RAM Cartridge for the Yamaha DX7 Synthesizer (Part III: Testing)

DX7 RAM cartridge version 1 (left) and 2 (right). Surprisingly(?) neither works.

My DIY project to build my own RAM cartridge for the Yamaha DX7 digital synthesizer is currently in limbo. I have fixed everything that I found to have made wrong with my prototype board, and it still does not work. I had the following problems with my board:

• 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.

Despite all of the above, the board fails consistently with a Write Error! message. I can briefly see the message Under Writing flash by, which led me to the missing address pull-up resistors. That didn't fix it, though. I'm a bit at a loss, because the circuit isn't that complicated, and I can see no reason why it shouldn't work. Perhaps I have simply fried the 28C64 EEPROM? It is a surface mount device, and I'm not the most expert solderer. For me, it means to go back to the drawing board.

In the meantime, you might be interested in the DX7 RAM Cartridge project by blogger Brian Durocher.