Magnus Oberheim Page


These are some material I have collected regarding my OB-8. I have also received material on the Xpander and Matrix-1000 from Johan Boberg.

OB-8 Service Manual

I've scanned the OB-8 service manual. Actually, I have two of them, slightly different but curiously enougth carrying the same part number and the same revision. After a quick look the only difference you see is that the format differs, one is A4 and one is A3 style sizes. So, if you assume that one is the copied-down version of the other then? Big misstake! The Table of Contents is adapted to give correct page numbers of that format. Also, other layout aspects of the pages will differ to make them equalent except for the format. Below is the scan of the US. Letter (A4 style) version.

Front sheets

I Front cover
II Inner cover
III Table of Contents


1 Introduction


3 OB-8 Calibration Procedure
4 Calibrating the DAC
5 Setting the bend trimmers
6 Adjusting envelope offsets (please see ECO 427)
8 Oscillator Volts per Octave adjustment
9 Led test, Switch test
10 Oscillator drift test
11 Remaining bend switches, Filter calibration
12 Output VCA volume adjustment


13 Freerunning the OB-8 processor board


14 Connector D, Connector E and Connector G
15 Connector H and Pot Board interconnections


16 1679B Sheet 1/4 OB-8 Processor Board
17 1679B Sheet 2/4 OB-8 Processor Board
18 1679B Sheet 3/4 OB-8 Processor Board
19 1679B Sheet 4/4 OB-8 Processor Board
20 1680B Sheet 1/1 OB-8 Bend Board
21 1681B Sheet 1/1 OB-8 Pot Board 1&2
22 1682B Sheet 1/3 OB-8 Voice Board
23 1682B Sheet 2/3 OB-8 Voice Board
24 1682B Sheet 3/3 OB-8 Voice Board

Parts Layout

25 1689B Part Layout OB-8 Voice Board (in 600 DPI as provided by Tony at Encore and Fred who sent me it)
26 1690B Part Layout OB-8 Processor Board
27 1691B Part Layout OB-8 Pot Board #1
28 1692B Part Layout OB-8 Pot Board #2

OB-8 OS releases

These are the tabulated form of the OS releases that I know of.

OB-8 OS Releases
Rel. NoDateReason for changeRef ECO
A4821222To add the output 3080 (U503) offset calibration procedure to the test software.410
A6830411To add separate calibration storage and retreival from tape and clearing calibration.425
A7830509Implement two new tests and improve offset cal.427
A8830712Add new features.434
B3840131Add OB-8 MIDI interface to OB-8.436
B5840815Adds MIDI features.439

Many thanks goes to BJ for tracking down the B5 EPROMs and doing the sweettalking to a local dealer in order to allow me to copy them. Thanks to these efforts I am now able to bring it onto the web.

OB-8 Factory Patches

Factory patches cortesy of IOtech and is available at IOtech and here.

OB-8 Engineering Change Orders

These are the tabulated ECOs that I have. I do not know of any other ECOs that relates to the OB-8. Naturally, if you do know of any such thing, I'd love a paper copy.

OB-8 Engineering Change Orders
ECO No.DateReason for changeDrawings affectedRef ECO
402821209To eliminate noise when controling the volume with an external low impedance voltage source.1679A Sheet 2/4
403821207To match schematic to board layout.1682A Sheet 2/3
404821210Increase the active range of the front panel pots.1681A Sheet 1/1
405821212Improve temperature stability and tracking of VCO2 filter env. modulation.1682A Sheet 2/3 & 3/3See 409
406821212The offset calibration routine of the service center test can correct the TL084 for positive or negative offset.1682A Sheet 2/3
407821215To reduce power-down glitch at outputs.1679A Sheet 2/4
408821222On revision A processor boards, when a sequencer is connected, distorsion becomes present in the noise generator.1679A Sheet 3/4
409821222To keep Osc 2 from going sharp when the sawtooth wave is selected.1682A Sheet 3/3Mod. 405
410821222To add the output 3080 (U503) offset calibration procedure to the test software.(A4)1682A Sheet 2/3Req. 405, 406 & 409
411830106Removes noise modulation from bender lever.1680A Sheet 1/3
412830118When the new power supply transformer (REF.561021) is used, this change decreases the output thump on the power off.1679A Sheet 4/4
413830118Remove leak of noise generator when a voice is gated.1682A Sheet 2/3
414830120Resistor at UX06 pins 10, 18 marked RX68 should be RX61. Capacitor in 2-pole/4-pole configuration drawing marked C20 should be C14.1682A Sheet 3/3
415830126To eliminate noise at output at low master volume levels.1682A Sheet 1/3
416830203To filter out digital noise picked up along the filter pedal line (VCFP) when a sequencer (DSX) is connected.1682B Sheet 3/3
417830211Rationalize the circuitery.1682B Sheet 3/3
418830211Bring the tracking trimmer in the middle.1682B Sheet 3/3
419830211Improves synchro sound.1682A Sheet 3/3See 431
420830211Remove cuts and jumpers.1682B Sheet 3/3Impl. 413, 417, 418, 419
421830222To center OSC 2 detune control when LED goes off with knob significantly counter clockwise of center..1681B Sheet 1/1
422830310Improves matching of timing between envelopes of upper and lower tray.1682B Sheet 2/3See 433
423830328Improve margin of "PUP" circuit to inadvertent turn-off, especially at 50 Hz.1679B Sheet 1/4 and 4/4See 429
424830411To avoid possible memory drop caused by short power drop.1679B Sheet 1/4
425830411To add separate calibration storage and retreival from tape and clearing calibration.(A6)
426830411To remove AC hum on audio output.
427830509Implement two new tests and improve offset cal.(A7)1682A Sheet 3/3See 433
428830523Tighten tolerence on envelope speed capacitor.1682A Sheet 3/3
429830621Avoid memory drop.1679B Sheet 1/4Mod. 423, Ref. 424
430830407To correct OB-8 voice board schematics.1682A & 1682B Sheet 2/3
431830627Allows use of signetics 4016 & identifies components.Mod. 419
432830627Avoid missing pulse on arpegiator external clock.1679B Sheet 2/4
433830705The new envelope calibration software starting with A7 (see ECO 427) does not require those resistors to be matched.1682B Sheet 2/3Mod. 422, Ref 427
434830712Add new features.(A8)1682A Sheet 3/3Req. 405, 406, 409 and 427
435831017Remove noise on "VCFP" (VCF PEDAL) line. [Contributed by Kevin Lightner]1682B Sheet 1/3
436 12345840131Add OB-8 MIDI interface to OB-8.(B3)
437840615To stop loss of offset in memory on power down. (like DSX)1679A Sheet 4/4
439 12345678840815Adds MIDI features. (B5)
440840907Remove noise modulation from vibrato lever and correct schematic.1681B Sheet 1/1
441841004Lower initial frequency of 3340 to accommodate a wider range of characteristic variations on the 3340.1682B Sheet 3/3

Tips and Issues

Filter Trimming
The OB-8 service manual has a flaw in its description of the filter trimming. The filter has a static downshift of 10 octaves and these are being scaled by the scale trimmer T106, now if you try to set the scale factor one has a large constant value which confuses you and you may turn out to tune yourself much worse then when you started. The trick is to start by playing the highest possible note first. You do this by setting your keyboard one octave up (press Transpose Up) and play the highest C. Take notice of how much "off" the tone is (numerous methods for this exist). Then play an C one or more octaves down and adjust the scale by trimming T106. Once the scale seems nice its time to trim the initial frequency by trimming T105 till you hit a reference note. There are naturally other detailed methods to fix this, but the common aspect is that one has to be aware that for higher notes the offset is better canceled and thus will scale trimming work by stepping down than stepping up, which is the usual assumption.


main  |  inside  |  patches
I used to have an Oberheim Xpander. It was a fantastic synth but i sold it to finance my Buchla. I will keep these documents up here though. Matrix 1000 users: check out the patch documentation at the bottom of this page.

This is a transcript of a document that came with my Xpander that i haven't previously seen on the Internet, so here it is:
- Xpander MIDI controller notes

If you need to update the software in the Xpander you'll need these files (and a EPROM burner):
(Main 1.2, Voice 1.4 )
- Xpander ROM images

Here is a PDF describing the 0ddernmart audio in modification on the Xpander.
I havn't tried this myself. If you do, please let me know the results ;-)
- Oddernmart Audio Input Xpansion

The early Oberheim Xpanders were manufactured in the US, the later ones in Japan. Here is a bunch of photos from the inside of two different Xpanders:
- Xpander inside
These photos can be helpful if you need to service your Xpander.

Even more helpful perhaps:
- Xpander Users manual
- Xpander Service manual

Most of the components used in the Xpander are easy to find and replace. But there are some that isn't. The rotary encoders are pretty rare, i've seen them on eBay but not very often. If they start jumping numbers twist them around for a minute and they will most often behave well again. The displays are very rare but no one seem to have any problems with them either so no need to worry. The VCO chip (CEM3374) is also rare, but it does not seem to fail often. The most common problem is the filter. If the filter fails the voice sometimes die. Replace the chip (CEM3372) and usually it starts to work again. (Hint: try swapping filterchips between voices to see if this was the problem) The filter chip is luckily still easy to find. Another common problem these days is failing power supplies. This is a major problem because it can destroy other parts of the synth. Make sure to replace the large electrolytic capacitors in the PSU every 15 years or so (uh-oh, the xpander was made 1983). The battery should be replaced too because it can start to leak. The acid from the battery can do seriuos harm to your Xpander.

Lots of patches to download here:
- Xpander patches

HI MOM!, I found the little easter-egg "Page 3" on my Xpander:

Matrix 1000:
All the names of the original Matrix-1000 patches in a convenient format:
- Microsoft Excel version
- HTML version to view online
- HTML version in a zip archive for download.


Jeff Bergman's OB-Xa webpage

Ersatz Planet Oberheim Databank
Good place for SEMs and SEM-base X-voices machines. Do visit!

SynthZone's Oberheim department

Oberheim Heaven
Site with OB-8, Xpander, Matrix-6 and Matrix-1000 material.


IOtech's OB-8 Factory Patch Tape Dump
There's a WAV file in that ZIP file.

OB-X Service Manual

SEM, Two Voice, Four Voice, Eight Voice and misc advertisement material.

Magnus Danielson <cfmd at bredband dot net>