Sequential prophet 5 User manual

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TECHNICAL
MANUAL
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About This Manual and Servicing The Prophet
The Prophet is a sophisticated instrument, and SEQUENTIAL CIRCUITS INC. issues its technical
manual for use by qualified technicians only.
Of course the manual will also be read by Prophet owners and others interested in synthesizer design.
And we realize it will also be used by some to modify their instruments.
While we support this innovative attitude in spirit, we cannot support it financially. Modifications or
unauthorized service void the Prophet's warranty. They also invariably extend service time (thus,
cost) if factory repair is required.
Familiarize yourself thoroughly with this manual before attempting any work on the Prophet. This
will at least help you judge whether you should be working on it at all. If in doubt please contact our
Service Department.
The manual is organized as follows:
SECTION 1 describes the Prophet's physical organization and procedure for its complete disassembly.
SECTION 2 covers principles of the system (hardware and software) design.
SECTION 3 contains complete schematics and pictorials identifying all components.
SECTION 4 explains how to tune and adjust the Prophet for optimum musicality.
SECTION S offers aid in identifying and correcting malfunctions.
SECTION 6 is included for you to order replacement parts from SCI.
Your response to the Manual User’s Questionnaire on the next page will help us monitor our
publication’s usefulness.

MODEL 1000
PROPHET-5 SYNTHESIZER
TECHNICAL MANUAL
By STANLEY JUNGLEIB
Wine Country Productions, Inc.
1572 Park Crest Court. Su4e #505
San Jose. CaWomia 95110 USA
p nooe [phone] PAX [phone]
SEQUENTIAL Product Soecw/.sTs Since 1987

058
MANUAL NO. TM1000C
Issued: January. 1980
Copyright © 1980 by
SEQUENTIAL CIRCUITS. INC
All rights reserved. Printed in USA.
The contents o< this manual are the property of SCI and are not to be copied or reproduced in any
whatsoever without our prior written permission.
manner

Table of Contents
SECTION 1
SECTION 2
SECTION 3
MECHANICAL PROCEDURES
1-0 GENERAL.
1-1 PRECAUTIONS .
1-2 SERVICE POSITION.
1-3 PC84 VOICE BOARD SWINGOUT OR REMOVAL
1-4 PCB3 COMPUTER BOARD REMOVAL.
1-5 PCB1 AND 2 CONTROL PANEL REMOVAL ....
1-6 PCB6 POWER SUPPLY BOARD REMOVAL-
1-7 KEYBOARD REMOVAL.
PAGE
. 1-1
. 1-1
. 1-3
. 1-4
. 1-5
. 1-6
. 1-7
. 1-8
THEORY OF OPERATION
2-0 OVERVIEW.
2-1 SYNTHESIZER BACKGROUND.
2-2 THE PROPHET.
2-3 VOICE FUNCTION .
2-4 OSCILLATOR A (OSC A).
2-5 OSCILLATOR B (OSC ..
2-6 FILTER ENVELOPE GENERATOR (FILT ENV GEN) .
2-7 FILTER (FILT).
2-8 AMPLIFIER ENVELOPE GENERATOR (AMP ENV GEN)
AND AMPLIFIER (FIN ..
2-9 AUDIO OUT (AUD OUT).
2-10 COMMON ANALOG.
2-11 MICROCOMPUTER SYSTEM.
2-12 CV AND S GENERATION .
2-13 MATRICES .
2-14 POTENTIOMETER MULTIPLEXER (POT MUX)
AND ANALOG TO DIGITAL CONVERSION (ADC) . . .
2-15 TUNE.
2-16 HARDWARE DESCRIPTION.
2-17 BD001 INTERCONNECTION.
2-18 SD211 CONTROL PANEL-POTS .
2-19 SD212 CONTROL PANEL-SWITCH AND LED MATRICES
2-20 SD311 COMPUTER-CPU. MEMORY. I/O.
2-21 SD312 ADC. DAC, COMMON ANALOG.
2-22 SD313TUN MUX.S/HS. AUDOUT.
2-23 SD411-15 VOICES 1-5.
2-24 SD711 BACK PANEL AND PCB6 .
2-1
2-1
2-4
2-7
2-7
2-8
2-8
2-8
2-11
2-11
2-11
2-12
2-12
2-16
2-18
2-19
2-20
2-20
2-21
2-22
2-22
2-26
2-27
2-27
2-32
DOCUMENTS
3-0 DOCUMENT LIST .
3-1 DOCUMENT NOTES

SECTION 4 TRIMMING AND TUNING
4-0 GENERAL.4-1
4-1 PREPARATION . 4-2
4-2 R603 +15V ADJUSTMENT .4-3
4-3 R301 DAC GAIN . 4-3
4-4 R302 ADC GAIN .4-3
4-5 R304 WHEEL-MOD NOISE VCA 8ALANCE. 4-4
4-6 R305 WHEEL-MOD LFO VCA BALANCE . . 4 -4
4-7 R306 LFO TRIANGLE SYMMETRY. . 4 -4
4-fl R307 MASTER SUMMER OFFSET . 4-6
4-9 TRIMMINC THE VOICES .4-6
4-10 R401 AMPLIFIER ENVELOPE GENERATOR TIME CONSTANT.4-6
4-11 R402 FILTER ENVELOPE GENERATOR TIME CONSTANT.4-7
4-12 R403 FILTER ENVELOPE AMOUNT VCA BALANCE .4-7
4-13 R404 POLY-MOD FILTER ENVELOPE AMOUNT VCA BALANCE ... 4-7
4-14 R405 POLY-MOD OSCILLATOR B AMOUNT VCA BALANCE _4-7
4-15 R413 OSCILLATOR B TRIANGLE SYMMETRY .4-8
4-16 R415 FINAL VCA BALANCE.4-8
4-17 FILTER TUNING.4-8
4-18 OSCILLATOR TUNING.4-9
4-19 R416 VOICE LEVEL.4-13
4- 20 R303 SEQUENCER CV OFFSET.4-13
SECTION S SERVICE NOTES
5- 0 GENERAL.5-1
5-1 POT MUX/ADC .5-1
5-2 KEYBOARD (KBD).5-1-
5-3 SWITCH MATRIX.5-1-
5-4 LED MATRIX.5-2
5-5 COMPUTER.5-2
5-6 NON-VOLATILE PROCRAM MEMORY.5-3.
5-7 DAC. CV OMUX. S/HS.5-4
5-8 COMMON ANALOC.5-4
5-9 AUDIO OUTPUT.5-4
5-10 VCOS .5-4
5-11 VCAS.5-4.
5-12 ENV GENS .5-4
5-13 VCFS.5-5
5-14 POWER SUPPLY.5-5
SECTION 6 PARTS LIST
PCS!.6-1
PCB2.6-1
PCB3.6-2
PCB4.6-7
PCB6.6-10
ASSEMBLY 7.6-11
SECTION 7 GLOSSARY

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Section I
Mechanical Procedures
1-0 GENERAL
This section shows how to remove the Prophet's main assemblies. Not all of the procedures given
here should be necessary at any one time. For most service situations you will only need to separate
the top and bottom panel assemblies and arrange them as shown in Figure 1-0. This configuration,
discussed in paragraph 1-2, allows access to all trimmers on PCB 4, VOICE BOARD (Figure 7-7).
For more extensive service you may have to "swingout" or completely remove PCB 4. Procedures
below show how to do either, also, how to remove the remaining assemblies: PCB 3, COMPUTER
BOARD. PCB 2. LEFT and PCB 1, RIGHT CONTROL PANELS. PCB 6. POWER SUPPLY BOARD (PSB),
and the KEYBOARD (KBD).
Generally, each procedure gives disassembly or removal instructions only. It is assumed that re¬
placement procedures are just the reverse.
Sections 2-5 of this manual assume that you are familiar with the Prophet's mechanical design as
covered in this section.
1-1 PRECAUTIONS
Observe the following precautions when working on the Prophet:
• To prevent damage to the top panel, keyboarder woodwork, use a carpetted or similarly-covered
work surface when opening the cabinet.
• Set S701, power switch, OFF, and check S702,115/230V, on the back panel of units 184 and above
before connecting to power outlet.
• Never switch line voltage with instrument power on.
• Trimming and tuning must, of course, be performed with power on. So, avoid the power supply
primary circuitry, which conducts lethal voltages.
• Switch power off before disconnecting or connecting any internal circuitry, or removing or in¬
stalling PCBs.
• Do not bend or strain the PCBs. Otherwise may cause tiny breaks in the printed circuit traces
which will be extremely difficult to find.
• To replace soldered components, switch power off. remove the PCB completely from the instru¬
ment and desolder from both sides. Use a vacuum syringe or DIP desolderer. Don't overheat the
pads. WORK CAREFULLY.
• Remember that in service position, the top panel assembly balances on its rear edges. Don’t
upset this balance by pushing too hard when trimming.
1-1

TOP
■ 'PANEL
ASSEMBLY
l
CONTROL
PANEL
BACK.
PANEL
BOTTOM
— PANEL
ASSEMBLY
J709/P402
PCB6
WARNING
LETHAL VOLTAGE PRESENT
WHEN INSTRUMENT IS ON
Figure 1-0
Service Position
2

*9 •• 9 % .
•; TL-:;
ttSJflMWi
PCB 3
COMPUTER
BOARD
W703
PCB 3/4
INTERFACE
PCB 4
VOICE
BOARD
BACK VOICE
PANEL POWER
CABLE CABLE
1-2 SERVICE POSITION
To set up the Prophet for service first switch power off and turn instrument over to expose bottom
panel. Figure 1-0 identifies (•) screw hole locations around the per imeter of the bottom panel.
Remove eleven bottom panel screws.
Holding top and bottom panel assemblies together, turn the Prophet right-side-up again. Remove
four screws (•) along the top edge of the back panel. Slowly slide the top panel assembly forward
— about six inches — so when raised, the control panel knobs will dear the power supply capacitors.
Raise the top panel assembly to service position shown in Figures 1-0 and 1-1. For best stability both
top panel side back edges should rest on the bottom panel. The top panel upper edge rests on a
cable from the back panel. This is normal; but a small (half-inch) prop may be added to support the
top panel edge, and perhaps improve stability.
WARNING
LETHAL VOLTAGE IS PRESENT IN THE POWER SUPPLY PRIMARY CIRCUITRY. BE CAREFUL.
The instrument can operate normally in this position. As shown in Figure 7-7, PCB 4 is completely
accessible for trimming (see Section 4).
Figure 1-1
Service Position

1-3 PCB 4 VOICE BOARD SWINGOUT OR REMOVAL
Although most trimmers are accessible when the Prophet is opened, PCB 4 may be swung out to
access PCB 3 trimmers which are otherwise concealed. For this setup, switch power off. remove
five screws identified (•) in Figure 7-7, and position PCB 4 as shown in Figure 7-2. Do not strain the
audio output cable which connects to the back panel. Power may be switched back on in this con¬
figuration.
To remove PCB 4 completely, switch power off and disconnect voice power cable at PCB 3 (Figure
7-7). Also remove W703. with a gentle “see-saw” motion at P303. Then detach audio output cable
at quick-disconnects J709/P402 on back panel 1701 AUDIO OUT (Figure 7-0). When reconnecting
voice power cable, be sure tabs interlock.
PCB I
RIGHT
CONTROL
PANEL
PCB 3
TRIMMERS
AUDIO
OUTPUT
CABLE
Figure 1-2
PCB 4 Swingout
1-4

1-4 PCB 3 COMPUTER BOARD REMOVAL
PCB 3 is held by the six screws identified (•) in Figure 7-3. and by direct connection to PCB 2. behind
it. (See J301. Figure 1-3 and P201, Figure 1-4.) To remove PCB 3. disconnect back panel and wheel
cables, remove screws, and pull at the standoff indicated, also using a “see-saw" motion to mini¬
mize stress along the board.
When replacing PCB 3 be sure J301/P201 pins are correctly mated before fastening screws. Align¬
ment is insured by matching screw holes with their standoffs. Remember to reconnect the wheel
cable, too, before replacing PCB 4. When reconnecting back panel cable, be sure tabs interlock.
J30I * STANDOFF *
WHEEL * BACK
CABLE PANEL
CABLE
Figure 1-3
PCB 3 Removal
1-5

1-5 PCB 1 AND 2 CONTROL PANEL REMOVAL
Once PCB 3 and 4 are removed, CONTROL PANEL removal involves pulling off all knobs, unscrew¬
ing all potentiometer mounting nuts for either or both panels (using a half-inch nutdriver), and
removing the screws identified (•) in Figure 1-4.
You may be able to avoid removing both panels by taking advantage of W701. the rlexible intercon¬
nect between them (Figure 1-4). Also. Section 3 of this manual includes pictorials identifying panel
components from the solder side — that is, as the control panels are normally mounted.
When replacing be sure all potentiometers fit correctly through the front panel and check switch
action before tightening any potentiometer nut.
W702
PCB 2
LEFT
CONTROL
PANEL
W70I PCB I
RIGHT
CONTROL
PANEL
Figure 1-4
PCB 1 and 2 Removal
1-6

1-6 PCB 6 POWER SUPPLY BOARD REMOVAL
PCB 6 is held to the bottom panel assembly by two capacitor clips and by five regulator mounting
screws. So, removal for component replacement requires unmounting all regulators from the back
panel. Figure 1-5 details regulator installation. Note that the regulators are electrically insulated
from the back panel.
After repair, you can check power supply output before installing on back panel.
CAUTION
DO NOT OPERATE THE UNLOADED POWER SUPPLY FOR LONG PERIODS WITHOUT HEAT¬
SINKING THE REGULATORS. NEVER OPERATE THE SUPPLY UNDER LOAD WITHOUT HEAT-SINKING.
When replacing PCB 6 check regulator insulation from back panel with an ohmeter before apply¬
ing power.
4-40 X 3/8
FL HD SCREW
innoo
BACK
PANEL
MICA INSULATOR
REGULATOR
TAB -4-40
HEX NUT
HEAT SINK
COMPOUND
BOTH SIDES
NYLON SHOULDER WASHER
REGULATOR
Figure 1-5
Regulator Installation
1-7

1-7 KEYBOARD REMOVAL
After removing PCBs 3 and 4. turn the top panel assembly over and disconnect W702 {Figure 7-4).
Figure 1-6 details keyboard mounting. Remove keybed supports, both ends, by first removing key-
bed screws then rear keyboard mounting screws. Then remove front keyboard mounting screws.
The entire keyboard will slide out of the case — and back in — as shown in Figure 7-7.
NOTE
During this operation the )-wires (Figure 1-6) for the lowest and highest keys are vulnerable to damage
through collision with the keyboard mounting brackets. When replacing, check J-wues at each end
of keyboard for contact with bus bar. Also, remember to attach wheel ground lug.
R702
MOD
WHEEL
FRONT
KEYBOARD
MOUNTING
SCREW
R70I
PITCH
WHEEL
J-WIRES
WHEEL
GROUND
LUG
KEYBOARD
MOUNTING
BRACKET
REAR
KEYBOARD
MOUNTING
SCREW
Figure 1-6
Keyboard Mounting

Figure 1-7
Keyboard Removal
1-9

Figure 2-0
Monophonic Additive Synthesis
2-2

Figure 2-1
Monophonic Subtractive Synthesis
Besides a dynamic envelope, a musical voice usually has a timbre which consists of a fundamental
pitch and a number of harmonics - all of varying relative strengths. Pitch and timbre synthesis raises
a distinction between two techniques, shown in figs. 2-0 and 2-7. The first additive synthesis might
create a timbre by summing the output of several sine-wave VCOs for the fundamental and each
harmonic. In contrast, subtractive synthesis can start with one sawtooth-wave VCO generating the
fundamental with extensive harmonics, then obtain the desired timbre by subtracting unwanted
harmonics with a low-pass filter.
The additive and subtractive techniques have encouraged the development of two types ot instru¬
ments. roughly, "studio", and "peformance". Since one can individually control each h f r ^°" ,c '
additive synthesis may be potentially more accurate for synthesizing a particular sound. Whether
they are additive or subtractive, studio synths may be configured from dozens of modules intercon¬
nected by "patch cords". The modules have knobs to establish the initial settings of VC-parameters
such as frequency (INIT FREQ), pulse width (PW). and resonance (RES). But the flexibility and com¬
plexity of modular synths discourages their "live" use on stage because significant sound changes
often require repatching modules and precisely checking knobs. Favorite, comp ex sounds take a
long time to create, and almost as long to recreate on a modular synth. So these often monophonic
synths instead feed multi-track recorders on which polyphonic interpretations or compositions are
actually orchestrated.
A comparison of the number of modules and interconnections depicted in figs. 2-0 and2-T shows
why the subtractive configuration has become the popular technique for performance synt s.
Obviously, subtractive synths may be far more portable and their patches will not be as elaborate.
Originally, these synths were monophonic. Or they exploited organ technology so more than one
note could be played at a time. "Preset" switches that select fixed patches supplanted many modular
controls. Though one could certainly change sounds quickly using them, many players found preset
synths unsatisfactory because they eliminated an essential part of synth musicianship. Some manu¬
facturers have offered partially-programmable instruments. But before the Prophet appeared it was
not possible for a keyboardist to instantly select his or her own customized synth sounds and play
them polyphonically.
2-3

Figure 2-2
Prophet Block (Diagram
THE PROPHET
The Prophet is a subtractive, performance and studio analog synth. It provides instantaneous patch
repeatability and polyphonic capability without the limitations of organ technology or fixed presets.
Fig. 2-2 shows the Prophet at the most general level. Instead of controlling the synth directly, the
keyboard and most controls are processed through a microcomputer system. The microcomputer
system provides a way to store all of the switch and knob settings which form a patch, and solves the
problem of generating five sets of independent CVs and GATEs from a single keyboard. Common
analog circuitry mixes the few controls not processed by the system with processed control signals
from the voices. These patch the five voices to sound fundamentally alike (homophonous) with,
typically, pitch differences corresponding to (at most) five simultaneously held keys.
Fig. 2-3 specifies the principle functions of the four main blocks. Beginning with the voices, their
outputs are combined and overall volume set by a VCA controlled directly from the front panel.
Each voice is a complete synth with two VCOs. a MIXER. VCF. FINAL VCA. and two ENV GENs but they
are patched homophonously by common switch (S) signals and CVs. Each voice also has its own
KEY CV which provides polyphonic pitch information, and GATE, which signals that the key is being
held. All S signals and most CVs are generated by the computer. The common CVs include master
tuning (MTUN), pitch-bend (P-8ND), wheel controlled modulation (W-MOD), which are mixed in
the COMMON ANALOG circuitry. The COMMON ANALOG circuitry also requires a few S signals.
The microcomputer performs the tasks of voice assignment. It decides which held keys sound which
voices through their KEY CVs and GATEs. The computer assigns VOICE 1 to the first key held, VOICE 2
to the second key. and so on. After the five initial assignments the system is “last note priority’'. The
earliest used voice is reassigned to each new note played. Repeated notes key the same voice. For
example, holding C. D, E, F, and G. sustains voices 1-5, respectively. Adding A "steals” VOICE 1 from
the C which "disappears”, even though it may still be held.
2-4

COM*
ANAL
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Figure 2-3
Prophet-5 Functional Diagram

2-S/2-6