Oberheim ob 8 service manual alt

Page 1

INTRODUCHON

The (13.2} is the latest development in the evolution of Uberheim
synthesizers. The design philosophy behind it was to take all of the
features of the DB-Xa, add as many no features 95 were economically
feasable, and redesign all of the circuitry for increased reliability
and lower cost. The result is a synthesizer "it 9 3"" calibrations
and ll loos circuit boards, and with "WY Or the remaining calibrations
microprocessor assisted. The circultrY and 3 trimmers that ""9
eliminated have been replaced with aortwarc functions 1 5
microprocessor which simulate the rerllifl':ed hardware. 50" 0f 9
functions are explained below.

VCO TRACKKNG

The volts per octave (scaling) parameters of the 16 oscillators in
the 03-8 are adjusted by the microprocessor menever the AUTO button is
pressed. The processor samples 5 different frequencies 70 "Ch
oscillator and calculates the proper EOFNCUW voltage 0 bring "C"
oscillator in tune. This voltoge changes depending on the final pitch
desired from the oscillator. There it? 9 mg trimmer adjustment for
each oscillator-'8 volta per octave in 0rd t0 9'3t the 0591115? "mu"
the range in which the processor can calibrate it. If this calibration
is out of the acceptible range, it may be mticed by Ell-h" AU WE
failing that oscillator, or excessive beating "99" 1t and 0
oscillators mile it is anvrcu half steps. The reason for this is that
the processor compensates for scaling of the DST-4118 5" Ch half
step, requiring that the oscillator be 115th "011 1°09 5° that
the tracking between half steps is a v01t per OCUVE- If m 13 Wt r
calibration it can also be peroived as atepa 1" the 0 dual-1°"-
since the oscillator will jump slightlY in pitch 59 0 "MUHW"
amount reachés each half step.

title

All or the Lrog in the 03.5 are generated in software. What this
means is, the voltage necessary at any given time to simulate U'D
modulation is output through the 0M: to vhatevsr destination is
selected. If, for example, the triangle wave of the LFD should be at
its peak, then a high voltage N111 be output to the destination. When
the up should romp down, the voltage is decreased- All of the
electronic switches and VCAs normally "Wife With 5 hardware W) 1"
order to determine us gunman, and amplituda are eliminated since
these functions are non accomplished in 90ft?!-

INTRODUCTION/I

Pl TEH BEND

The pitch bend and vibrato levers are scanned by the microprocessor
and their relative positions are calculated to determine the voltage
neceaary to bend a pitch 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 amp offsets, and scaling
trimmers. There are 2 rough trimmers used to bring the levers into
range so that theprocessor can read them. Once they are in range. the
processor re-calculatos the center dead-zone of each lever each time
AUTO is pressed.

A-PDLE FIUER

The DB-Xa had aeperate-Z-polo and t-pole filters for each voice,
each filter utilizing a CEMDZO with different external components to
implement the filter functions. In the 08-8, There is one 3320 per
voice. and the external components are electronically switched to
generate either a 2-polc or a 4-pole slope. This eliminates the need to
calibrate the two filters separately, since they are now the some
f llter.

0P AMP OFFSETS

Many sample .and hold up amps can affect a parameter if their offset
is large. To correct for this, the processor uses a software
calibration procedure described later to assist in setting the offset to
0 volts. This adjustment is most critical for the envelope time
parameters. since the CEMDIO envelope chips require a control voltage
range from D to 300 millivolta. The software is able to correct for the
offsets by outputting a voltage to the sample and holds that will
compensate for the ,offaetfi If an op amp has +30 millivolts of offset,
the processor.uill output 30 millivolts less than the final voltage
required for that sample and hold, effectively canceling the offset.

Page 2

08-8 CALIBRATION PROCEDURE

The following calibration instructions are those followed by the
technicians at Dberhaim Electronics prior to the shipment of an 03-8.
The microprocessor assists in many of the necessary calibrations by
indicating which direction to turn a trimmer, and indicating when a
trimmer is calibrated by using the test LEDa located on the inside of
the front panel circuit boards (they are visible than the lid is open).
Even though these calibrations seldom need adjustment, it is a good idea
to check them whenever servicing an 08-8.

A digital voltmeter with b A 1/2 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 03-8, it is necessary to
remove the right wood endbell completely and to remove the top two
screws from the left endbsll. The four screws holding down the circuit
board with the Upper four voices should also be removed, to allow access
to the Lower Voices.

lhc teat procedures to follow assume the (TB-8 has software version
M 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 Line
being displayed in the PROGRN'MER section now show the current version
number. If the Hills light up as version Al or A2. a few spacial
procedures are required which will be mentioned later in the teat
descriptions. For software version A}. the only difference is that the
23151; volume offset cannot be calibrated (unless updated to M with [C0

First, power the unit on. Verify all voltage sources (*5, -5, +15,
and -15l at connectors I and J. Calibrate the +15 supply using T2 to +
or -iBmv. Calibrate the -5 supply using Tl to e or - Saw. The +5 and
the ~15 supplies do not require calibration, but should be verified to
be within v or - 5: of their rated value.

Now, enter the calibration mode by turning the TEST 1 switch on
(Up). This switch is located inside the synthesizer at the lower left
corner of the front panel. All tins should now be off except for the
05C 1 Mil) LED in the Bend Box. The Bend Box in conjuction with the two
lads 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 a different test or calibration procedure and assists in
calibrating 28 of the 55 trimmers in the 09-8.

CALIBRATION /2

CALIBRAT INC THE DAC

Pressing the, 05C. 1 M00 button in the Band 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 a voltmeter to the DAC OUTPUT and
AGND test points on the processor board. If there are no test points,
the DAB output can be found at pin 20 of U45. Be sure to connect to a
ground closest to the DAC. Press CD on the keyboard. Adjust the DAB
LFfSET trimmer (U) until the meter rsada 0.0le volts. Now press 00.
Adjust the DAC SCALE trimmer (Tb) until the meter reads 10 volts 4/-
va. Then press C0 again and check that the meter still reads {2.000
volts and recallbrate if necessary. mm: 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 SEITING ENVELOPE (YFSETS.