|DX7 RAM cartridge version 1 (left) and 2 (right). Surprisingly(?) neither works.|
- Edge connector is too narrow. It appears to have a 3+ mm pitch. Mine has a tenth inch (2.54mm) pitch. I fixed it by gluing a separate correctly sized/pitched double sided PCB to the board. At the same time I also fixed...
- Swapped connectors. There was a 50-50 chance as to which side of the edge connector faces which way. My guess was wrong: the side with the power supply faces backwards. To fix that, I had to wire the pins of my glued-on PCP to the corresponding pins on the prototype PCB crosswise back-to front and front-to-back.
- Missing pull-up resistors. For a reason that totally eludes me, I forgot to add pull-up resistors for address lines 8 to 10. I soldered them at the back of the PCP using the power pin on a near resistor network.
- Missing 2.2uF/16 polar bypass capacitor. I know from various synth DIY projects that you're supposed to add bypass capacitors near the power supply of ICs. I didn't add one on the prototype board, because the design on yates.ca doesn't include one either.
In the meantime, you might be interested in the DX7 RAM Cartridge project by blogger Brian Durocher.