
Amplifier
Input
j: -
4.3.1 SIGNAL FLOW
Although the mixtures, signals and filter settings are
different for each voice in the instrument, all voices
share a similar signal flow. Each of the four groups
of three signals coming from the Keyer Board are
mixed together and prefilterd, resulting in four
signals: Upper Short, Upper Long, Lower Short, and
Lower Long (US, UL, LS, LL). Each of these four
signals is filtered once again and then sent to a 4-to-
2 mix circuit. The two outputs from this circuit are
then set into the resonator bank which has one audio
output. This output is processed through the phaser
circuit which has stereo output.
4.3.2 SHORT MIX CIRCUIT
The Short Mix Circuit combines the three footages
(8', 4' and 2') from the Upper and Lower Keyer
Boards in one of four different weightings and com-
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VOICE BOARD SIGNAL FLOW
SHORT FILTERS
OUTPUTS
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binations. For some of the voices, these sign; is are
mixed and sent directly to the Short Filter Circuit.
For others,the signals are individually filtered before
they are mixed by C3, 5, 7, 9 and 11. The following
chart lists the different possible combine)lions.
Z1
Address
Code
(14) (15)
A B
Z1
Mix
Selected
Footage Combination
Z2 is the mixing Op amp for the Short Mix Circuit.
Z3A and Z3B attenuate the outputs of Z2 wh. n the
soft pedal is depressed which lowers the amo nt of
Short signal in the final mix.
4.3.3
SHORT FILTERS
In all, there are 8 different filters in the Short Filter
Circuit. Four are used for the upper half of tf.j key
board and four for the lower half. Z7 is the hicjvpass
filter with a switchable cutoff frequency. 2 3 and
Z9 are fixed band-pass filters and Z11 is a low-pass
filter, also with a switchable cutoff frequency. The
outputs from the Short Mix are sent to all i ,ur of
these filters but only one is selected at a time b Z12.
The following chart lists filters and their appro a imate
frequencies:
Z12A
Address
Code
(03) (04) Filters Filter
A B Selected OP AMP Ref. No.
Two outputs from the Short Filter Circuit are sent
to the 4-to-2 Mix Circuit (see the voice flow charts
for the signal flow for each voice).
4.3.4 LONG MIX AND LONG FILTERS
The Long Mix Circuit combines the Long 8', 4', and
2' signals from the Upper and Lower Keyer Boards.
Unlike the Short Mix, the signals are always pre-
filtered. Z5A sums the Upper Long signals and Z5B
sums the Lower Long signals. The following chart
lists various selections available.
Code Line
(16) (17) Footage Selected (Long)
8' Prtfilttrtd
6'. 4' Prtfilttrtd
8*. 2' Prtfilttrtd
8', 4' 2' Prtfilttrtd
In the Long Filter Circuit there are only four filters;
two for the upper keyboard and two for the lower
keyboard. Z13A and B are band-pass filters and
Z16A and B are low-pass filters with selectable cut
off frequencies. The following chart lists their
approximate frequencies.
UPPER LONG FILTERS
Z7A selects either the low-pass filters or the band
pass filters and routes the signal to the 4-to-2 Mix
Circuit.
4.3.5 THE 4-TO-2 MIX CIRCUIT
The outputs of the Short and Long Filter Circuit
are mixed together in the 4-to-2 Mix Circuit. For
some of the voices, the Upper and Lower Short
signals are combined. For others, the Long and Short
signals for each half of the keyboard are combined.
One of the signal paths inserts a special equalizer
circuit (Z18B) which is used on voices P, 2 and 9
after the Short and Long signals have been combined.
The following chart illustrates the combinations:
Z19A"X"Mixdown
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Z20 sums the two signals and sends them to the
resonator circuit via the envelope follower circuit.
4.3.6 ENVELOPE FOLLOWER CIRCUIT
For voices P through 13, the Envelope Follower is
not used and the two signals (X and Y) are routed
around the circuit via Z35 and Z36 (See the voice
flow charts for the signal paths). CR5, 6, and 7
decode the voice code so that there is a logic 0 on
Z36, pin 9, only for voices 14, 15 and 16, thus
routing the outputs of the Envelope Follower directly
to the resonator bank. The Envelope Follower can
process either the X or the Y signal from the 4-to-
2 Mix Circuit (selected by Z35A) but not both.
The Envelope Follower is a two pole, band-pass
filter whose resonant frequency is determined by
two LDRs (Light Dependent Resistors) which are
inside of photo module PM1. Z36C decreases the re
sonance of the filter on voice 14 (only).
4.3.7 ENVELOPE FOLLOWER CONTROL
CIRCUIT
Two 2' Short audio signals are taken directly from
the Keyer Boards and filtered by CR1 and C1 in the
Short Mix Circuit and are used to generate an
envelope to sweep the filter. This envelope is sent to
Z32B and the Envelope Follower Control Circuit
which powers the LED and PM1. This LED, in turn,
"sweeps" the envelope filter at the decay rate of the
2' audio. R169 sets the sensitivity of the Envelope
Follower.
4.3.8 RESISTOR MATRIX AND RESONATOR
BANK CIRCUITS
The Resonator Bank consists of 6 band-pass filters,
tuned to 125Hz, 250Hz, 500Hz, 1KHz, 2KHz and
4KHz. A "straight" unfiltered signal path is also
available. The voice code generated by the control
panel addresses four demultiplexers (Z1 through Z4)
on the Resistor Matrix Board which routes the X
and Y signals from the 4-to-2 Mix Circuit to resistors
which set the amount of signal going to each band
pass filter in the Resonator Bank (see the voice flow
charts for signal paths). Z49B sums the outputs of
the resonators and sends it to the phasor circuit via
the volume and tone controls on the control panel
(tone feed J11-1).
4.3.9 PHASE SHIFTER
The Phase Shifter operates in two modes. 1) When
the phasor is turned on the control panel, it provides
traditional phasor effects (deep notches and high
resonance peaks), 2) when the phasor is off, it
provides*a subtle stereo animation by slowly sweeping
with shallow notches and no resonance. The phasor
always processes the audio signals, whether it is turn
ed on or off by the control panel.
The output of the Resonator Bank is sent to Z30B
which drives 6 phase shift stages. The amount of
phase shift is varied by 5 LDRs (Light Dependent
Resistors) inside a photo module PM2. The resistance
of these LDRs is determined by the control LED in
PM2, which is driven by the Phasor Control Circuit.
When the phasor is turned off on the control panel,
the output of the phase shift stages are routed
through Z54C to the left audio output. The right
channel receives a straight unphased signal through
R228 and R54A. The mono output is a combination
of straight phased outputs. Also the resonance feed
back path is broken by Z36A. When the phasor is
turned on, the left output has both straight and
phased signals summed along with the resonance
feedback. The right and mono outputs are the
straight and phased signals averaged together in
different amounts. The resonance feedback path is
through Z55B, the resonance control on the control
panel, Z36A and Z30A (which inverts the signal).
4.3.10 PHASOR CONTROL
A low frequency triangle wave is supplied to the
Phasor .Control Circuit from the control panel. C72
filters the triangle wave (and causes it to decrease
in amplitude at higher speeds). Z32A drives CR8
through 12 to simulate an exponential response. This
response is desirable to create an even sweep effect to
the ear as the LFO oscillates. The LED and Photo
module PM2 are driven* by Z33B. It establishes the
resistance of the 6 LDRs in the Photo module. One
of the LDRs (PM2B) is on the input of Z33A, pro
viding an effective feedback path from the output of
Z33B to the input of Z33A. In this configuration,
the resistance of PM2B will be automatically adjusted
to be inversely proportional to the current supplied
by Z32A, thus establishing the phasor tuning. As the
LFO from the control panel oscillates, the LED and
PM2 increase and decrease the resistance of all 6
LDRs in the Photo module, thus varying the amount
of phase shift.
4.3.11 +6 VOLT POWER SUPPLY
All of the audio circuits on the Voice Board, with the
exception of the Output Amplifiers and Headphone
Circuit are referenced to +6 volts instead of ground.
The +6 volts is supplied by Q11, 12 and 13. This
voltage reference is used to keep the audio signals
between the power supply voltages supplied to all of
the CMOS switches (Ground and +12V).
13
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4.3.12 OUTPUT AMPLIFIERS
The three audio outputs from the Phasor Circuit are
supplied to the output buffers (Z56 and Z57A)
through N channel FETs (Z55). These FETs turn off
the audio when a voice is selected and during power
up and power down to mute the outputs. The
external audio inputs are supplied to the lefi and
right output amplifiers. The three output buffc s are
referenced to ground, as is the headphone amplifier.
Note that the ground path for the headphones s not
from the Voice Board; ground is supplied to the yoice
panel via the green ground cable from the power
supply panel.
4.3.13 MUTE CIRCUIT
The Mute Circuit controls the FETs which intt rrupt
the audio output of the instrument when the voices
are changed or when the instrument is turned on or
off. During a voice change, a strobe pulse is generat
ed by the control panel which momentarily tuns on
Q5, which turns off the 3 FETs. When the i istru-
ment is first turned on, the voltage supplied from
the power supply on P9-6 (power sense) beg .is to
rise. R146 and C70 slow down the rise of the volt
age, thus preventing the output from turning on
immediately. CR4 is a quick discharge path to turn
off the 3 FETs fast, when the power is turne i off.
4.3.14 ROM LATCHES AND ROM LATCH
CONTROL CIRCUITS
The signal paths and the filter settings for each of the
16 voices in the instrument are stored by a Read Only
Memory (ROM, Z22). A total of 16 lines is used to
set the different parameters of the instrument. Since
16 lines are necessary to program the instrument and
there are only 8 outputs on the ROM (Z22), 8 output
states are read out of the ROM at a time, then they
are stored by a latch (Z27). The ROM is immediately
addressed again and the second 8 output states are
stored in latch Z9, thus storing the total of 16 lines.
Once the 16 lines are stored, the ROM is no longer
needed and the power to it is turned off, since the
device draws a lot of current.
Controlling the access of information stored in the
ROM is a strobe pulse and a 4 bit (4 line) address
code provided by the control panel. These signals are
generated each time a new voice is selected or when
the instrument is first turned on. The voice code
from the control panel is actually the complement of
the code which is used by the ROM. The code is
inverted by Z21. The following chart lists the codes
for each voice.
When the strobe from the control panel is generated,
it momentarily turns on Q2 in the ROM Latch
Control Circuit to clock Z31A. Z31A and Z31B are
1-shot pulse generators and have a pulse duration of
about 3 microseconds. The output of Z11A in turn
clocks Z11B, thus producing two pulses (CO and C1,
sequentially). The first pulse (CO) is used to clock
Z27 in the ROM Latch Circuit, which latches the first
8 data lines. The second signal (Z1) is used to clock
the second latch (Z28) which stores the second 8
data lines. The C1 pulse is also used as th MSB (Most
Significant Bit) in the 5-bit code necessary to address
the ROM. The other 4 bits are supplied by the
control panel. (See the ROM truth table in the sche
matic section of this manual.) Z24 and Z25 are used
as inverter buffers to drive the latches.
STROBE
Q1, Collector •
CO
00-07 Bits addressed
C1
10-17 Bits addressed
Q3 and Q4 in the ROM Latch Control Circuit are
used to supply power to the ROM (+5V) only during
the pulses from the 1-shots (Z51A and B).
4.4 Control Panel Board
4.4.1 AUDIO CONTROLS
The output of the Resonator Bank on the Voice
Board is sent via the Tone Send line to the Control
Panel. There, the tone control (R2) boosts or cuts
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the treble and the volume control attenuates the
signal. The Tone Return line sends the signal back to
the phasor input on the Voice Board.
+6V
4.4.2 TUNING CONTROLS
The Master Tune Control supplies a voltage between
+12 and -12 to the CV input of the high frequency
VCO on the supply via the tune line (P10-15). If
selected, the vibrato triangle wave is supplied to the
tune line also.
4.4.3 VIBRATO LFO, SWITCH
Z3A and Z3B are a low frequency oscillator which
produces a square wave and a triangle wave. Its
frequency varies between 3 and 7Hz , depending on
the setting of the speed control. R7, the vibrato
speed control, varies the current supplied to integra
tor Z3A and thus sets the frequency. The square
wave output is supplied through R33 to the Trill line.
It is used only for voice 5, to create the trill effect.
The triangle output of the vibrato LFO is supplied
from Z3A,.Pin 1, through the vibrato depth control
(R8) to the input of the vibrato VCA (Z5A, B and
Z9B). This VCA is used to slowly turn on the vibrato
when the left foot pedal is depressed, thus insuring
that the vibrato does not abruptly snap on when it
is selected.
When the vibrato on/off line from Z13F and 12 goes
low, Q6 turns off and C6 begins to discharge through
R43 toward -12 volts. As the voltage on C6 goes
more negative, more current is drawn through the
vibrato VCA via R42 and the vibrato triangle wave is
gated out to the Tune line. Z5E is turned off, which
allows the vibrato LED (CR20) to light when the
vibrato on/off line is low (OV).
4.4.4 PHASOR LFO
The Phasor LFO operates in two modes. When the
phasor is turned off, it oscillates at about .2Hz and
its speed is not controllable. When the phasor is
turned on, speed is increased to between .1 and
6.3Hz, depending on the speed control setting.
The Phasor LFO is not unlike the vibrato LFO.
Z1A is an integrator which supplies a rising or falling
voltage (a triangle waveform) to a hysteretic switch
composed of Z1B and Z6A. Z6A is used to provide
accurate output voltage of either 0 or +12 volts which
is necessary for precision control of the LFO output
swing. The hysteretic switch is set to switch at
+5.9 volts when the voltage on Z1A, Pin 1, is rising
and 0 volts when it is falling.
When the phasor is turned on, the P signal from the
Switch Latch Circuit is high (+12V) and the P signal
is low. This turns on the N and P channel FETs
connecting the phasor speed control to the input of
integrater Z1A. Z5D is also turned off enabling
CR7 to light. When the phasor is turned off, R10 is
connected to the input of Z1A, setting a fixed speed.
Z4A generated +6 volts as a reference voltage so that
the circuit is compatible with the Voice Board
circuitry.
4.4.5 VOICE SELECT LOGIC AND DEBOUNCE
LOGIC
This circuit debounces and latches the state of the 19
switches on the control panel, lights the indicator
LEDs, generates a strobe pulse for the Voice Board
ROM Circuit, and processes the left foot pedal con
trol signal. The 16 voice switches are arranged in a
2 by 8 matrix (voices P through 8 and 9 through 16).
Q1 senses whether the voice being pressed is in the
upper-or lower bank; the collector Q1 is low when
voices P through 8 are pressed and high (+12V)
when voices 9 through 16 are pressed. The 8 switch
lines from the control touch panel are connected to
an encoder (7 through Z7) which reduces the 8 lines
to a 3-bit code. This 3-bit code and the bank code
from Q1 make up the voice code. This code is
supplied to latch Z8. (See the voice code chart on
Page 20.)
NEW VOICE STROBE PULSE GENERATION
ssr --JIJITLTLTL
Z9 "D4" input
Voice button held down
Z9 "Q4" output
(and "D5" input).
Z9 "D5" input
Z8-8
Z14Apin3.
Strobe Pulse
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To be sure that signals from the control panel are
valid, a free running debounce oscillator (Z13A, B
and C), oscillating at approximately 50Hz, clocks
latch Z9. One of the lines on the input of this latch
is from the encoder (Z7, Pin 14, GS line). This
line goes high whenever there is a signal present on
any of the inputs from the switches (thus indicaiing a
voice button is being pressed). When a button is
pressed and when the debounce oscillator clocks a
high into the 'D4' input of Q9 (Pin 13), the next
clock signal from the debounce oscillator produces
a high on the 'Q5' output, then clocks the voice code
latch Z9. The voice code is thus stored.
Z10A and Z10B decode the voice code to a 4 by 4
matrix of LEDs to indicate the voice which has been
selected. The complement of the voice code (QO, Q1,
Q2 and Q3) is sent to the Voice Board.
4.4.6 PHASOR ON/OFF
When the phasor button is pressed, a high is swit to
the 'D' input of latch Z12A. This latch is also clock
ed by the debounce oscillator. The output of the
latch is sent to flip/flop Z12B, which toggles up
alternately high or low (Phasor on or off). The P and
P outputs are sent to the Phasor LFO Circuit and the
P signal is sent to the Voice Board on P10, pin 10.
4.4.7 VIBRATO ON/OFF
When the Vibrato button is pressed, +12 volts is sup
plied from J12, Pin 3, to the 'D2' input of latch Z9.
When the debounce oscillator clocks Z9, a high is sent
out on '02' (Z9, Pin 7) to toggle flip/flop Z16B
alternately high or low. The output of Z16B drives
Z13F through Z15D to turn on the vibrato.
4.4.8 PEDAL AND VIBRATO LOGIC
The left foot pedal can be assigned by the control
panel to either turn on the vibrato or to act as a
'soft' pedal. When the Pedal Select button is prussed,
+12 volts is supplied to 'D3' (Pin 11) of the Z9. The
debounce oscillator then clocks a logic 1 to 'Q3' (Pin
10) output which toggles Z16A. This signal is also
sent via Z15C to reset the vibrato on/off flip/flop
Z16B to the "off" state. If the output (Pin 1) of
Z16A toggles high, then the red LED (CR19) lights to
indicate the vibrato pedal function is selected. Pin 12
of Z14B is high, and when the left pedal is depressed
so that +12 volts from P10, Pin 9, is also high, the
output of Z14B also goes high, thus turning on the
vibrato through Z15B and Z13F. When Z16A, Pin
1 is low (soft mode), Pin 2 of Z13F is high, which
lights the green LED. This enables Z14C to gate the
pedal signal to the Voice Board. Z16A is reset to
the 'soft' state whenever the vibrato flip/flop Z16B is
toggled via Z15B.
SCHEMATIC, PEDAL ASSEMBLY
4.4.9 STROBE (DEBOUNCE LOGIC)
A strobe signal is generated for the Voice Board when
the instrument is first turned on and whenever a voice
is selected. When the power is first turned on, C10
and R66 causes the 'D1' input (Pin 4) of Z9 to
briefly rise to a logic 1 state. Once it has reached a
high state (above +6V) and the debounce oscillator
clocks high to 'Q1' (Pin 5), the output is sent to reset
and initialize the phasor flip/flop (C12B) to the off
state and the soft/vibrato select slip/flop Z16A to
the 'soft' state and the vibrato on/off to the 'off
state. The high on the 'Q1' output of Z9 is clocked
on the next debounce oscillator cycle to the 'QO' out
put. When this happens, the 'DV input is low once
again and the 'Q1' output is clocked low. This
provides a high on the input of Z14B through Z13E
so that Z14B's output goes high, creating a strobe
pulse. Similarly, a strobe pulse is generated from the
outputs of 'Q5' and 'Q4' of Z9 when a voice is select
ed.
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SECTION 5 MAINTENANCE
General
The ARP 16-Voice Electronic Piano is all-electronic
so no regular maintenance is required. The following
procedures should be followed if a problem occurs.
5.1 Key Actuator Adjustment
ADJUST KEY ACTUATOR WHEN A KEY
CONTINUES TO SUSTAIN ( AS THOUGH
THE "DAMPER" WERE STUCK) WHEN
THE KEYS AND THE PEDAL ARE RE
LEASED.
1. Remove the top cover and top three
nuts (only) of the Keyer Board. Gently
lift the Keyer Board to gain access to
the keyboard action.
2. Identify the faulty key and slightly
loosen the actuator screw on the rear of
the hammer arm. Gently push the
actuator spring in the hammer arm
slot toward the rear of the instrument
to apply more pressure to the switch.
THE ACTUATOR SHOULD REQUIRE
NO MORE THAN 1/16" ADJUST
MENT
3. If the problem persists, note the loca
tion of the actuator wheel. It should be
within the top switch square (see illus
tration, page 18). If it is above or below
this square, remove the key and adjust
the capstan screw on the end of the key.
5.2 Switch Replacement Procedure
MOST PROBLEMS WHICH APPEAR TO
BE RELATED TO THE KEY SWITCHES
ARE ACTUALLY ON THE KEYER
BOARDS. REPLACE THE KEY SWITCH
ONLY AFTER TESTING THE CONTINU
ITY OF THE SWITCH WITH AN OHM-
METER. (NOTE: CONTACT RESIST
ANCE IS TYPICALLY BELOW 40 OHMS.)
There are three 18 key switch assemblies and one 19
key switch assembly. The 19 key switch assembly
is for the bottom 19 keys. BE SURE THAT YOU
HAVE THE CORRECT KEY SWITCH ASSEMBLY
BEFORE STARTING THE REPLACEMENT PRO
CEDURE.
1. Remove the top cover, Voice Board and
Power Supply printed circuit boards.
2. Remove the six nuts on the front and
rear of both Keyer Boards and discon
nect the six tail connectors on each
Keyer Board.
3. Remove the three switch plate mount
ing screws, the two retaining bolts, and
remove the entire switch assembly.
4. Peel off the defective switch assembly
and clean the metal surface with dena
tured alcohol. BE SURE THE SUR
FACE IS CLEAN AND FREE OF ANY
BUMPS.
5. Peel the backing from the keyswitch
and align it with the other keyswitches.
6. Put the switch plate in place on the key
bed and insert but do not tighten the
three key switch mounting screws.
7. Be sure the rollers are in a line of sight
with a deviation of no more than
+ 1/32" vertically and +1/16" horizon
tally. Adjust as needed.
•8. Gently move the switch plate toward
the keys until the plate just touches all
of the rollers on the keyboard action,
then move the plate exactly 3/32"
towards the action and tighten the
mounting screws. This will put a load
of approximately 45grms. on each
keyswitch. Replace the two bolts
through the switch plate and keybed.
9. Remount the Keyer Boards, carefully
insert the 12 switch tails and remount
the Power Supply and Voice Boards.
10. Turn on the instrument and test each
key. Adjust individual key switch
actuators if necessary. See diagram,
page 18.
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KEY ACTION & SET-UP DIAGRAM
OPTIMUM
KEY AT REST
SET-UP SPECIFICATIONS
1. Hammer at rest: Bottom front of hammer to base - 1.45".
2. Damper bar height: Base to top of bar - 3.90".
3. Normal switch plate location from action rail: Rail to bottom of plate - 2.531" typical.
4. Normal key dip: 3/8" + 1/32".
REMOVE THESE NUTS
FOR ACCESS TO THE ACTION
OPTIMUM
KEY HELD DOWN
LOCATION
P.C. BOARD HINGES HERE
TYPICAL
MAXIMUM EXCURSION
(HAMMER IN FLIGHT)
ACTUATOR
ADJUSTMENT
SCREW
SWITCH PLATE
CAPSTAN SCREW ACTION RAIL
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19
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