RAM Cartridge for the Yamaha DX7 Synthesizer (Part I: Introduction)

When I was a teenager I spent hours on end in the keyboard section of music instrument shops and played those keyboards and synthesizers that I knew from the back of album covers, where musicians used to list the equipment they used. Of course, the famous flagship synthesizers like the Roland Jupiter-8, the Oberheim OB-8 and the likes were completely out of my price range. Finally, a friend of mine who owned a small monophonic synthesizer himself told me that there's a new synthesizer that's perfect for me: the Yamaha DX7. A few weeks later my mom drove me across the border into Germany where I bought a DX7 for under CHF 3000. Despite being over 30 years old, it still works perfectly, and I still play it frequently.

The Yamaha DX7 was the dominant synthesizer of the 1980s with its idiosyncratic sounds such as the electronic piano, bass, and marimba voices. If you happen to see a music video of that era and watch closely what keyboards the guys with the strange hairdos mime playing on, 9 out of 10 times it will be a DX7.

The DX7 has internal memory for 32 voices, which even for the 80s was relatively little. It has an expansion slot, however, where cartridges can be plugged in to provide direct access to further 32 voices. The DX7 shipped with two ROM cartridges, each containing 2 preset voice banks (2 x 32 voices) selectable by a switch on the cartridge. At the time, Yamaha also sold RAM cartridges onto which single voices or the full contents of the internal memory could be stored, sort of an external RAM extension. Unfortunately, the cartridges are very hard to find these days, and even the used ones sold on eBay are very expensive.

The publicly available circuit diagrams [PDF] of the DX7, e.g. on Dave Benson's DX7 page, reveal that the cartridge design is very simple. It is really only an external memory chip, and the pins on the cartridge slot are just the address, data and signal buses to control the reading and/or writing of data from or to an EEPROM.

The simplicity of the circuit suggests that it should be possible to build oneself such a memory cartridge. EEPROMS with 64kbits (8k x 8), that's room for 2 voice banks, can be found for less than USD 5. The other parts (resistors, switches and an AND gate for the combined chip enable lines) cost perhaps a dollar or two. The PCB is another story, but there are prototype PCB manufacturers like OSH Park that do them for USD 10 apiece or so. Don't even ask about the enclosure.

As you can guess, I'm going to try to design and build such a cartridge. In the following weeks I will be reporting on the progress of this project. Peek preview: The prototype boards have already been sent away for manufacture.

Update [19.08.2014 08:42 CEST]: I just got a notice from OSH Park that the PCBs have been shipped!

So stay tuned!