Cloning the TR-909 Cymbals
If you take a look at the schematics for the TR-909 cymbals you'll see the samples are stored on HN61256P devices.
The 61256s are mask programmed ROMs.
You can use standard 27256 EPROMs in place of them in a clone, bearing in mind a couple of necessary changes.
Roland, for reasons known only to themselves, chose to invert the sense of pin 22, Chip Enable, for the Hi-Hat samples.
On the Hi-Hat ROM pin 22 is active HIGH.
On the Crash and Ride roms, the pin is active LOW (as usual).
The way round this is to use the NOT Q output on the 4013 that drives the Hi Hat CE line.
You'll see that the NOT Q output is used for CE on the other 2 ROMs.
Also, pins 20 & 26 need to be swapped. On the Roland ROMs, pin 20 is a Chip Select line, and pin 26 an address line.
The address and data lines on the Roland ROMs are also jumbled up, presumably to simplify PCB routing.
Or maybe to make it more difficult to rip off the samples ?...
I didn't bother working out the counter bit to address pin mapping.
Instead I just extracted the data byte by byte and used the nominal address lines for my clone.
I only had to work out which bits were which on the data lines.
I was planning to make proper PCBs for the 909 cymbals, but Trevor Page saved me the effort.
You'll find his siteon my links page.
Last update: 19th April 2003