Sequential Pro One User manual

MODEL BOO
Manual No. TM100A
PRCLONE SYNTHESIZER
TECHNICAL MANUAL
by Stanley Jungleib
1 '
. Sequentid Circuits, Inc.
i
3951 North 1st Street
/
Sari Jose, CA 95134
[phone]
TELEX: 36441 2 INTR 706

PRO-ONE SYNTHESIZER
TECHNICAL
MANUAL
By Stanley Jungleib
Manual No. TM 1 OOA
Issued: December, 1981
~o~~ri~ht@1981 by
SEQUENTIAL CIRCUITS, INC.
All rights reserved. Printed in USA.
The contents of this manual are the property
of SCI and are not to be copied or reproduced
without our prior written permission.
Data sheets reprinted in the Appendix are courtesy
of RCA and of Curtis Electrornusic Specialties.

Table of Contents
SECTION 1 FUNCTIONAL TESTS
INTRODUCTlON
PKEPARATION
KEYBOARD TEST
OSCILLATOR A TEST
OSCILLATOR B TEST
MIXER AND NOISE TEST
PITCH WHEEL AND GLIDE TEST
FILTEK TEST
AMPLIFIER TEST
MODULATION TEST
BACK PANEL TEST
TUNING FLOW CHART
SECTION 2 THEORY OF OPERATION
INTRODUCTION
GENERAL
OSCILLATOR A
OSCILLATOR B
M1XER/NOISE/EXTERNAL
FILTER
FINAL VCA
ENVELOPE GENERATORS
MODULATION SOURCES
LFO
MICROCOMPUTER
KEYBOARD/SWJTCH MATRIX
DAC
GLIDE
GATE AND TRIGGER LOGIC
POWER SUPPLY
3-0 DOCUMENT LIST
3- 1 DOCUMENT NOTES
SECTION 4 MECI-IANICAI,
INTRODUCTION
4-0 i 4-1 -
4- 1 OPENING CASE AND REMOVING PCB 4-1
4-2 REVISION LO/ 1 .I PCB MOUNTING MODIFICA+ION 4-2
4- 3 REVISION 1.01 1.1 POWER CONNECTOR MODIFICATION 4-4
4-
4 REPLACING AND ADJUSTING WHEELS 4-4
TMIOOA 12/81
iii

SECTTON 5 SERVICE
5-0 INTRODUCTION
5- 1 DAC SCALE
5-2 TUNING
5-3 AMPLIFIER BALANCE
5- 4 DIGITAL INTERFACE
SECTION 6 PARTS
6-0 CHASSIS
6- 1 PCB 1
SECTION 7 GLOSSARY
APPENDIX DATA SHEETS
CA3280 DUAL OPERATIONAL TRANSCONDUCTANCE AMPLIFER
CEM 33 10 VOLTAGE CONTROLLED ENVELOPE GENERATOR
CEM 3320 VOLTAGE CONTROLLED FILTER
CEM 334013345 VOLTAGE CONTROLLED OSClLLATOR

SECTlON 1
FUNCTIONAL TESTS
1-0 INTRODUCTION
I
This section contains procedures for complete functional testing of the Pro-One. The
functional tests check all signal paths and the range of the oscillators. A flow-chart
for selectively checking tuning- is on page 1-12. To correct oscillator tuning, see the
Service procedures in Section 5.
Instruments to be serviced should be completely tested beforehand. This will reveal
related or unrelated malfunctions, and provide a basis for troubleshooting, If a trim
can't
be achieved service will: certainly be required. In such cases, consult the theory
of operation and diagrams in Section 2, and schematics in Section 3 to isolate the
failure.
Mechanical procedures are in Section 4. When operating disassembled it can be
difficult to read the controls: Figure 3-1 may be of help. Also, when disassembled it is
often convenient to leave the wheel cable disconnected. The Pro-One will operate
normally, that is, without pitch bend or modulation.
These procedures are presented in the order recommended for a Pro-One assumed to
be completely operational. Real-world problems may require you change the order of
some tests. Therefore each is written to be performed independently.
You must play the Pro-One throughout these tests (or use the ascending and
descending scales preprograrnrned into SEQ 1 and SEQ 2 on power-up). All tests are
performed by ear, the object generally being that the
knobs adjust smoothly and
switches work. Toggle the panel switches repeatedly to check for intermittents.
1-1 PREPARATION
I. Connect headphones, or mono cable between back panel AUDIO OUT jack and power
amplifier,
2. Check back panel 1 14/23O line voltage selector.
3. Check that back panel power switch is off.
4. Ef operating disassembled, testing may be facilitated by placing the knobs lightly
back on their shafts..
5. Connect power cable to properly-grounded outlet.
I
- -
6. Switch power on. ,J
WARNING! LETHAL VOLTAGE IS PRUENT IN THE POWER SUPPLY AREA
OW THE PCB (IN EARLIER MODELS) OR ON THE BACK PANEL (IN LASER MODELS).

7. Center MASTER TUNE and PITCH wheel and set MOD wheel to minimum (unless
*the wheel cable is disconnected).
8. Adjust VOLUME as required, below.
1-2 KEYBOARD EST
I. Patch according to Figures 1-1 or 1-2. The essential features of these patches are a
fully "open" AMPLIFIER and FILTER, the sawtooth of one oscillator is on, and there is
no modulation.
2. Check that all keys work. Make a note of scratchy, intermittent, or inoperative
keys, Oxidation, particularly of lesser-used keys, can cause keys to go "dead,rt and may
be aggravated by ambient conditions or handling. Check also for contact bounce by
hitting each key hard and repeatedly while listening for break-up or double triggering.
For either condition, cleaning ar careful readjustment of the "J-wirestr is often a1that
is necessary. After the top panel is removed, the keyboard can be detached from the
bottom panel for cleaning by simply removing four screws. Wipe the contacts carefully
with an alcohol-
or freon-wetted cotton swab. Donqt use any abrasive techniques, as
this will simply remove the gold plating. Replacement J-wires can be ordered as SCI
#S-051.
3. If the keyboard doesn't work at all, check its connector. The GATE LED should light
whenever a key is pressed. If still there is no sound, check Final VCA (para. 1-8).
4. Switch DRONE an. You should hear a continuous tone. The frequency will change as
you play various keys, but the tone itself is not interrupted. Switch DRONE off.
5. Switch REPEATJEXT on. The frequency of the last key played repeats at a rate set
by the LFOJCLOCK FREQUENCY knob. Keys will not be heard when played, but
repeated pitch will change. Switch REPEATIEXT back to NORMAL.
6.. To check or demonstrate low-note priority, hold a key and hit one above it. The
higher key will not sound. Now hold the higher key and hit the lower one. The rower
key sounds when pressed.
5. Switch MODE from NORMAL to RETRIG. Note that this switches from low-note to
last-note priority. The newest key sounds though a lower key may be held.
6. Hold several notes and switch ARPEGGIATE UP, then UP/DOWN to check function.
While arpeggiating, switch up SEQUENCER RECORD and remove hand from keyboard.
The arpeggiator should remain "latched." Switch ARPEGGIATE off.
7. With SEQUENCER RECORD still on, select SEQ 1 and record a sequence. Play it
back. Repeat for SEQ 2.
i
/'

1-3 OSCILLATOR A TEST
Figure 1-1
OSC A TEST PATCH
1. Patch according to Figure 1-1.
2. Check that 0SC A FREQUENCY knob range exceeds one octave. For reference,
play C2 (which is one octave below the highest key, C3) and remember pitch. Then
hold C1 and raise OSC A FREQ. The pitch should increase smoothly, approaching the
reference pitch at 7 - 8 on the dial. At 10 the pitch should be at least a whole step
above the reference.
3. Turn OSC A FREO to 0. Check OSC A OCTAVE switch function by playing C2 and
.. .
switching octaves down to 2, 1, and 0. NOTE: When switching octaves with a key
depressed, it is normal to hear the pitch "jump" between octaves.
' "
4. Switch OSC A OCTAVE to 2, and switch OSC A SYNC on. Rotate OSC A FREQ
knob 0 - 10 to check sync function.
5. Switch OSC A SYNC off. Switch OSC A SAWTOOTH off. Switch 05C A PULSE on.
The difference between the 'Ibrassylr sawtooth and tfwoody" pulse should be obvious.
Rotate OSC A PULSE WIDTH knob 0 - lo, listening for the timbral effect of varying
harmonics. The pulse will degenerate to dc (or very nearly dc--99% duty cycle) at the
extreme knob settings, resulting in no (or very little) sound. Find the setting where the
second harmonic drops out (50% duty cycle). This normally occurs between dial
markings 5 and 6.

1-4 OSCILLATOR B lF5T
Figure 1-2
OSC B TEST PATCH
1. Patch according to Figure 1-2.
2. Check that OSC B FREQUENCY knob range exceeds one octave. For reference,
play C2 and remember pitch. Then hold C1 and raise OSC B FREQ. The pitch should
increase smoothly, approaching the reference tone at 7 - 8 on the dial. At 10 the pitch
should be at least a whole step above the reference.
3. Turn OSC B FKEQ to 0. Check OSC B OCTAVE switch by playing C2 and switching
octaves down to 2, 1, and 0. NOTE: When switching octaves with a key depressed, it is
normal to hear the pitch "jump" between octaves.
4. Switch OSC B OCTAVE to 2. Switch OSC 0 SAWTOOTH off. Switch OSC B
TRIANGLE on. The difference between the ''brassy" sawtooth and "dull1' triangle wave
should be obvious. The triangle waveshape has very little harmonic energy. Switch OSC
I3 TRIANGLE off.
5. Switch OSC B PULSE on. Rotate OSC B PULSE WIDTH knob 0 - 10, listening for the
timbral effect of varying harmonics. The pulse will degenerate to dc (or very nearly
dc--99% duty cycle) at the extreme knob settings, resulting in no (or very little) sound.
Find the setting where the second harmonic drops out (50% duty cycle). This normally
occurs between dial markings 5 and 6.
6. Switch OSC B LO FREQ on. Raise 6SC B FREQ to about 6 to more easily hear OSC
B "track" the keyboard at lev$ frequencies. NOTE: he OSC B LO FREQ switch extends
OSC B's FREQ range to sub-audio. It is therefore normal qhat if OSC B FREQUENCY
is set to 10, the LO FREQ switch will appear to have nd effect. Switch 05C B LO
FREQ to NORMAL. /
7. Switch OSC B KYBD off and check that OSC B ceases "tracking" the keyboard.

1-5 MIXER ad NOEE TElST
Figure 1-3
MIXER and NOISE TEST PATCH
1. Patch according to Figure 1-3,
2. Hold a key and turn MIXER OSC A knob 0 - 10. Check for smooth vohme increase.
Leave knob set to 0.
3. Turn MIXER OSC B knob 0 - 10. Check for smooth voEurne increase. OSC B should
be as loud as OSC A. Leave knob set to 0.
4. Turn MIXER NOlSE/EXT knob 0 - 10. Check for smooth noise volume increase.
(Since noise is unpitched, it does not track the keyboard). Leave knob set to 0.
5. External audio input is checked during the BACK PANEL test (para. 1-10).
TMlOOA 12/81

1-6 PITCH WHEEL AND GLIDE EST
OE IIVI
Lullr, "GIOII*LI 'rnILO.I XI" I... u
LWY*. -nu*,
mm
1.
am" mu,.
PaII11* "ELL.,.
. ' 6
1
, v*mL .ts..l
rm.u*, *bull
mn'
OlCll IYTI.I* .CLT.s.
LiO
%
Figure 1-4
PITCH WHEEL and GLIDE TE5T PATCH
1. If the wheel cable has been disconnected, switch power off and reconnect to test
PITCH wheel. Otherwise, skip steps 3 and 4.
2. Patch according to Figure 1-4.
3. At this point you will probably Rear "beatsM resulting from the slight difference
between OSC A and OSC E3 pitch. To "zero-beat," raise OSC A FREQUENCY (if OSC B
is sharp) or OSC B FREQUENCY (if OSC A is sharp). It is not possible to maintain
absolute zero-beat over more than a few seconds--but get as close as you can.
4. Slowly raise the PITCH wheel while listening for beats, then lower the wheel.
Beating should not increase significantly as the pitch is raised or Powered. Increased
beating probably indicates a difference in oscillator scaling, which can be corrected by
retuning. The wheel range should be at least a fourth above and below the center
pitch.
5. Check that the PITCH wheel is centered. Alternately play C3 and CO. There should
be no detectable GLIDE. Now advance GLIDE RATE to 5 and repeat, This should yield
a medium glide. Turn GLIDE RATE to 10 and repeat. It should take a minimum of 3
seconds to skew 3 octaves.
6. Switch AUTO-GLIDE on. Alternately play C3 and CO. There should be no GLIDE.
Now hold C3 and play CO. Tlis should activate GLIDE.

1-7 FILTER TEST
4: '
-. z -.
nBC L P318
.....
CUTO,, nE,n,,mwcr rM"rLnmt Mtv.n.mm
LY"Y*T LY"""7
#YOUWT mrm I~LOYFYL*
9n11 ~TI~SW mrn =usr.nm arrx.mv
06C B W" I .
.'E
**mu.. a.u.t
rnl' . .
WT7.4 IIELIL.W
IFO
Figure 1-5
FBLTER TEST PATCH
1. Patch according to Figure 1-5. NOTE: Unless you play staccato or switch to
RETRIG mode, the envelope generator will not retrigger
2, Rotate FILTER CUTOFF knob 0 -
10, listening for sine-wave oscillation. The
superaudia range is encountered at about 7 on the dial.
3. Set FILTER CUTOFF to 3. Raise FILTER KEYBOARD AMOUNT. As this knob is
advanced, the notes heard will approach the diatonic scale tones. With KEYBOARD
AMOUNT set to 8, the scale should be fairly accurate. Leave knob set to 8.
4, Hold a key and rotate FILTER ENVELOPE AMOUNT 0 - 10. This should not detune
the pitch. If it does detune the filter, look into the Filter Envelope Generator circuit
far
the source of the offset. It is normal to hear an increasing transient as you advance
ENVELOPE AMOUNT. This is the voltage peak between the ATTACK and DECAY
periods--which becomes audible because the Envelope Generator is being triggered.
Although the knobs are set to 0, it still takes a finite time to cycle through the
ATTACK and DECAY periods. Leave knob set to 2.
5. Decrease FILTER RESONANCE to the point where oscillation stops and note dial
indication. It should
be between 5 and 6. Return knob to 10.
6. Increase FILTER ATTACK knob to 6. This should give about 1-sec attack time, as
indicated by the pitch which gradually rises then "snaps back" to its starting pitch.
Return to 0.
7. Increase FILTER DECAY knob to 6. This should give abyut 1-sec decay time. Return
knob to 0.
/
8, Rotate FILTER SUSTAIN knob 0 - 10. It should smoothly increase filter frequency.
Leave
knob set to 10.

9. The next step requires increasing the AMPLIFIER RELEASE time so you can hear
the FILTER RELEASE time. Start with AMPLIFIER RELEASE set to about 8. Set
FILTER RELEASE knob to 6. This should give about a 1-sec release time.
16. Set both the FILTER and AMPLIFIER RELEASE knobs to 10. Hit a key and check
that it takes a minimum of 15 seconds for the pitch to fully RELEASE.
1-8 AMPLIFIER TEST
Figure 1-6
AMPLIFIER TEST PATCH
1. Patch according to Figure 1-6.
2. With AMPLIFIER ATTACK set to 6, the attack time should be about 1, second, heard
as a rising volume which then "snaps backr9.to silence. Return knob to 0.
3. Increase AMPLIFER DECAY knob to 6. This should give about 1-sec decay time.
Return knob to 6.
4, Rotate AMPLIFER SUSTAIN knob 10 - 10. It should smoothly increase amplitude.
Leave knob set to 10.
5. Increase AMPLIFER RELEASE knob to 6. This should give about a 1-sec release
time.
6. Set the AMPLIFIER RELEASE knob to 10. Hit a key and check that it takes a
minimum of IS seconds for the note to fully RELEASE.

1-9 MODULATION TEST
This test checks the LFO waveshapes, the three MODULATION FROM AMOUNT knobs
and ROUTE switches, and the five destination (TO) switches. (The FIL ENV and OSC B
modulation sources were tested above.) Since this test includes the LFO, don't use the
arpeggiator or sequencer.
Figure 1-7
MODULATION SOURCE TEST PATCH
1. Patch according to Figure 1-7. (AMPLIFIER RELEASE must be greater than FILTER
RELEASE to hear the complete filter envelope.)
2.
Hold a key or use DRONE. Raise the LFO AMOUNT knob and note increasing
frequency sweep of OSC A. Since OSC A frequency is being controlled by the LFO
sawtooth, the frequency rises consistently, then snaps back to the t'initial"frequency
[para. 2-19).
3. Switch LEO SAWTOOTH off and LFO SQUARE on. This will give a futrillM effect,
with the interval expanding with increased LFO AMOUNT. At 4 - 5 on the dial the
higher note will be an octave- above the initial frequency. At about position 8 the range
will be two octaves, (para. 2-19)
4. Switch LFO SQUARE off and LEO TRIANGLE on. A symmetrical vibrato should be
heard. Adjust LFO FREQUENCY O - 10 to explore the range of vibrato available
(which is from about 1/4 to about 25 Hz). Leave LFO FREQUENCY set to about 6.
Return LEO AMOUNT to 0.
i
5. Adjust OSC B AMOUNT 0 - 10 to check OSC 0 as rnoddlation s6urce. The effect will
be a vibrato similar to that produced by the LFO. Adjust OSC B FREQUENCY to hear
the range, Leave OSC B FREQUENCY and OSC B AMOUNT set to 0.

6, If used, switch DRONE OFF. Play slowly while adjusting MOD EIL ENV AMOUNT
knob O - 10 to check the filter envelope as modulation source. The filter envelope has
been patched in Figure 1-7 with 1-sec ATTACK, DECAY, and RELEASE times.
Applying this envelope to OSC A FREQ gives a frequency sweep which rises for 1-sec,
decays at the same rate to the sustain level while the key is held, then releases tothe
initial
frequency at the same rate when the key is released. The range of the sweep is
increased as you advance the FIL ENV AMOUNT knob. Return FlL ENV AMT to 0.
7. Switch FIL ENV. OSC B and LFU ROUTE switches to WHEEL. Also switch TO OSC
A FREQ from DIR to WH. Test the FEL ENV, OSC B and LFO WHEEL-modulation
sources as in steps 4 through 6 (for DIRECT MOD). The WHEEL test results will be the
same as the DIREC-T results,-cxcept the MOD wheel is also effective as an attenuator
in series with any of the MODULATlON AMOUNT knobs. In other words, the MOD
wheel and the AMOUNT knobls) must both be advanced to engage modulation.
-
8. Now that the modulation sources have been checked with OSC FREQ A, the four
remaining destinations can be tested. Patch according to Figure 1-8.
Figure 1-8
MODULATION DES'FINATION TEST PATCH
9. Play and listen to pulse-width modulation of OSC A. Raise LFO AMOUNT to check
depth of modulation. Set to 10, the LFO triangle peaks may cut-off OSC A PULSE (by
forcing it to dc).
10. Switch TO OSC A PW off. Turn MIXER BSC A to' 0. Turn MIXER OSC 5 to 10,
Switch TO OSC B FREQ to DIR. This should create vibrato as heard in step - 4, above.
Check depth by adjusting LFO AMOUNT.
L
11. Switch TO OSC B FREQ off. Switch TO OSC B PW to DIR.' This should produce
pulse-width modulation as heard in step - 9.

12, Switch TO OSC B PW off, Switch TO FILTER to DIR. The LFO should now
modulate the filter cutoff frequency, giving a "wah-wah" effect.
13. Switch LFO ROUTE from DIRECT to WHEEL and repeat steps 8 - 12 except switch
the destinations to WHEEL (instead of DIR) to test the WHEEL desfinazon paths.
1-10 BACK PAEL TEST
,I
1. If desired, connect electric piano, guitar, or microphone to AUDIO IN. Switch mode
to EXT. Raise MIX NOISE/EXT knob and adjust for adequate gating by the external
signal.
2. With a suitable patch, plug one end of a mono phone cable into 36 FJLT CV IN. This
should disable the FILTER KEYBOARD AMOUNT knob. Connect other end to CV OUT,
Unit should play normally. Disconnect cable.
3. Plug one end of mono phone cable to 52 CV IN. This should disable the keyboard.
Connect other end to CV OUT, Unit should play normally. Remove cable.
4. If a GATE source is avaitable connect to GATEjCLK IN, and switch MODE to
REPEATJEXT check that the envelopes are gated and sequencer advanced with each
pulse.
5. GATE OUT can be checked with a voltmeter. The output goes from 0 to +5V when a
key is pressed.
1-1 1 TONING FLOW CHART
Figure 1-9 on the next page is a flow chart for selectively checking Pro-One tuning.
Especially because most tuning trimmers are accessible to the owner, whenever the
Pro-One Is grossly out of tune it is probably best to completely retune according to
Section 5, below or in the the Operation Manual, CMIOOB. But often only a couple of
trimmers may need "touching-up," in which case following the chart may save you
significant time,

PRO-BNE TUN1 NG CHECK FLgW-CHRRT
OFFSET
SCRLE
SCRLE
Figure 1-9
PRO-ONE TUNING FLOW-CHART

SECTlON 2
THEORY OF OPERATION
2-0 INTRODUCTION
This section explains the Pro-One's hardware theory of operation. The microcomputer
program (software)
is not generally discussed because it is proprietary and because the
program details are usually not-relevant to service problems. That is, you canlt "fixt'
the software (you - can replace it, by replacing the Microcomputer IC). Note that if you
first understand the hardware surrounding the microcomputer, you will be able to
deduce the basic program functions.
2-1 GENERAL
1
The Operation Manual CMlOOB covered general function and signal flow of the voice
and modulation sections. The Pro-One is a monophonic synthesizer in which the analog
voltage-controlled sound sources and modifiers are controlled by a microcomputerized
keyboard, as shown in Figure 2-0.
Figure 2-0
PRO-ONE GENERAL BLOCK DIAGRAM
r
- KYUU LV
AUOlQ
The microcomputer generates the keyboard control voltage (KYBD CV), which is the
main determinant of oscillator pitch, and the GATE and TRIGGER (TRIG) signals,
which controi the
filter and amplifier envelope generators.
KEYW&D
Figure 2-1 shows more functional detail. The integrated microcomputer contains a
central processor unit CCPU), read-only memory (ROM3, and random-access memory
(RAM). The CPeJ foliows the program in ROM by which it f'reads" the keyboard,
SEQUENCER, ARPEGGIATE, and MODE switches and forms the appropriate output
signals, The CPU uses RAM to keep track of the keyboard and switch status and to
store latched arpeggios and sequences.
To play a note the microcomputer first reads the keyboard ta find out what key is
pressed, then sends the key'number in parallel, 6-bit binary form to the digital-to-
analog converter
(DAC). When it receives the DAC ENbBLE (EN) signal, the DAC
converts the key number to the analog KYBD CV for OSC I'A, OSC, 8, and the FILTER.
The Microcomputer then issues the- TRIG and GATE signah which enable the Filter and
Amplifier Envelope Generators.
The TRIG only appears briefly when the key is first
struck.
When the key Is released, the GATE goes off, initiating the RELEASE period(s1.
h.
, DATA
9
For the remainder of this section, please refer to Figure 2-2, Abstract Schematic, and
the Interconnect Diagram and Schematics {SD 121JSD 112) in Section 3.
MICRO GATE
COMWTER TRIG

2-2 OSCILLATOR A
The Pro-One's three oscillators (OSC A, OSC B, LFO) use the CEM 3340 integrated
voltage-controlled oscillator
(VCO). As in the Prophets and most standard synthesizer
modules, these
VCOs operate in exponential mode (as opposed to linear mode), scaled
at BVloctave. This means a CV change of exactly 1V ideally producmitch change of
exactly one octave, i. e. a doubling of frequency. The basic range of OSC A (and OPC
B) is seven octaves, resulting from up to 3V provided by the keyboard, up ta 3V
provided by the OCTAVE switch, and up to a little more than 1V provided by the
FREQUENCY knob.
The VCO itself is of course a complex of other circuits, Basically, the summed CY
input drives an exponential control current generator, which charges the external
timing capacitor.
The increasing positive charge produces the ascending portion of a
triangle wave (by direct integration). When this charge reaches a specific level, a
comparator switches- in a discharge path for the capacitor, producing the descending
portion
of the triangle wave. The sawtooth is obtained from the triangle via another
comparator and
switch. The pulse is created by a cornparatar which toggles as the
sawtooth level matches
the pulse width CV input,
See SD112. Looking at the various signals summed into U102 OSC A's control pin 15,
R123 sets the initial frequency, R119 OSC A FREQUENCY knob provides over 1V of
control through R131. R120 OSC A OFFSET trimmer adjusts initial frequency through
R124, When centered, R1210 MASTER TUNE is +7.5V, summed through R126.
R133 adds the OSC A OCTAVE voltage, which is switched in precise 1-volt steps.
Precision resistors are used throughout this circuitry to allow octave transposition
without
having to retune R119 (OSC A FREQ). The switch circuit itself takes
advantage
of the summing ability of the op amp. The divider containing RlllE -lV
trimmer, RlllQ, and Rll09 places -lV on the input of Buffer U107-7. Figure 2-3
shows the function of 5105 05C A OCTAVE switch, which can also be thought sf as a
2-bit DAC. The low-impedance voltage source is switched through R-2R summing
resistors to an inverting summer, becoming a positive voltage at U104-14. R167, of
course, sets the gain to 1. C117 makes a low-pass filter to suppress switching noise.
Summed through
R 127, Rl PITCH wheel contributes a nominal OV when centered.
I3101 and Dl02 form a "deadband" circuit. The inherent diode voltage drop reduces the
requirement that the
PITCH wheel be exact1 centered to not detune the oscillators.
The wiper can be between +/- 700 mV be 741 ore t e oscillators are affected.
QSC A is always under contror of the KYBD CV through R121, which is precision-
matched to R133 OSC A OCTAVE summing resistor. (The origin of the KYBD CY is
discussed in par a. 2- 1 2.)
5104 MODULATIQN TO OSC A FREQ switch applies a modulation CV, mixed ever
either the WHEEL (WH) or DIRECT CDIR) modulation busses, through R125. (The
Modulation source circuitry is discussed in para. 2-8.)
L

Figure 2- 1
PReONE FUNCmONAL BLOCK DIAGRAM