Oberheim Xpander – Episode 1
- At April 16, 2021
- By amsynths
- In Synthesizer
8
Overview In 2020 I researched the original OB-Xpander analog synth and posted my story about the prototype here. I concluded that whilst it might be possible to recreate it from OB-8 PCB’s and modified OS software, the effort was not worthwhile for a 4-voice synth. However it might be worth recreating the production Oberheim Xpander with its 6-voices but the time needed to design the five PCB’s meant it was never going to be viable for me, although people have recreated the Jupiter 8 and Memory Moog from scratch.
Then it all changed! In Spring 2021 I noticed that Syntaur had NOS Oberheim Xpander PCB’s for very reasonable prices; both the Voice and Processor but no Pot PCB. They also had the Power and Display boards but these are not needed as I will use a modern PSU and new VFD replacement displays. Before buying the PCB’s I carefully researched the availability of components, especially rare DAC’s and old ROM, RAM, and concluded everything was easily available except the 14-bit DAC. Even the numbered buttons could be bought from Mouser (if 16mm square), and the CEM3374 and 3372 are available at just £20 each.
I priced up a full BOM, which came to a total of around £2000. This is an expensive project but cheaper than a second hand one at £4000, and I would get a brand new PSU and set of displays, so my new build would be future proofed. The downside is the resale value is probably low but then I don’t plan to sell it!
The Elephant This all sounds promising but the “elephant in the room” is the panel. Whilst I may be able to get a metal case manufactured, and I can buy new wood sides, I sure can’t make either a new Mylar film or a silk screened panel. The completion of the project is reliant on someone else doing this, as replacements for existing Xpander panels.
As at Spring 2021 there are two possible contender projects which seem active, although it could be a long wait for these projects to be in production! However it will probably take me 6 months to build the Processor and Voice boards, so I am hoping the timing works out! I will build a plywood base to mount the Voice and PSU and work out a temporary panel to hold the Processor and Pot Boards.
A smaller “elephant in the room” is that I don’t have a Pot PCB, and I will have to make one. This is tricky to get right in terms of dimensions before I have a proper and accurate front panel, so I will probably use a set of smaller prototype PCB to prove the new Xpander works and then get a full sized one made when I have a panel. A one off PCB this size is around £300, so its a major expense and something I have to get right.
Progress in April The Processor and Voice card PCB’s have been ordered from Syntaur and the CEM3372 and CEM3374’s acquired. I have started to identify the type of capacitors needed in the Voice board using detailed photos of the Xpander and Matrix 12 PCB’s. During May I hope to acquire the new VFD displays and some of the hard to find components (DAC’s, RAM, ROM’s). In terms of critical path I can build the Processor Board first but really need a Pot board working to see if the CPU boots up and loads the software. I also need a power supply!
Xpander Mockup Panel was created by Sunshine Jones and its his copyright – More in Episode 2!
Matthias
Hi,
I was searching about xpander mods and found your project about it. i‘ not understand much about electonic. My mod wish for the xpander would be to change the cv gate inputs to cv gate outputs. Do u think this would be a possible mod? The reason i like to change it is i would love to use the xpanders vco‘s to create a 6 voice poly synth with modular eg‘s, vcf‘s and vca‘s.
Best regards matthias
amsynths
Hi
Thanks for the comment. The VCO outputs can be easily wired into the CV outs, replacing the CV Ins. The gate signal would be tricky as there is no keyboard, maybe send the envelope outputs. It would be a shame to cut traces on the PCB’s and maybe better just to wire the VCO and EG outputs outs via a new set of x12 3.5mm jack sockets set into one of the wooden ends. Best Regards Rob
Keith
If I can lend you a Pot board Pcb to reverse enigineer, would you build me one for a parts unit that I have lying around???
A friend of mine would maybe be willing to lend out his pot board pcb if you are willing to clone it.
I had the EXACT same idea when I saw Syntaurs stocks of NOS boards…. but no por board pcb is the major roadblock.
Get back to me via email
amsynths
Hi,
Fantastic news! Yes I can clone the POT PCB and provide them for free. I have sent you an email, so we can sort out the details. I will use the VFD replacement and design a new power supply.
Best Regards
Rob
Keith
Ps- here is a VFD display board reproduction that makes finding those old displays no longer a headache- http://www.lenham.clara.net/sound/obvfd.html
Keith
Idk if my post sent but I may be able to convince my friend to let me lend out his pot board pcb so you could reverse engineer it.
Email me- keithmays93 (@) gmail dot com
teak
Amazing Rob,
I love this project. I was thinking the same when I saw those boards on Syntaur. I will be looking forward to future updates. Where did you source your cem3372s?
Thanks
amsynths
Hi,
I found the 3372 and 3374 on eBay. I have asked Alfa Rpar if they will remake the 3374 but its a no. They have remade the 3372. Rob