Monday, 3 November 2014

RAM Cartridge for the Yamaha DX7 Synthesizer (Part IV: Reboot)

This is part four of my report on building a RAM cartridge for the Yamaha DX7 digital synthesizer. It took a while since the last installment, because I've got several hard- and software projects that run in parallel. One is a hardware programmer for the DX7, a bit like this Jellinghaus DX programmer, but with far fewer knobs, but a graphical display instead. Another is a user interface (including graphical display) for a Yamaha XG synthesizer based on a DB50XG daughter board, and the third is a software librarian and editor for the DX7.

Anyway, I completely redesigned the RAM cartridge, this time using a correctly sized edge connector, correct pin assignment, and a through-hole EEPROM.


Furthermore, I tried to stay within the confines of the original cartridge board area. The packing is dense but not too crowded. Soldering the components was quick and painless. The board is suitable for both 64kbits (suitable for 2 DX7 voice banks) and 256kbits (8 banks) EEPROMs. To test the board I use the simpler 64kbit device and a an ordinary sliding switch in the position of the rotary coding switch (top left corner of the board), and only one additional resistor. I didn't include a pad for the 2.2uF polar capacitor, but managed to mount it between 5V and ground by soldering it to the power leg of the EEPROM and the bottom pad of R1 to the left of the memory chip. The finished board looked quite neat and tidy, a far cry from the mess of the previous prototypes. I couldn't wait to test it in my DX7.

Well, make that last one "I couldn't wait to test it in my DX7". It seems the board is too thick. I could possibly force it into the connector, but I don't want to ruin my DX7. The boards do have a solid if not heavy feel. Nonetheless, OSH Park's pricing and spec page states that the boards are 1.6mm thick. They're closer to 1.8mm. When I tested my previous prototypes I thought that even my 1.6mm thick hand crafted adapter was close to the limit regarding the force required to set the board into the socket, but these are just too thick.

This is very unfortunate, because I like the quality, price and ease of use of OSH Park, but for this project I can't use their service. I need to find a different prototyping service that allows specifying thinner boards, perhaps 1.4mm or so.


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