Octave Voyetra Eight Owner's manual

Voyetra Eight Hardware Maintenance Manual
Revision 1.0a
Revision Date Changed By Reason
1.0 ??/??/1984 Carmine J. Bonanno Original Revision
1.0a 06/19/2002 Joan Touzet
[email protected]
http://www.atypical.net/
Mistake correction
Revisions for clarity
Started schematic entry
“Copywright [sic] 1984 by OCTAVE-PLATEAU ELECTRONICS, INC.
51 Main Street
Yonkers, New York 10701”

Table of Contents
1INTRODUCTION............................................................................................................................... 4
1.1 TRACKING DOWN VOICES ............................................................................................................ 4
1.2 IS ANYTHING REALLY WRONG?................................................................................................... 4
1.3 DOES THE INSTRUMENT NEED A CALIBRATION?............................................................................ 4
1.4 CARD SUMMARY .......................................................................................................................... 6
1.4.1 Mixer Card.............................................................................................................................. 6
1.4.2 VCO Card ............................................................................................................................... 6
1.4.3 VCF Card................................................................................................................................ 6
1.4.4 DAC Card ............................................................................................................................... 6
1.4.5 DIGIMOD Card...................................................................................................................... 6
1.4.6 ANAMOD 1 Card.................................................................................................................... 6
1.4.7 ANAMOD 2 Card.................................................................................................................... 6
1.4.8 CPU Card ............................................................................................................................... 6
1.4.9 Pot Panel................................................................................................................................. 7
1.4.10 Switch Panel....................................................................................................................... 7
1.4.11 Jack Bay ............................................................................................................................. 7
1.4.12 Power Supply ..................................................................................................................... 7
1.4.13 Keyboard ............................................................................................................................ 7
1.5 SIGNAL FLOW DIAGRAMS............................................................................................................. 8
2MIXER CARD................................................................................................................................... 10
2.1 THEORY OF OPERATION.............................................................................................................. 10
2.2 VCA OPERATION ....................................................................................................................... 10
2.3 HEADPHONES ............................................................................................................................. 11
2.4 NOISE ......................................................................................................................................... 11
2.5 CALIBRATIONS ........................................................................................................................... 11
3VCO CARD ....................................................................................................................................... 12
3.1 THEORY OF OPERATION.............................................................................................................. 12
3.2 VCO OPERATION ....................................................................................................................... 12
3.3 AUTOTUNE FUNCTIONALITY ...................................................................................................... 13
3.4 DC VOLTAGES............................................................................................................................ 14
3.5 CARD PIN SIGNALS ..................................................................................................................... 14
3.5.1 Inputs .................................................................................................................................... 14
3.5.2 Outputs.................................................................................................................................. 14
3.6 TROUBLESHOOTING TIPS ............................................................................................................ 14
3.6.1 Step 50-71: Waveform Tests................................................................................................. 15
3.6.2 Step 72-73: Sync Check......................................................................................................... 15
3.6.3 Step 74-75: Linear FM Check............................................................................................... 15
3.6.4 Step 76-79: Modulation Test................................................................................................. 15
3.7 TYPICAL PROBLEMS AND SOLUTIONS ........................................................................................ 15
3.7.1 Letter “A” appears ............................................................................................................... 15
3.7.2 Letter “B,” “C” or “D” appears ......................................................................................... 16
3.7.3 Letter “E” appears or other pulse width problems .............................................................. 16
3.7.4 No sub octave........................................................................................................................ 16
4VCF CARD ........................................................................................................................................ 17
4.1 THEORY OF OPERATION.............................................................................................................. 17
4.2 VCF OPERATION ........................................................................................................................ 17
4.3 CALIBRATIONS ........................................................................................................................... 18
4.4 VCO MULTIPLEXING SYSTEM .................................................................................................... 19
4.5 DC VOLTAGES............................................................................................................................ 19

4.6 CARD PIN SIGNALS ..................................................................................................................... 19
4.6.1 Inputs .................................................................................................................................... 19
4.6.2 Outputs.................................................................................................................................. 20
4.7 VCF CARD TROUBLESHOOTING TIPS ......................................................................................... 20
4.7.1 Step 00-02: Resonance (Q) Tests ......................................................................................... 20
4.7.2 Step 03: DC Feedthrough Check ......................................................................................... 20
4.7.3 Step 04-11: Initial Fcand V/Oct Calibration ....................................................................... 20
4.7.4 Step 11-27: VCO Volume Test ............................................................................................. 21
4.7.5 Step 28-31: Fc& Q Modulation ........................................................................................... 21
4.7.6 Step 32-35: VCO1 & 2 Modulation...................................................................................... 21
4.7.7 Step 36-39: Noise Audio Path.............................................................................................. 21
4.7.8 Step 40-43: VCO Bypass VCF ............................................................................................. 21
4.7.9 Step 43: TL068 Drift Test..................................................................................................... 21
5DIGIMOD CARD ............................................................................................................................. 23
5.1 THEORY OF OPERATION.............................................................................................................. 23
5.2 DEMULTIPLEXING ENABLE SIGNALS .......................................................................................... 23
5.3 MODULATION SYSTEM SCAN CLOCKS........................................................................................ 24
5.4 DEMULTIPLEXING /REMULTIPLEXING ........................................................................................ 24
5.5 MONO CONTROLLER MULTIPLEXER ........................................................................................... 25
5.6 MISCELLANEOUS SWITCHING FUNCTIONS .................................................................................. 25
6KEYBOARD...................................................................................................................................... 26
6.1 THEORY OF OPERATION ............................................................................................................. 26
6.2 CIRCUIT OPERATION................................................................................................................... 26
6.2.1 Power Retrieval .................................................................................................................... 26
6.2.2 Clock/Strobe Retrieval ..........................................................................................................26
6.2.3 Digital Data Acquisition ....................................................................................................... 27
6.2.4 Analog Data Acquisition....................................................................................................... 28
6.2.5 Keyboard Pressure Sensor.................................................................................................... 28
6.2.6 Joystick.................................................................................................................................. 29
6.3 CALIBRATIONS ........................................................................................................................... 29
6.4 KEYBOARD TROUBLESHOOTING TIPS ......................................................................................... 29
7POT PANEL ...................................................................................................................................... 30
7.1 THEORY OF OPERATION.............................................................................................................. 30
7.2 KEYBOARD CLOCK/POWER INTERFACE...................................................................................... 30
7.3 KEYBOARD DATA INTERFACE .................................................................................................... 31
7.3.1 Digital Data Retrieval........................................................................................................... 31
7.3.2 Analog Data Retrieval .......................................................................................................... 32
7.3.3 Controllers ............................................................................................................................ 32
7.4 PARAMETER TRIMMER MULTIPLEXER ........................................................................................ 32
7.5 MASTER VOLUME AND MASTER TUNE CONTROLS..................................................................... 32
7.6 CALIBRATIONS ........................................................................................................................... 33

1 Introduction
1.1 Tracking Down Voices
From time to time, this manual will refer to calibrations for a particular
voice. These voices are numbered from 0-7 and are indicated on the
trimmer locations diagram. However, when troubleshooting, it is often
necessary to determine which voice is defective. The Set Page (see
appendix) provides a feature for disabling voices which may be used to
locate which voice is sounding.
1.2 Is Anything Really Wrong?
The Voyetra is a very complicated instrument and at times may generate
sounds which may seem to indicate a malfunction when none actually
exists. In case of question, the easiest way to determine if the instrument
is functioning properly is to load the Null Patch by pressing A440 and
then pressing Call Right and Call Left. This loads the instrument with a
known sound and the editing pages can be used to check for specific
features.
Alternately, the Quality Control/Calibration test tape may be loaded into
the instrument and the documented test procedure may be used to test
all system functions for proper operation.
It is strongly suggested that a problem be verified prior to attempting any
system troubleshooting.
Examples of non-existent problems are “puzzling” things like a changing
timbre that doesn’t always happen. Remember that the keyboard is
velocity sensitive and the timber change could very well be part of the
program. Other modulation routings can cause “weird” non-existent
problems such as detuning oscillators, strange tracking, etc.
1.3 Does the instrument need a calibration?
The Voyetra will rarely need calibrating since the most sensitive part of
the instrument, the VCO, is automatically calibrated when Auto Tune is
pressed. However, the VCFs, VCAs and Mod system do have
calibrations that may have to be tweaked.
The following list of sample problems may help in determining what
symptoms require specific calibrations:
One voice sounds “different” than the others.

First, determine what it is about the voice that is “different.” Is it the
“tone?” Then it may need a VCF calibration. Is it the “level?” Then it may
be a VCA calibration. Is it the “tuning?” Then it may need a Mod System
calibration, or a VCO may be defective.
If the tone is different, refer to the VCF section of the manual and
calibrate or troubleshoot the VCF circuits. If the level is different, refer to
the MIXER section and troubleshoot the VCAs. If the Mod System is at
fault, refer to the Mod System overview and then see whether it’s the
ANA1, ANA2, or VCF card. If there’s a VCO problem, refer to the VCO
section of this manual.
A strange “clicking” or “thumping” is heard whenever a key is pressed.
Is the click part of a patch? Play the null patch and see if it goes away.
Try turning off the VCOs in the null patch to see if it’s still there. Maybe
the VCA offsets on the mixer card need calibrating. Usually, only one
voice is out, so you’ll have to use the SET page to find the culprit.
One program is out of tune but the rest are O.K.
This sounds like a classic Mod System “problem” where VCO modulation
is on ever so slightly. A problem on “one program” is usually an indication
that the program rather than the instrument is at fault.
Pitch bend on the left is different than on the right.
This may be a Mod System setting, but it can also be a calibration on
ANA2 or the POT PANEL.

1.4 Card Summary
1.4.1 Mixer Card
Contains eight VCAs (one per voice) and a stereo mixer that sums the
system outputs. Calibrations include CVA offset trimmers and channel
gain trimmers. The MIXER also contains the Noise Source and various
other minor system functions.
1.4.2 VCO Card
Contains four VCOs. The system has four of these cards, two per side.
1.4.3 VCF Card
Contains four VCFs and modulation circuitry. The system has two of
these cards, one per side. Calibrations include filter scaling, initial filter
cutoff and initial filter resonance for each VCF.
1.4.4 DAC Card
Contains the Voice on/off functions, and the A/D and D/A functions,
which allow the CPU to control the analog system components.
Calibrations include DAC offset adjustment, reference trim and mod
system offset trim.
1.4.5 DIGIMOD Card
Contains system clocking logic for the modulation and DAC subsystem.
1.4.6 ANAMOD 1 Card
Contains the mod subsystem routing circuitry.
1.4.7 ANAMOD 2 Card
Contains high speed multiplexed VCAs for the mod subsystem.
Calibrations include VCA offsets and gain as well as an adjustment for
pitch bend LEFT and RIGHT balance.
1.4.8 CPU Card
Contains the host computer and memory circuitry for controlling the
Voyetra and memorizing patch settings.

1.4.9 Pot Panel
Contains the Keyboard and front panel parameter trimmer interface
circuits. Calibrations include pitch bend balance and gain, and X axis
gains.
1.4.10 Switch Panel
Contains LEDs and switches with interfacing circuits for connecting them
to the CPU.
1.4.11 Jack Bay
Contains interface jacks and an AGC circuit for the Tape input.
1.4.12 Power Supply
Contains regulators and fuses for +/- 15V, +5V, +11V, +22V, as well as
power-down detection circuitry for the battery backed-up RAM on the
CPU.
1.4.13 Keyboard
Contains digital and analog circuits to interface the keys and controllers
to the CPU via the Pot Panel.
FAN
JACK
Power Supply
ANA1 ANA2 DAC VCO VCO VCO VCO DIGI VCF VCF CPU MIX
MIX VCO VCF VCO VCO VCF VCO DAC DIGI ANA1 ANA2 CPU
POT
Figure 1-1 Card Placement inside the V8 Chassis

1.5 Signal Flow Diagrams
The following diagrams are included to assist in understanding some of
the Voyetra 8’s signal flow paths.
Mod Bank
VCAs
VCO1, VCO2 FcQ
22
2 2
VCO1
VCO1
2 2
2 2
VCO2 VCO2
Mono Contr.
MIX VCO VCF VCO VCO VCF VCO DAC DIGI ANA1 ANA2 CPU
POT
+/-x, Press (Mono Controllers)
Figure 1-2 Modulation System Signal Flow
JACK
MIX VCO VCF VCO VCO VCF VCO DAC DIGI ANA1 ANA2 CPU
VCO 1,2 VCO 1,2 VCO 1,2 VCO 1,2
Figure 1-3 Audio Signal Flow

Jack Bay
Mixer
Card
8
VCO 1
8
VCO 2
VCA
VOL 1
8
VCA
VOL 2
8
VCO Cards
8 VCF VCA
(8)
8 8
VCA VCA
VCA
VCF
Cards
Phones Volume
Phones Out
+15
Noise Gen
MM5837
VCA 2
Vol L/R
Right Mono Left
Master Outputs
Buffers
VCA VCA Programmable
CPU Vol L (Parameter Trim Volume)
CPU Vol R
Buffer
External Volume Pedal
+15
Master
Volume
+15
Pot Panel
Schematic 1-1 System Audio Path

2 MIXER Card
2.1 Theory of Operation
The MIXER Card consists of eight identical Voltage Controlled Amplifier
(VCA) circuits followed by mixing circuitry. In addition, certain other
related functions are placed on this card.
The functions on this board include:
•Eight channel demultiplexer for ADSR 2 voices 0-7
•Demultiplexing for Volume L, R, Noise volume L, R, and slave
synthesizer Gate, CV
•Noise generator with associated VCAs and Low Pass Filter
•Headphone Amplifier
2.2 VCA Operation
The VCAs on the MIXER Card are made up of SSM 2024 Quad VCA ICs.
The audio inputs to these VCAs come from the VCF card audio outputs.
Whenever a key is pressed and a voice activated, an ADSR voltage is
generated at the appropriate CD4051 output and drives the VCA for that
voice. This causes the VCA to pass the VCF audio at its input to the
summing amp at its output. The gain is set by a trimmer in line with the
ADSR signal.
The VCA outputs are mixed in four groups of two such that VCA 0 & 1
come from one op amp output, VCA 2 & 3 from another op amp output,
and so on. When the keyboard WHOLE 8 Mode is selected, the output of
sub-mixer 0, 1 is hard panned to one side of the VOYETRA output, while
the output of sub-mixer 6, 7 is hard panned to the other side. The
outputs for 2,3 and 4,5 are panned to be slightly offset from the center of
the stereo spectrum. When the SPLIT or LAYER modes are selected,
the outputs of 0, 1, 2, 3 are hard panned to one output while the 4, 5, 6, 7
outputs are hard panned to the other side. Of course, the MONO output
always contains the sum of all eight voices.
The switching to accomplish this output panning is accomplished by
using a DG308 analog switch that is turned on by a 0, +8V control signal
called SPLIT/WHOLE 8 generated at the DAC CARD.
The L and R output mixers consist of two VCAs that are used to set the
output level and serve as noise gates. When no keys are pressed, the
CPU turns off these VCAs to quiet the system output. The output level is
set by the Master Volume Control on the POT PANEL and the Parameter
trimmers. The CPU digitizes the parameter trimmers and demultiplexes

them on the MIXER CARD at the points called VOLUME L, R. The
Master Volume Pot is routed to the JACK BAY for processing by the
external Volume Pedal, and then routed to the MIXER CARD to the point
labeled MASTER VOL. These control signals are multiplied by VCAs that
convert them to control currents used to derive the volume/noise gate
VCAs as illustrated on the schematic.
The A440 tone comes from the DAC CARD and is turned off by the CPU.
It is injected into the system audio by an inverter transistor designed to
eliminate the digital noise that is present on all DAC CARD signals.
2.3 Headphones
The outputs of the VCA subchannel mixers are routed to another VCA,
followed by an opamp used as a headphone driver. The front panel
Headphone Volume control is used to send a control voltage to this VCA
to set the headphone volume.
2.4 Noise
The noise source is generated by an MM5837 digital noise generator.
This output is sent to two VCAs which set the noise L, R volume, and
whose outputs are routed to the VCF cards L & R. The noise is also
passed through a low pass filter whose low frequency noise output is
routed to the ANAMOD 1 CARD for use as a controller in the Modulation
System.
2.5 Calibrations
Any VCA will have a certain amount of DC feedthrough from its control
input to the output which causes an output signal that follows the DC
control signal. This sounds like a “thumping” or “clicking” when fast attack
and release ADSRs are used. To compensate for this, each VCA on the
MIXER CARD has an offset adjustment trimmer which must be set for
minimum DC feedthrough. This includes eight voice VCAs and two for
the Left Right output VCAs.
To assure that the voice volumes are all the same, individual channel
volume trimmers are also adjusted.

3 VCO Card
3.1 Theory of Operation
The Voltage Controlled Oscillator Card (VCO CARD) consists of four
oscillator circuits based on the CEM 3340. The system uses four
identical cards, two for the LEFT side of the instrument and two for the
RIGHT side of the instrument. The VCOs on the card correspond to
VCO1 and VCO2 for two voices. Thus, the left and right side of the card
are identical. The cards may be interchanged to facilitate troubleshooting.
The functions on this board include:
•Four VCO circuits each with Voltage Controlled Frequency,
Voltage Controlled Pulse Width and Voltage Controlled Tracking.
•Waveform mixers for each oscillator
•-5V reference circuit
•VCO control voltage demultiplexers
3.2 VCO Operation
The CEM 3340 is an integrated oscillator IC that generates sawtooth,
triangle and variable width pulse waves. The output of the sawtooth wave
is differentiated and sent to a CD4520 divider IC to generate a square
wave that is one octave below the oscillator frequency. The waveforms
are summed by an LF347 wide bandwidth op amp. Since this summer is
inverted, it is followed by an inverter to form positive waveform outputs
that are routed to the VCF card.
The triangle and sawtooth waveforms are routed through CD4053 analog
switches which are turned on or off by the DAC CARD. The pulse wave
is turned off by the CPU by adjusting the control voltage that varies the
pulse width to the 100% width value. This causes the pulse width to go
high, which is inverted by a transistor that feeds the mixer. The sub-
octave is turned off by the DAC when it toggles the rest line on the
CD4520 divider.
SYNC and LINEAR FM are also turned off/on by the DAC driving a
CD4053 analog switch.
Oscillator frequency is determined by the CPU tracking control voltage,
CPU initial pitch control voltage, modulation input control voltage (from
VCF) and pitch bend control voltage (from ANAMOD 2.) These voltages
are summed directly onto the CEM3340 via 1% scaling resistors. Note
that earlier model VCO cards use “blue” 1%, 100K resistor packs for this

summing network, while newer versions have this network replaced by
discrete 1% 100K resistors.
3.3 AutoTune functionality
When the front panel AUTOTUNE button is pressed, the CPU CARD
calibrates the VCO tracking (V/Octave), initial pulse width, and initial
frequency. The CPU first turns off all of the VCO waveforms except for
the Sawtooth and disables the VCO pitchbend lines coming from
ANAMOD 2. Then, it selects VCO1 in voice 7 (designated on the front
panel displays as VCO 15) by turning on the proper slot on the VCF
tuning multiplexer (see VCF CARD technical description), switching the
VCO L/R switch on ANAMOD 2 (see ANAMOD 2 technical description)
and selecting VCO1 on the DAC CARD (see DAC CARD technical
description.) The sawtooth is then converted to a square wave by a
comparator on the DAC CARD and the frequency is measured by a
68B40 timer IC on the DAC. The CPU uses this frequency to compute
the correction voltage it has to send to the VCO Control Input to have it
generate a frequency of 440Hz when the middle A key is pressed on the
keyboard. This voltage is added to the keyboard voltage when it is sent
to the VCO.
After all this, the CPU turns off the sawtooth, turns on the pulse, and
uses the same system routing to measure the pulse width. It then
computes the initial pulse width voltage necessary to get a 50% pulse.
These measured values are stored in the Batter Backed-up RAM on the
CPU (the same RAM that remembers the program settings) so that the
next time AUTOTUNE is pressed, the computer knows that it can begin
its calculations from the last calculations it made, even if the power is
shut off.
After it finishes with VCO1 voice 7, the CPU selects VCO2 voice 7
(called VCO 14), and performs the same calculations for it. Then VCO 1
voice 6 (VCO 13) is done, etc. until all 16 VCOs (15 through 0) are
calibrated.
If problems are found while the CPU is trying to tune the VCOs, a letter is
displayed next to the VCO number as follows:
Letter Problem
A Can’t sense VCO
B ?
C ?
D ?
E Pulse wave won’t turn off

Each letter corresponds to a different problem, but the only serious one
is “A” since it indicates that the VCO can’t be sensed by the CPU, either
because it is very low in frequency, has a DC offset on it, or is completely
missing.
If you get other letters coming up, they usually go away after the second
or third time you press AUTOTUNE. This is because it’s possible that the
CPU didn’t get the right reading on the previous try, or that the
instrument isn’t warmed up yet. Persistent letters indicate a true VCO
problem.
If any letters are displayed, the CPU will disable the voice in which the
defective VCO resides, so that the instrument may still be played with
one less voice until it can be repaired.
3.4 DC Voltages
The card uses +/- 15V to drive the CEM 3340s and opamp ICs. The
CEM 3340s also require a stable -5V reference, which is derived on
board to minimize system interaction between cards. The +10V reference
from the DAC is also used on this card to provide a current reference for
the CEM 3340s.
3.5 Card Pin Signals
The signals coming into and out of the card are as follows:
3.5.1 Inputs
Modulation inputs for the VCOs coming from the VCF CARD
+10V reference from the DAC CARD
Power supply inputs +/- 15V and analog ground
CPU control signals for the CD4051 demultiplexers (Enables and AL0, 1,
2) coming from DIGIMOD
DAC control lines (SAW, TRI, SUB, FM, SYNC)
Pitch bend input from ANAMOD 2
Multiplexed DAC signal for 4051s
3.5.2 Outputs
VCO1 & 2 outputs for two voices
3.6 Troubleshooting Tips
These troubleshooting tips assume that the VCO test tape REV 1.0 has
been loaded into the VOYETRA. If this tape is not available, the

suggestions will still prove useful if the theory section has been
thoroughly understood.
3.6.1 Step 50-71: Waveform Tests
These steps check that all of the VCO waveforms are present and are of
proper amplitude and frequency. Since the instrument has been
AUTOTUNEd and the middle A key is down, the outputs should all be
pretty close to A440. The setting of the MASTER TUNE control, the
pitchbend joystick and pitchbend pedal will all affect this.
Problems with the waveforms can be traced to the CD4053 for the saw
and triangle, the CD4520 for the suboctave and the pulse width control
voltage coming from the CD4052 for the pulse wave.
3.6.2 Step 72-73: Sync Check
The sync function is checked here. The distinct “sync” waveform should
be apparent as VCO1 is locked onto VCO2’s frequency. Problems may
be caused by the CD4053 switch and the CEM3340.
3.6.3 Step 74-75: Linear FM Check
Linear FM is when the output of the VCO2 mixer is routed to the linear
control current input on VCO1. Problems can be caused by the CD4053
switch or if the output of VCO2 is defective.
3.6.4 Step 76-79: Modulation Test
VCO1 and 2 modulation comes from the VCF card (see VCF technical
description) and is fed into the VCOs via summing resistors on the CEM
3340s. The problem of no modulation can be caused by improper
resistor values or shorts on the modulation lines.
Defective demodulation buffers on the VCF card can cause jittery
oscillators if the VCF has not been checked for proper operation for the
modulation demultiplexers. If jittery oscillators are found, try switching
VCF cards to see if the problem follows the VCF. If so, chances are it’s in
the Modulation system and not on the VCO card.
3.7 Typical Problems And Solutions
3.7.1 Letter “A” appears
This means the VCO can’t be sensed at the DAC. Check if the VCO has
a sawtooth at PIN 9. If so, trace it to the waveform mixer output that

feeds the VCF card. If not, switch the CEM 3340 with a good one and
see if the problem is a bad IC.
3.7.2 Letter “B,” “C” or “D” appears
This problem may not be on the VCO but instead may be coming from
the VCF. First switch VCF cards to see if the letters appear on other
voices. If not, then pull out the ANAMOD 1 card to make sure it’s not
caused by a defective modulation system. If you still have problems,
chances are the problem is on the VCO card, but just to make sure,
switch the bad card with a good one and see if the problem is still on the
bad one. If it is, then the card is defective.
These problems can be caused by a bad CD4051, bad buffers or bad
CEM3340. First switch the CD4051 with one on a voice that has no
problems. If the problem now appears on the other voice, the CD4051 is
bad. Then try switching the suspect CEM3340 with a known good one. If
this isn’t it, you’ll have to try trouble-shooting the CD4051 buffer circuits
and support circuitry for the CEM3340.
3.7.3 Letter “E” appears or other pulse width problems
The voltage at the CEM3340 pin 5 should be about +6V or the Pulse
wave won’t turn off.
3.7.4 No sub octave
Check if the input to the CD4520 has a negative spike on it. If it doesn’t,
then check the coupling cap and bias resistors on the input to the
CD4520. If it is, check if the reset pin on the IC toggles when the
suboctave is turned on and off. If it does, the CD4520 is probably bad.

4 VCF Card
4.1 Theory of Operation
The Voltage Controlled Filter Card (VCF Card) consists of four identical
four-pole (-24dB/Octave) VCF circuits based on the SSM 2044
Integrated Filter. One card is used for the LEFT side of the instrument
and the other for the RIGHT side of the instrument. Both cards are
identical and may be interchanged to facilitate troubleshooting.
The functions on this board include:
•Four filter circuits each with Voltage Controlled Cutoff (Fc) and
Resonance (Q).
•Voltage Controlled Oscillator (VCO) volume amplifiers for VCO1
and VCO2 on four voices.
•Modulation system demultiplexers for VCO1, VCO2 Filter Cutoff
(Fc) and Resonance (Q).
•VCO1 and VCO2 multiplexing for the modulation/tuning system.
4.2 VCF Operation
The SSM2044 is a four-pole filter with a current input and current output.
The input consists of the output currents of the VCO volume VCAs and
the noise voltage dropped across the 100K noise input resistors, while
the output current is converted to a voltage by the voltage-to-current
converter opamp inverter TL082 U12. This output is routed to the mixer
card.
The filter audio inputs include VCO1, VCO2 and Noise. The noise source
is generated on the MIXER card and is routed to all four VCFs
simultaneously. The VCO1 and VCO2 volume levers are determined by
two SSM 2024 Quad Voltage Controlled Amplifier (VCA) ICs. The control
voltage for these ICs is generated by the CPU at positions 4 and 5 on
demultiplexer CD4051, U3.
VCO1 may bypass the VCF using the MC14551 quad switch to route the
output current directly into the current-to-voltage converter at the output
of the SSM 2044. The position of this switch is determined by the control
signal on pin 6 which is generated on the DAC and controlled by the
CPU. The resistive current divider at the VCO 1 output to the VCF is
used to balance the relative gains of VCO1 in the normal and bypass
modes.

The VCF Fccutoff is determined by the output of the control summing
amp comprised of the four opamps in the LM324 U6 and associated
resistor networks. The control voltages include:
•Modulation control
oDetermined by the modulation system and demultiplexed
by CD4052 U1.
•ADSR 1 control
oDetermined by the CPU and demultiplexed by CD4051 U4
oNote that the ADSR signals are generated unconventionally
with respect to the standard Demultiplexing scheme (see
DIGIMOD theory)
•Initial Fc setting
oDetermined by trimmer calibration
•Keyboard Control and panel Fc setting
oDetermined by the CPU and demultiplexed by CD4051 U3
The sum of these control voltages determines the VCF cutoff while the
adjustable divider network feeding pin 13 on the SSM2044 determines
the filter tracking (Volts/Octave). Increases in control voltage at the
summing amp input produce decreases in control voltage at the divider
network since the summer is an inverting network. The trimmer labeled
“V/Oct” is set so that a change of one volt at the summer input produces
a one octave change in filter frequency. The control voltage is scaled
down to less than +/- 100mV at the SSM2044 input pin 13 to obtain the
full sweep range of 20 Hz – 20 kHz.
Filter resonance is determined by the control voltage at pin 2 on the SSM
2044, which in turn is derived from the difference amplifier LM324 U7.
This difference amplifier receives a signal from the CPU (demultiplexed
at CD4051 U3) that determines the initial resonance for all four VCFs on
the card. The modulation signal coming from the CD4052 U1 comes from
the mod system and is the result of the “Q Destination” settings in the
mod banks. This signal is subtracted from the initial Q voltage and is
scaled by the “Q CAL” trimmer that feeds pin 2 of the SSM 2044.
Because of this subtraction, the resonance increases as the modulation
signal in the mod banks decreases, thus an increasing resonance is
obtained with an inverted source.
4.3 Calibrations
The VOLTS/OCTAVE trimmer is used to set the VCF keyboard tracking.
When set properly, one octave change on the keyboard should closely
correspond to a one octave change in Fc. The “FcINITIAL” trimmer is
used to set all VCFs to the same cutoff for the same key down. When set
properly, the VCFs should all produce an Fcof A440 when the middle A

key is pressed on the keyboard (assuming that the test programs have
been loaded.) The “Q CAL” trimmer is used to set all VCFs so that they
oscillate at the same resonance setting. When set properly, all of the
VCFs should burst into oscillation at the same point on the front panel Q
control pot.
4.4 VCO Multiplexing System
The CD4052 U19 is used to multiplex four VCO1s and four VCO2s to
two independent lines called VCO1 MUX and VCO2 MUX. These signals
serve two distinct purposes.
First, when the CPU is autotuning the VCOs, this multiplexer allows it to
route one of the VCOs to the DAC card for frequency measurement.
Secondly, it allows the modulation system to use the VCOs as
modulation sources. The scanning clocks for the multiplexer, labeled
VMA1 and VMA2 are derived from the CPU during autotune and from the
modulation system during normal operation. This clock selection is
determined by the CPU on the DIGIMOD card. The LF347 buffer
amplifiers that precede the CD4052 inputs prevent the scanning clocks
from leaking into the VCF audio inputs.
4.5 DC Voltages
The card uses +/- 15 Volts to drive the SSM ICs and associated audio
opamps. The LM325 ICs are driven from +8V and -7V to limit their range
to within the specifications of the SSM 2044 control inputs. The CD4052s
used in the modulation demultiplexing are also biased with these
voltages as their outputs must be bipolar while their maximum bias must
be less than 16V. The FcINITIAL trimmers are biased using a cascade
transistor pair to derive a buffered reference for the trimmers as well as
the -7V card voltage. The +8V is derived from a 78L08 voltage regulator
driven from the +15V line.
The 15V modulation scan clocks and VCF bypass control signal are
converted to 8V by the CD4050 U5 buffer to be compatible with the bias
on the CD4052 demultiplexers and the MC14551 switch.
4.6 Card Pin Signals
The signals coming into and out of the card are as follows:
4.6.1 Inputs
•Modulation multiplexed inputs for Fc, Q, VCO1 and VCO2 coming
from ANAMOD 1.
•Power supply inputs +/- 15V and Analog Ground

•CPU control signals for the CD4051 demultiplexers (EN 15, EN 9,
AL0, AL1, AL2) coming from DIGIMOD
•Modulation scan clocks (MODEN, MA0, MA1)
•VCO multiplexer clocks (VMA0, VMA1)
•DAC control line (Bypass)
•VCO1 & VCO2 audio inputs for four voices from VCO cards
•Noise input from MIXER
4.6.2 Outputs
•VCO1 & VCO2 modulation outputs for four voices
•Four VCF audio outputs
•Multiplexed VCO1 & VCO2 outputs
4.7 VCF Card Troubleshooting Tips
These troubleshooting tips assume that the VCF test tape REV 1.0 has
been loaded into the VOYETRA. If this tape is not available, the
suggestions will still prove useful if the theory section has been
thoroughly understood.
4.7.1 Step 00-02: Resonance (Q) Tests
These steps check that the Q control is within a specific tolerance.
Problems may be caused by a defective SSM 2044, but the LM324 that
derives the Q control voltage should first be suspected. Note that the four
Q modulation outputs from the CD4052 U7 (and the four Fcmodulation
outputs) should all be at 0 V. Thus, Q calibration problems where Q will
not turn off should take this into account.
In steps 00 and 01 the Fcoutput controls from U6 LM324 should be
about 2V on pins 1, 7, 9 & 14. As in any inverting opamp circuit, the
negative inputs, pins 2, 6, 9 & 13 should be at 0 V.
In step 02, pins 1, 7, 9 & 14 should have a sweeping sawtooth. Pin 13 on
the SSM 2044 should have a sawtooth of about 50 mVpp.
4.7.2 Step 03: DC Feedthrough Check
Ideally, the SSM 2044 should not have any output if no audio input exists.
However, if the IC is defective, changes in Fccontrol voltage will reflect
onto the output even with no audio at the input. Thus, a square wave
control signal (derived from the Fcmod system) is used to check for this
feedthrough. ICs which have an output square wave leakage of greater
than 1 Vpp should be rejected and replaced.
4.7.3 Step 04-11: Initial Fcand V/Oct Calibration
These steps all check for proper Fcsumming amp (LM324 U6) operation
by calibrating the initial Fcsetting, filter keyboard tracking (V/Oct) and