The mini cartridges I did a few years ago for the TOM turned out to be an unexpected hit, given how few TOMs were made. They were convenient but designed in somewhat of a hurry, each only holding a single 32kB ROM with 7 sounds. I was never really satisfied with the bare circuit board either.
I realised that it wouldn’t be much more expensive to roll several cartridges into a single multi-cartridge with a bank selection system, and also that the physical dimensions of the cartridge cutout on the TOM are almost exactly the same as on the Commodore 64, which makes sense as Sequential themselves had produced several cartridges for the C64. I presume they used the same plastic shells for their TOM cartridges as they had used for the Model 242 MIDI Interface cartridge. As the C64 still enjoys a large homebrew scene, there are several vendors of aftermarket plastic cartridge shells, and it was possible to adapt one of these for this project.
The multi-cartridge holds 8 banks, which are cycled through by pressing the small button in the corner. A microcontroller reads the button input; cycles through the banks, also showing the current selection in binary form on 3 LEDs; and after advancing a bank, tells the drum machine to re-read the header data so it knows what sounds there are and how long they are.
Note to Tauntek TOM16 users – I’ve been informed that a small modification is required to the V1.1 board for the cartridge to work fully (otherwise you may get clicks at the ends of sounds). Details can be found here. V1.2 already has the modification.