Oberheim OB-8 User manual

INTRODUCTION
The 0B-8 is the latest development in the evolution of Oberheim
synthesizers. The design philosophy behind it was to toke all of the
features of the OB-Xe, add as many new features as were economically
feasable, and redesign oil of the circuitry for increased reliability
and lower cost. The result is asynthesizer with 90 fewer calibrations
and 11 less circuit boards, and with many of the remaining calibrations
microprocessor assisted. The circuitry and the trimmers that were
eliminated have been replaced with software functions in the
microprocessor which simulate the replaced hardware. Some of these
functions are explained below.
VCO TRACKING
The volts per octave (scaling) parameters of the 16 oscillators in
the OB-8 are adjusted by the microprocessor whenever the AUTO button is
pressed. The processor samples 5differentfrequencies for each
oscillator and calculates the proper correction voltage to bring each
oscillator in tune. This voltoge changes depending on the final pitch
desired from the oscillator. There is arough trimmer adjustment for
each oscillator's volts per octave in order to get the oscillator within
the range in which the processor can calibrate it. If this calibration
is out of the acceptible range, it may be noticed by either AUTO TUNE
failing that oscillator, or excessive beating between it and other
oscillators while it is BETWEEN half steps. The reason for this is that
the processor compensates for scaling of the oscillators for each half
step, requiring that the oscillator be callibrated well enough so that
the tracking between half steps is avolt per octave. If this is out of
calibration it can also be percived as steps in the LFO modulation,
since the oscillator will jump slightly in pitch as the modulation
amount reaches each half step.
LFOs
All of the LFOs in the OB-8 are generated in software. What this
means is, the voltage necessary at any given time to simulate LFO
modulation is output through the DAC to whatever destination is
selected. If, for example, the triangle wave of the LFO should be at
its peak, then ahigh voltage will be output to the destination. When
the LFO should ramp down, the voltage is decreased. All of the
electronic switches and VCAs normally required with ahardware LFO in
order to determine its destination and amplitude are eliminated since
these functions are now accomplished in software.
INTRODUCTION/I
PITCH BEND
The pitch bend and vibrato levers are scanned by the microprocessor
and their relative positions ore calculated to determine the voltage
necesary to bend apitch up or down, or how much vibrato to add. This
eliminates all problems associated with matching 100k resistors in order
to send equal voltages to all voices, op emp offsets, and scaling
trimmers. There are 2rough trimmers used to bring the levers into
range so that the processor can read them. Once they are in range, the
processor re-calculates the center dead-zone of each lever each time
AUTO is pressed.
4-POLE FILTER
The OB-Xa had seperate 2-pole and 4-pole filters for each voice,
each filter utilizing aCEM3320 with different external components to
implement the filter functions. In the OB-8, There is one 3320 per
voice, and the external components are electronically switched to
generate either a2-pole or a4-pole slope. This eliminates the need to
calibrate the two filters seperately, since they are now the same
filter.
OP AMP OFFSETS
Many sample -and hold op amps can affect aparameter if their offset
is large. To correct for this, the processor uses asoftware
calibration procedure described later to assist in setting the offset to
0volts. This adjustment is most critical for the envelope time
parameters, since the CEM3310 envelope chips require acontrol voltage
range from 0to 300 millivolts. The software is able to correct for the
offsets by outputting avoltage to the sample end holds that will
compensate for the offset. •' If an op amp has +30 millivolts of offset,
the processor. will output 30 millivolts less than the final voltage
required for that sample and hold, effectively canceling the offset.

CALIBRATION /2
OB-8 CALIBRATION PROCEDURE CALIBRATING THE DAC
The following calibration instructions are those followed by the
technicians at Oberheim Electronics prior to the shipment of an OB-8.
The microprocessor assists in many of the necessary calibrations by
indicating which direction to turn atrimmer, and indicating when a
trimmer is calibrated by using the test LEDs located on the inside of
the front panel circuit boards (they are visible when the lid is open).
Even though these calibrations seldom need adjustment, it is a good idea
to check them whenever servicing an OB-8.
Adigital voltmeter with 441/2 digits is required to perform some
of these calibrations. The rest can be done without any test equipment.
For access to all voices when servicing an OB-Q, it is necessary to
remove the right wood endbell completely and to remove the top two
screws from the left endbell. The four screws holding down the circuit
board with the Upper four voices should also be removed, to allow access
to the Lower Voices.
The test procedures to follow assume the OB-8 has software version
A4 or above. To determine the software version, press the CHORD/PAGE 2
button twice and hold it down the second time (the LED should now be
lit), and while holding it, press and hold the SYNC button. The LEDs
being displayed in the PROGRAMMER section now show the current version
number. If the LEDs light up as version A1 or A2, afew special
procedures are required which will be mentioned later in the test
descriptions. For software version A3, the only difference is that the
output volume offset cannot be calibrated (unless updated to A4 with ECO
410).
First, power the unit on. Verify all voltage sources (+5, -5, +15,
and -15) at connectors Iand 0. Calibrate the +15 supply using T2 to +
or -lOmv. Calibrate the -5 supply using T1 to +or -5mv. The +5 and
the -15 supplies do not require calibration, but should be verified to
be within +or -5S£ of their rated value.
Now, enter the calibration mode by turning the TEST 1switch on
(up). This switch is located inside the synthesizer at the lower left
corner of the front panel. All LEDs should now be off except for the
OSC 1MOD LED in the Bend Box. The Bend Box in conjuction with the two
leds on the inside of the pot board and the first eight keys of the
keyboard can now assist in many of the necessary calibrations. Each
button performs adifferent test or calibration procedure and assists in
calibrating 28 of the 56 trimmers in the OB-8.
Pressing the OSC 1MOD button in the Bend Box enables the DAC
calibration mode. This procedure allows for the DAC offset and scaling
to be calibrated, and should be done before any other calibrations
(excluding the power supply). Connect avoltmeter to the DAC OUTPUT and
AGND test points on the processor board. If there are no test points,
the DAC output can be found at pin 20 of U45. Be sure to connect to a
ground closest to the DAC. Press CO on the keyboard. Adjust the DAC
OFFSET trimmer (T3) until the meter reads 0.000 volts. Now press DO.
Adjust the DAC SCALE trimmer (T4) until the meter reads 10 volts +/-
5mv. Then press CO again and check that the meter still reads 0.000
volts and recalibrate if necessary. NOTE: The sample/holds to the
attack, decay and release controls of the filter and VCA envelopes are
enabled during this test so that op-amp offsets may be checked. This is
necessary only if version A1 is being used. The procedure is described
under SETTING ENVELOPE OFFSETS.

3
SETTING THE BEND TRIMMERS
When the OSC 2MOD switch is selected, the Bend Box trimmers can
be calibrated. These trimmers are used to roughly center the Pitch Bend
and Modulation Levers. First, flick the levers back and forth alittle
to make sure that they are in their rest position. Press CO. The LOWER
LED has come on to indicate that the Modulation Lever trimmer can now be
adjusted. Either one or both of the inside Pot Board LEDs (hereafter
called test: LEDs) will be on. If both LEDs are on, this trimmer is
already properly calibrated. If only one LED is on, Adjust the RIGHT
trimmer on the Bend Board until both test LEDs are on. Now press DO.
The UPPER LED has come on to indicate that the Bend Lever trimmer can
now be adjusted. Adjust the LEFT trimmer on the Bend Board until both
leds are on. The Bend and Modulation Lever trimmers are now calibrated.

4
ADJUSTING ENVELOPE OFFSETS
The following procedure should only be done if the envelope parame-
ter times are not matched between the upper and lower four voices. This
can by determined by listening for more than a2second variation from
voice to voice when holding anote with the filter attack set to maximum
and the filter sustain and decay set to minimum. This calibration is
stored in the memory of the microprocessor and should not have to be
reset unless the memory, the battery, or U33, U35, or U27 have been
removed or replaced. The purpose of this calibration is to allow the
microprocessor to compensate for the offsets of the envelope sample and
holds, since these offsets can cause the envelopes to have different
time constants. This calibration procedure exists only on software
versions A3 and above. For units with A1 or A2 software, envelope
matching is done by selecting low offset op amps for use at U33 and U35.
The offsets of these op amps can be checked while in the DAC CALIBRATION
mode, and should be selected to as close to 0volts as possible.
Pull up on the RATE knob. The RATE LED should come on to indicate
that the Envelope Offset mode has been selected. Connect the DVM to U33
pin 14 on the Lower Voice Board. Connect the ground of the DVM to a
ground near pin U33 (The ground side of one the nearby electrolytic
capacitors will do). Be 3ure that the DEPTH knob ia pushed in so that
the Lower Voice Board is selected. Press CO. Using the UPPER and LOWER
DJ::°na 'adjust the offset until the meter reads 0.000 volts, +or -1
*. To adjust it up or down fast, hold down the MODE button
wnue holding down UPPER or LOWER. When not holding the MODE button,
fk wil1 move up or down slowly. When the offset is set, press
the ARPEGdATE button to write the offset into memory. None of the
aorations set in this mode will be remembered unless each is written
into memory. Now attach the DVM to U33 pin 0. Press DO. Adjust for
0.000V offset using the UPPER and LOWER buttons and write it into memory
with the ARPEGGIATE button. Using the diagram as aguide, repeat this
procedure for the Filter and VCA Decay, and the Filter and VCA Attack by
selecting the proper parameter using the first six white keys on the
keyboard. Do not adjust the Filter Sustain or the Volume Offset yet.
Now, pull up on the DEPTH knob to select the Upper Voice Board.
Connect the meter to the Upper Voice Board and repeat the procedure of
the Lower Board* Be sure to write each offset into memory with the
ARPEGGIATE button.
After the Release, Decay, and Attack parameters have been
calibrated, the Filter Sustain can be calibrated. This calibration only
requires that the Upper and Lower Voice Boards be matched, but it is not
necessary for them to be adjusted for 0.000 volts offset. With the
DEPTH knob still pulled up, connect the meter to U27 pin 1on the Upper
Voice Board. Press B0. Adjust the offset for zero volts, or as close
os it can be adjusted to zero volts. If the offset is not zero volts,
remember the offset value. Write the offset into memory with the
ARPEGGIATE button. Push down the DEPTH knob and connect the meter to
U27 pin 1on the Lower Voice -Board. Press B0. Adjust the offset so
that it is the same as the Upper Voice Board. If the offset will not go
low enough, set it as low as it can go, store it in memory, and readjust
the Upper Board offset to match the Lower Board.
The final offset. calibration is the output volume offset. This
adjustment is made to insure equal volume between Voice Boards at any
volume setting. This calibration only exists in software versions A4
and above. Like the Filter Sustain calibration, the important thing is
to match the 2boards, even if they are not set for 0offset. Make
sure that the MASTER VOLUME and trimmers T501 on each Voice Board are
all the way up before making this adjustment. First on the Lower Voice
Board, connect the meter to U503 pin 6. This is ahigh impedence signal
so be sure to use ashielded cable on the meter. Press the DEPTH knob
into its down position and press Cl on the keyboard .Using the same
procedure as for the Filter Sustain, adjust the offset until the meter
reads 0.000 volts or is as close to 0as possible. Write the offset
into memory with the ARPEGGIATE button. If the offset is more than 150
millivolts, replace U503 with another 3080 and repeat the procedure. If
the offset is below 150 mv but above 0, remember the offset. Repeat the
procedure for the Upper Voice Board by pulling up on the DEPTH knob,
pressing Cl, and connecting the meter to U503 on the Upper Board.
Adjust the offset so that it matches the Lower Board. If the offset
cannot be adjusted low enough, set it as low as it will go below 150 mv
(otherwise it should be replaced), write it into memory with the
ARPEGGIATE button and reset the Lower Board so that the offset matches
the Upper Board. Write the new offset into memory. After all of the
offset calibrations have been completed, press down on the RATE knob.

5
OSCILLATOR VOLTS PER OCTAVE ADJUSTMENTS
To calibrate the volt* p.ie octave of oscillator 1of each voice,
press the OSC 2ONLY button. The voice to be calibrated is selected
using the first 8white keys on the keyboard (C0-C1). The gate LED will
be lit on the voice that i3 currently selected. Select voice 1by
pressing CO on the keyboard. If both test LEDs are on, this oscillator
is in calibration. If only one LED is on, adjust the volts per octave
trimmer (T10 1)until both test LEDs are on. If both test LEDs are off
or seem to be flashing randomly, then this oscillator is not functioning
properly and should be repaired or replaced. Select DO now to calibrate
voice 2and repeat the procedure stated above for voices 2through 8
using trimmers T201, T301, T401, etc. After calibrating all 8voices,
both test LEDs should be on whenever pressing any of the first 8white
keys. To calibrate oscillator 2of each voice, press the BEND AMOUNT
button. Follow the procedure for calibrating oscilator 1using the
first 8white keys to select which voice is being calibrated. Adjust
oscillator 2’s volts per octave trimmers (T103, T203, T3Q3, etc.) until
all 8voices are calibrated. To calibrate the amount of filter envelope
modulation into oscillator 2, press the DOWN TRANSPOSE button and select
avoice using the first 8white keys. Adjust the F-ENV trimmer for each
voice (T102, T202, etc.) until both test LEDs are on. Again, if both
test LEDs are off or flashing randomly, oscillator 2of the selected
voice is not functioning correctly. NOTE: The volts per octave for Osc
2and the envelope offsets must be calibrated before this adjustment is
made. Also, on software version A1 and A2, the F-ENV calibration may
not be able to be calibrated so that both LEDs are always on. This is
due to the tempature instability and the nonlinearity of the CEM3360 VCA
on each voice (U109). If this is aproblem, it can be rectified by
implimenting ECOs A05 and A06 end replacing the eproms (U21-U24) with
the most recent version of software. This change requires many cuts and
jumpers as well as component value changes and should only be attempted
if absolutely necessary.

OSCILLATOR DRIFT TEST
w
LED TEST
To test all of the LEDs, press the MODE button. This teat will
turn on all of the LEDs on the front panel and in the Bend Box except
for the CASSETTE LED which must be turned on with the Cassette Enable
switch. Any LEDs that do not light are not functioning properly. To
leave this mode, select another test.
SWITCH TEST
Enter the switch test mode by pressing the ARPEGGIATE button. When
in the switch test mode, each switch on the front panel will light its
respective LED when pressed. The AUTO switch will light the detune LED.
This test, in conjunction with the LED test, can be used to determine
whether aswitch or aLED is not functioning. To leave this mode,
select another test.
This test is used to determine if any of the oscillators drift an
unreasonable amount over agiven time and temperature. To enter this
mode, press the UPPER button in the Bend Box. The UPPER LED will flash
and the processor will wait 10 minutes to allow the internal temperature
to stabilize, afterwhich it will autotune all of the voices. The UPPER
LED will continue to flash as the processor waits another 10 minutes,
and then autotunes the oscillators again, comparing them to the last
tuning. After the second tuning, the UPPER LED will stay on to indicate
that the test is over, and if any of the oscillators have drifted more
than +or -10 cents (hundredths of asemitone), the processor will
light acorresponding LED for the drifting oscillator(s). The bottom
row of LEDs in the programmer section of the front panel except for the
MANUAL LED will show which oscillator (s) drifted out of range. The
SPLIT LED is for voice 1oscillator 1, the DOUBLE LED is for voice 1
oscillator 2, LOWER is for voice 2osc 1, UPPER is voice 2osc 2, GROUP
Ais voice 3osc 1, GROUP Bisvoice 3osc 2, GROUP Cis voice 4osc 1,
GROUP Dis voice 4osc 2, and PROGRAM 1-8 are for voices 5-8. If any of
these LEDs came on, the test should be performed again to verify that
the lit oscillator is bad, since this test requires that the temperature
remains stable to determine accuracy, snd could fail an oscillator due
to room temperature changes. If the oscillator fails the test twice, it
should be replaced and recalibrated, and the test should be performed
again, to insure that the new oscillator is within stability range.
This test is accomplished by tuning each oscillator at A-Sharp 3
(466.16 Hz), waiting .10 minutes, tuning again at A-Sharp 3and comparing
the amount of correction necessary to tune. If the difference between
the 2tunings is more than plus or minus ahalf acycle, the oscillator
is displayed. It is important to note that this test can only check for
long term drift, and not short term stability since the tuning is done
only twice within the 10 minute interval. If it is desired to interrupt
this test while it is in process, press any of the other test switches
to enter anew test.

REMAINING BEND SWITCHES
The remaining Bend Box switches (LOWER, UP TRANSPOSE) do not
currently have any test function and will be ignored when pressed.
These switches have been left for possible future test procedures.
All of the microprocessor assisted calibrations and
tests have now been done. Return the synthesizer to
its normal operating mode by turning the TEST 1switch
inside the front panel to its off. position before
performing the following callibrations.
FILTER CALIBRATION
There are three more calibrations necessary per voice before the
voices are calibrated, and these are for the filter. This calibration
procedure can be done by ear, or with astrobe tuner. First, put the
front panel into manual mode, and turn off all of the switches except
TRACK in the filter section. Turn the RESONANCE, VOLUME ENVELOPE
SUSTAIN, and PROGRAM VOL/BAL knobs all the way up, center the MASTER
TUNE control, and turn everything else all the way down. Set the master
volume to adesired listening level. Go into page 2by pressing the
CHORD/PAGE 2button twice, and turn off all of the voices except the one
to be calibrated by using the program 1-8 buttons. Play anote on the
keyboard and adjust the resonance trimmer (T 104 for voice 1) up until
sound can be heard. While playing alternate octaves on the keyboard,
adjust the volts per octave trimmer (T106) until the interval is one
octave. Verify this adjustment by playing notes 2or 3octaves apart
and determining that the octaves are in tune. Next, adjust the
resonance trimmer (T104) down while holding down akey until just after
the filter stops oscillating. To adjust the filter initial frequency
(T105), Turn on oscillator 1and adjust the trimmer for maximum volume.
This sets the filter to the same frequency os the oscillator. Repeat
this procedure for all Bvoices by enabling the voice to be calibrated
and following the steps stated above. After all 8voices have been
calibrated, turn all 8voices bock on and verify that all 8voices sound
the same.
7
OUTPUT VCAVOLUME ADJUSTMENT
The only two remaining adjustments are the final volume trimmers
(T501) on each Voice Board). While listening in MONO, turn both
trimmers to maximum volume (clockwise). Turn the MASTER VOLUME and the
PROGRAM VOL/BAL knobs to maximum, and check for even volume between the
two Voice Boards by playing through the voices and seeing if four voices
are louder than the other four. If uneven, adjust the trimmer on the
board that is louder to match the other. While still in mono, turn the
Master volume half way up. Check for about the same volume between
Voice Boards. If the difference is very noticable, the output volume
offsets may need to be calibrated. This procedure was mentioned under
ADJUSTING ENVELOPE OFFSETS. If the software version is Al, A2, or A3,
replace U503 on the louder board with a3000 with low offset and
recalibrate T501.

FRGGRUNNING/8
FREERUNNING THE 00-8 PROCESSOR BOARD
By removing the jumper plug at location U25 on the OB-8 Processor
Board the Z80 will be placed in afreerun mode. To do this turn off AC
power ,remove the jumper and reapply AC power.
Theory:
In cases when the uP locks up, troubleshooting can be anightmare.
Because the uP bus forms acomplex feedback loop, failure here causes
many components to behave abnormally or vice-versa. The answer is to
break the feedback path. By removing the jumper plug, the data bus is
isolated from the system. The pull-down resistors cause the uP to see
only NOP instructions (00H for aZ80 CPU). Therefore on power up the
first instruction fetch is aNOP. This instruction increments the
program counter and causes afetch of the next instruction (another NOP
in this case). This technique forces the processor to address the
entire memory-address space despite failures in the bus, address decoder
or ROM.
Troubleshooting Technique:
The test set-up used is relative to the “Instruction Op-Code Fetch"
cycle (refer to any Z80 data book). With the uP freerunning attach the
EXT. trigger of an oscilloscope to the RD#signal (pin 21 on the Z80),
trigger on the falling edge.
An acending binary count can now be observed on address lines A00
through A15. Address decoding can be verified by checking all logical
outputs for any type of transistion.
If the uP is not stepping through the address field (no movement on
the address pins of the Z80) then afailure is likely in one of the
following:
The Z80.
The system clock.
The BUSRQ* is stuck low.
The power supply.
The RESET input is stuck low.

CONNECTORS/
9
CONNECTOR 0CONNECTOR ECONNECTOR GCONNECTOR H
Computer Interface Bend Box Connector Pot Board Connector Voice Board Connector
(Rear Panel) (On Processor Board) (on Processor Board) (on Processor Board)
1-HRD* El _15 G1 _AGND HI NOISE
2-GROUND E2 --15 G2 -ANLGIN H2 _VCFP
3-GROUND E3 -+15 G3 -AGND H3 .. AGND
4-GROUND E4 -DGND G4 --15 H4 _AGND
5-GROUND E5 -+5 G5 _VOIP H5 ANLGOUT
6-GROUND E6 -DGND G6 _+15 H6 _AGND
7-DSC MUX E7 -AR1 G7 _SWENF* H7 AGND
8-HD 4E8 -+5 G8 -P0T1* H8 _AGND
9-HD 5E9 -ARO G9 -AR3* H9 _AGND
10 -HD 6E10 -AGND G10 -DGND H10 _VOLPOT
11 -HD 7Ell -P0T3* Gil -+5 Hll _CASSIN
12 -HA 0E12 -AGND G12 -ARO H12 _OSCMUX
13 -HA 2E13 -ANLGIN G13 .LCOO H13 V0ICE3*
14 -HA 4E14 -AGND •G14 -AR1 H14 _CASSIN
13 -HA 6E15 -BSWO* G15 -LC02 H15 _V0ICE4*
16 -HA 15 E16 -VIB G16 -AR2 H16 D3A
17 -HA 14 E17 -BLEDO* G17 -LC01 H17 _VOICE 2*
18 -HA 13 E18 -BLED1* G18 -P0T2* H18 D7A
19 -HA 12 E19 -BSW1* G19 -CEN* H19 _VOICE 1*
20 -HMRQ* E20 -D1A G20 -POTO* H20 D2A
21 -HWR* E21 -D5A G21 -SWD7* H21 _A3
22 -HINT* E22 -DOA G22 -SWDQ* H22 _D6A
23 -BUSAKA* E23 -D3A G23 -LR07 H23 _A2
24 -3USRQ* E24 -D4A G24 -SWD1* H24 D1A
25 -HRV* E25 -D2A G25 _LR06 H25 _A1
26 -HD 3E26 -BSWEN* G26 -SWD4* H26 _D5A
27 -HD 2G27 -LR05 H27 CLR*
28 -HD 1G28 -SWD2* H28 _DOA
29 -HD 0G29 -SWD6* H29 _A4
30 -HA 1G30 -SWD5* H30 _D4A
31 -HA 3G31 -LR04 H31 _A5
32 -HA 5G32 -SWD3* H32 _DGND
33 -HA 7G33 -LR03 H33 _A6
34 -HA 8G34 -LR02 H34 _DGND
35 -HA 9
36 -HA 10
37 -HA 11
POT BOARD INTERCONNECTIONS
NOTE:
These connectors are not labeled. The
connectors sre described from top to bottom
with the unit opened up.
BOARD JLPUT BOARD 2
A1 SWD7* A1
A2 LEDR5 A2
A3 SWD5* A3
A4 SWENC* A4
A5 LEDR3 A5
A6 SWD6* A6
A7 SWEN9* A7
A8 SWD3* A8
81 LEDR1 B1
B2 LEDCO B2
83 LEDC7 B3
B4 SWD1* B4
B5 SWDO* B5
B6 N.C. B6
B7 SWD4* B7
B8 LEDR2 B8
Cl SWD2* Cl
C2 SWENA* C2
C3 LEDC1 C3
C4 LEDC3 C4
C5 LEDRO C5
C6 LEDR4 C6
C7 +15 C7
C8 +5.6 C8
D1 AGND D1
D2 AGND D2
D3 MOD 1D3
D4 MOD 2D4
D5 RATE D5
D6 PORT D6
D7 DETUNE D7
D8 TUNE D8
D9 BALANCE D9
DIO VOLPOT DIO

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LOWER
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