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Using M-64C cartridge is the only way to load/dump tones and patches on a JX-10 with factory firmware. M-64C cartridges are pretty hard to find these days and they go for obnoxious prices on eBay. At least considering the 8K of static RAM inside a small plastic box. So I acquired an M-16C which can generally be found for half the price and opened the box with the goal of converting the M-16C into an M-64C. If anyone would like to follow the same procedure here are the steps.
This is what the original M-16C circuit board looks like:
Besides the TC5517 (which is a vanilla 8 x 2K static RAM chip) the board does not contain much more. The address lines 11 and 12 required to access full 8K address space reserved for memory cartridge are not connected.
The next step is to desolder the 2K static RAM to make room for a bigger (8K) chip leaving the board even less impressive:
The main problem with substituting an 8K static RAM for a 2K one is that the larger chip has 28 pins while the original one only has 24. Consequently there are 4 holes missing on the board, therefore legs 1, 2, 27 and 28 should be bent up at 90 degrees and the narrow part cut off. Standard 2K and 8K static RAMs are almost pin-compatible so the substitution is almost straightforward - except for pin 21 (R/W) on the 2K chip. On a 8K chip A11 has taken this position so pin 23 on the 8K chip has to be bent up and trimmed as well.
TC5517APL has 250nS access time so I guess any contemporary standard 8K x 8 chip qualifies as a substitute (mine happens to be HM6264LP-70). The only suggestion here is to pick one in plastic package (the reason for this will become clear below).
Now some additional wiring has to be added to take care of the new pins (from now on the pin numbers refer to the new 8K RAM chip):
- Pin 1 is not connected to anything
- Pin 2 (A12) is wired to the edge connector on the bottom side, 2nd pin from left (the wire crosses the side through battery hole)
- Pin 23 (A11) is wired to edge connector on the component side, 2nd pin from right
- Pin 26 (CE2) is wired to Pin 28 (Vdd) with a short bridge
- in 27 (R/W) is wired to the hole beneath Pin 23 (where Pin 21 of the original chip used to be). This wire was actually soldered in place BEFORE the RAM chip itself and is located under the chip but it could be put on the bottom side if this seems easier.
The new address lines (A11 and A12) lack pull-up resistors. Fortunately there are some spare ones in the resistor arrays so two more wires are added. They connect two lowest (closer to the edge connector) legs of the centermost resistor array on the bottom side with edge connector pins (2nd one from left on the bottom side and 2nd one from right on component side, respectively). Sorry for the green wires - they are a bit hard to follow on the pictures, but look pretty in real world :)
That's it. What's left is to squeeze the modified board back into the plastic cartridge. This is a tight fit as the new RAM chip is larger. I even had to cut off some innards of the cartridge and to file some plastic off the edge of the RAM chip. This was the most complex part of this mod, no kidding, so if you don't feel comfortable with knife, file and pliers consider getting a factory M-64C instead :) Any comments and suggestions are welcome.