Abstrakt Instruments Oberheim OB-X User manual

Date: March 4, 2019
Doc: D00282-01
Rev: 1
OB-X Voice Expansion Kits
INTRODUCTION
The Oberheim OB-X was sold in 4-voice, 6-voice and 8-voice variations. Our voice expansion kits were
developed for those wanting to upgrade a 4-voice or 6-voice model, and for those wanting to add multimode
filter support.
Please note: There are also video installation & calibration guides in addition to this pdf document:
OB-X Custom Parts Installation
OB-X Calibration
OB-X Additional notes on filter calibration
CONTENTS
4-Voice Expansion
(4) OB-X Voice Card Assemblies
(1) OB-X Motherboard Assembly (Expansion)
(1) Motherboard Tray
(1) Filter Control Board
(4) 10-Position Connectors
(1) 5-Position Connector
(1) 4-Position Connector
(1) 4-Position Power Connector
(1) 4-Position DF13-style connector (pre-plugged to voice motherboard)
Mounting hardware for mounting motherboard tray, voice cards, and filter control board
8-Voice Kit
(8) OB-X Voice Card Assemblies
(1) OB-X Motherboard Assembly (Primary)
(1) OB-X Motherboard Assembly (Expansion)
(1) Motherboard Trays
(1) Filter Control Board
(7) 10-Position Connectors
(1) 10-Position Connector w/ long wires for gate & CV I/O
(1) 5-Position Connector
(3) 4-Position Connectors
(1) 4-Position Power Connector
(2) 4-Position DF13-style connector (pre-plugged to voice motherboards)
Mounting hardware for mounting motherboard tray, voice cards, and filter control board

INSTALLATION - BOTTOM MOTHERBOARD
The new bottom assembly for voices 1-4 comes partially-assembled & fully calibrated.
If you are only installing a 4-voice expansion please skip to the section Top Motherboard Installation.
1 Remove the top tray assembly to access the bottom motherboard
a) Remove the (6) connectors between the upper voice tray and mainboard. The connectors can be
discarded as new connectors are included in the kit.
b) Remove the 5-position connector on the right side of the voice motherboard.
c) Remove (3) screws from the bottom of the OB-X that are holding the (3) 1 ½” standoffs to the left
side of the upper voice tray. Alternately, remove the (3) 6-32 nuts from the top side of the unit and
leave the standoffs in place for now.
d) Remove the upper tray assembly.
You should now be staring at the bottom tray assembly:

2 Remove the bottom voice assembly
a) Remove the (7) connectors between the upper voice tray and mainboard. The connectors can be
discarded as new connectors are included in the kit. Also remove the connector to the output jacks.
b) In order to remove the voice motherboard voice cards 2-4 must be removed first. To remove the
voice cards remove the (6) 6-32 screws holding down each card and pull out the cards.
c) After voices 2-4 are removed remove the (15) 6-32 screws holding down the voice motherboard:
d) Remove the voice motherboard assembly. With the voice motherboards removed you see the
bottom of the enclosure:

3 Install the new primary Voice Motherboard assembly
a) Place the new primary motherboard in the bottom of the unit and align the outer holes.
b) Note that voices cards 2-4 have been temporarily installed with (3) screws each. This is because
they need to be removed so that the motherboard can be screwed down. Remove the (3) screws
holding each card down and remove them (it is not important to reinstall the voices in the same slot).
c) Install (15) 6-32 x ¼” screws w/ lock washers to hold down the motherboard. All new hardware is
included –old hardware does not need to be re-used.
d) Once the motherboard is installed replace voice cards 2-4. They are hard to install! Make sure to
place even pressure on all connectors (especially “G”) and press until firmly seated.
e) At this time you don’t need to install the screws & lock washers to hold down the voice cards. Do this
later after the install has been tested.
f) Install the (7) new connectors between the voice motherboard and mainboard.
g) Note: The kit includes a new 10-position connector (connector L/M) with long wires to connect to the
CV & Gate I/O jacks. The original connector can be used if it is ok and you don’t want to solder the
new wires.
h) Reinstall the output connector to the output jacks.
i) Dress the small 12”long 4-position cable to the left of the motherboard. It will plug into the Filter
Control Board that will be installed later.

INSTALLATION - TOP MOTHERBOARD
1 Place the Upper Voice Motherboard assembly
a) The upper board ships completely assembled & calibrated.
b) If the unit has original 1 ½” hinged standoffs installed remove them.
c) Install the (3) 1”standoffs onto the brass hinges that are pre-installed on the motherboard tray:
d) Place the upper motherboard assembly above the primary motherboard and align the standoffs with
the holes in the bottom of the enclosure. Install (3) 6-32 screws and lock washers from the bottom of
the unit to attach the standoffs.
e) Do not install the screws on the right side of the tray yet. The tray swings open and it should be left
as-is until the voices are tested & verified.
2 Install Connectors
a) Install the (6) new connectors between the upper voice motherboard and the mainboard.
b) If upgrading from a 4-voice OB-X there may be a dummy load resistor attached to the +/-19V power
connector that goes to the upper voice motherboard. Simply unplug it from the old connector. It can
be removed but there is no problem just leaving it. Note: the wires have been cut off in the pic below:

c) Note: There is no connection to the 4-position header at the bottom of the upper voice motherboard.
d) Install the 5-position voice sum connector from the bottom voice motherboard to the upper board.
These are the connections at the bottom-right of each voice motherboard.
e) Dress the small 12”long 4-position cable to the left of the motherboard. It will plug into the Filter
Control Board that will be installed next.

INSTALLATION –FILTER CONTROL BOARD
1 Remove screw from CPU Board
Note: If the Encore MIDI Kit is installed it should be first removed to access the CPU board.
2 Swing open CPU Board and remove IC31 & screw shown

3 Install the included male-female standoff where shown
4 Install the Filter Control Board into socket IC31. Secure it to the standoff with a 6-32 screws &
lock washer

5 Connect the 4-position connector(s) from the motherboard(s). Either connector can be used
w/ either cable - they are in parallel

FILTER CONTROL BOARD –OPERATION
The two switches on the filter control board determine whether the NOISE FULL & NOISE HALF buttons
control the noise level or the filter mode.
1 When the left (HP) switch is in the up position the NOISE FULL button will select highpass mode and
noise full no longer works.
2 When the right (BP) switch is in the up position the NOISE HALF button will select bandpass mode
and noise half no longer works.
Please note: The 4-Voice expansions were shipped with the filter control board included. If it installed you
can use it to have different filter modes for the top four voices. The bottom four voices will not have filter
control and you will lose the noise half and/or noise full volume features if the bandpass and/or highpass
modes are enabled.