Oberheim DPX-1 Refurb 1

My DPX-1’s in 1998

Overview  I bought an Oberheim DPX-1 in July 2020 with a failed power supply, which is very typical as the original contains a weak spot – the Rifa X2 capacitor across the mains inlet power which burns out rather spectacularly. I originally had two of these sample players back in the late 1990’s along with a HDX-20 hard disk, which I never got to work (the hard disk has to be formatted at the Oberheim factory). I used it to play Emulator II samples when I ran the Emulator Archive, its a nice sound but not an exact replica.

Fast forward 20 years and the DPX-1 remains a very good way to replay Emulator II samples (and S900, Mirage, Prophet 2000), but with the added flexibility of a Gotek floppy emulator and 1000;s of sample banks. Hopefully this will be a quick repair and I can get this item into the studio.

Technology The Oberheim DPX-1 is an 8 voice polyphonic digital sample player, released in December 1986. It uses a Motorola 68000 microprocessor to reproduce 8 and 12 bit digital samples. The DPX-1 is entirely controlled by computer software which is loaded from internal EPROM at boot time (OS 2.2 is the latest version). The DPX-1 uses quite a large collection of TTL logic in addition to the SSM2045 VCF/VCA on the analog outputs. The analog channels also use fast settling JFET Op Amps, the LF400c which is now obsolete but a possible modern replacement is the OP42.

There are two neat and well laid out circuit boards, sitting on top of each other. The lower board has eight voice channels, and the upper board has the memory chips and microprocessor with 768 MB of sample memory (in 24 chips!). The DPX-1 can boot up without the analog board plugged in.

Refurb Plan I am hoping for a fairly simple refurbishment; a new power supply, a recap the analog and digital boards, and fitting a 3.5″ Gotek drive with access to lots of sample disks. However it did not go well, and I am documenting my long journey to repairing the DPX-1 to help other owners.

Power Supply First step is to replace the old PSU with a modern Meanwell RT65B or PT65B. Checking the old power supply it looks like all the electrolytics and the Rifa X2 capacitors need replacing, which adds up to £20. There are no obvious burn outs and the fuse is missing, but it does look well used and may need more parts replacing, so I decided to replace it.

The open frame PT65B has the right PCB width and length to slot into the old housing, but its a bit too high and the heat sink will touch the chassis and need isolating. The RT65B is closed frame and wider at 98 mm but will fit in the DPX-1 casing, and at £20 its a better investment than repairing the old one.

I have retained part of the original PSU housing for the power inlet socket and switch (which I upgraded to DPDT). The RT65B attaches to two new holes in the DPX-1 chassis with M3 screws.

The original Oberheim low voltage wiring is incorrectly colour coded. The floppy drives use the standard 4-pin Molex connectors and colour codes. Oberheim then added a brown wire for -12V into the analog board and decided to use a yellow wire as +5V into the digital board! I checked this before powering on, and moved the yellow wire to +5V on the RT65B. I will replace it with a new red wire during the refurb.

Gotek Drive Options The DPX-1 has two drives that could be replaced with a Gotek floppy diskette emulator. The lower 5.25″ drive can only read EII diskettes, whilst the upper 3.5″ drive can read S900, Mirage, Prophet 2000 and the DPX format backup disks. I have chosen to replace the 5.25″ drive as I have 1000’s of EII samples in .EII format, which I can load onto the USB stick, and that leaves the 3.5″ drive for using with the occasional S900 floppy disk.

To install the Gotek we need some parts:

• 5.25″ to 3.5″ connection adapter
• 5.25″ to 3.5″ plastic casing, buy a good one
• 5.25″ to 3.5″ power cable adapter

Once all these parts arrived I fitted them into a case with an OLED pre-fitted, as it has the wider display hole machined out of the case. I then flashed the bar Gotek drive with HXC firmware and configured the drive for the 5.25″ Emulator II rather than the 3.5″ DPX-1. I also added a rotary encoder and a spare Boss knob.

OS Versions Oberheim actively developed the DPX-1 and released OS upgrades via ROM chips. This development was clearly well planned as the digital board has jumpers that set the ROM size . Version 2.x uses larger 512kb ROM chips and needs the jumper setting changed from Version 1.x.

• 1.3 and 1. are bug releases
• 1.5 Added diskette backup to the internal 3.5″ DS/DD drive, using a proprietary DPX1 format. MIDI echo on MIDI out
• 2.0 Filter cutoff variable from 00 – 99
• 2.1 Support for the Akai S900
• 2.2 Support for the HDX-20 and CDS3 CDROM and individual voices

There also quite a few ECO changes to improve the quality of the sound (documented in the service manual) and the DPX-1 needs specific brands of chips in some locations.

Oberheim 3.5″ Diskette

Oberheim Factory Library Oberheim made a factory library of sample diskettes, some in 3.5″ and some in Emulator II 5.25″. Here is a list of the 5.25″ Emulator II diskettes:

• Woodwinds 1,2, Horns 1,2,3, Plucked 1
• Sound FX 1,2,3, Strings 1,2, Piano 1,2
• Drums 1, Woodwinds 1, Vox 1, Cymbals 1

CDROM Upgrade With OS 2.2 came the support for the uber rare RS422 CDROM player, eight individual voices and SCSI for the HDX-20. Oberheim released an upgrade kit with the hardware and new chips. Unfortunately Oberheim did not put a hard disk format capability into the OS (I guess there wasn’t sufficient space), so only Oberheim HDX-20 hard disk units can be used and the drive was formatted at the factory.

USB Sample  Transfer OMI produced just a RS422 interface card which attached to the analog board, so that there CXROM could be used. More recently a RS422 to USB interface has been developed, the Emuser. It looks possible to combine these two interfaces, and attach the Teensey microprocessor of the Emuser straight into the serial interface chip at U106, bypassing the RS422 TX and RX chips.

A simple daughter board on the analog PCB using the same J4 connector as the OMI board, would provide USB sample transfer into/from the free EMXP software running on a Windows PC. The advantage over the Gotek drive is that the computer can be used to transfer sample banks to the DPX-1, without have to use the front panel.

Outcome The new power supply worked perfectly and the DPX-1 got to Load Disk sage with Ld on the display, however it could not find the diskette drives, possibly because they are broken. Any subsequent power on did not make it this far, no display at all. I tried various drives but no luck. An “Episode 2” post will detail the next steps in getting the DPX-1 back to life!