© 2013 Benden Sound Technology

Made by Serif

Background

This synth was acquired by a regular client after a long period in storage. On arrival it was found to be quite dead, not powering up at all.

It transpired that part of the mains wiring had been crushed between the rear and side panels of the case, perhaps during the original assembly. Eventually this broke the wire and must only narrowly have avoided shorting mains to the earthed case.

With that resolved, there was still plenty to do:

- Replacement of several faulty ICs.

- Repair of dry solder joints on voice-card motherboard.

- Complete replacement of all electrolytic capacitors.

- Upgrading of the notoriously underrated bridge rectifiers in power supply.

- Replacement of lithium memory backup battery.

- Cleaning of multiway connectors and front panel pots.

- Rebushing of Pratt-Read keyboard mechanism.

- Recalibration according to factory procedure.

- Installation of Encore MIDI kit.

Observations

The OB-X is a big, heavy unit! In common with many of the early polyphonic synths not based entirely around a CEM/SSM chipset, they contain a lot of circuitry and if you are used to handling Polysixs, Roland Junos, etc. then you might be surprised by the bulk of the OB-X.

This particular unit was notable in that it was fitted with a JLCooper filter mod. For this, a small daughterboard (see photo) is added to each Oberheim voice card containing a Curtis CEM3320 configured as a 4-pole LPF and a 4053 CMOS analogue switch. The OB-X's little-used "Half Noise" button is modified to instruct the 4053 to switch between the original Oberheim 2-pole filter and the new 4-pole CEM filter.

There were also extra rear panel sockets added for direct CV/gate control of voice card 1.

The JLC mod required slightly different EPROM firmware, and this was potentially a problem since this would be replaced by the Encore MIDI kit. However, kudos to Encore Electronics who was able to supply a MIDI kit incorporating the JLC modifications.

Rebushing the Pratt-Read keyboard mechanism made a huge difference to the feel of the keyboard, though some care is required in realigning the key heights afterwards.

Sound

Listening to an OB-X for the first time confirmed all that I had read about them. They have a big, muscular, enjoyable sound, in-keeping with their substantial physical size. What they do, they do well, though it would probably be fair to say that other synths offer a broader sonic palette. The JLC mod goes some way to addressing this but, interestingly, both my client and I preferred the sound of the original Oberheim filter above that of the 4-pole JLC/CEM filter.  

Overall view of the Oberheim OB-X. Another four voice cards reside on a second tray underneath the one visible.

One OB-X voice card. Very close to being a voltage-controlled Oberheim SEM on a single board.

Close up of the JLCooper flter daughterboard. This adds a four-pole CEM3320 LPF to each voice card.

Oberheim OB-X CPU board with Encore MIDI kit installed.

Oberheim OB-X power supply after overhaul. Note new electrolytic caps and larger bridge rectifiers.