I think the 707 has some definite things going for it: great interface, lightweight, individual outs, onboard sound mixer pots, solid trigger outs. However, the things I always thought were odd were that it wasn't battery powered (such a lightweight chassis...why not?), and the sounds were good but not great (probably better than the 505 and 626, but the stepsister to the 808, 909, and possibly 606).

So this mod tries to fix both of those:

The first part is battery-power. I have an internal array of 10 AA cells that can be switched in and they'll power the unit for at least 10 hours. I use rechargeable NiMH cells for environmental friendliness. This enables sitting on the couch jamming with headphones and no wall wart leash!

The second part is a ZIF eprom socket mounted on the front panel where the M64-C memory cartridge used to go (something I know that I would never ever have used). This is wired into IC34 and IC35 on the voice board, which provide the ROM samples for all the drums except the crash and ride. Since IC34 and IC35 were 27c256 sized ROMs in parallel, they can be replaced with a single 27c512 64k ROM, which I've done here. No credit to me: Thierry on the Yahoo TR-707 group figured this out and posted his methods there for adding new sounds. To burn my own sounds, I got the WAVs from Hyperreal for the 808 and 909, downsampled them to ~22kHz and 8-bit, converted them to binaries, then adapted Thierry's BASIC program that organizes all the sounds into on 64K file per their stock organization in the TR-707.

This video shows me running from battery power (the only thing I plug in is the amp). I run through the exact same patterns using sounds first from a 27c512 EPROM with the stock 707 binaries, then my converted 808 EPROM kit, then finally a 909 EPROM kit (I didn't like how that last one turned out...anybody know of better sources for 909 sounds?).

I suppose doing the crash and ride eproms are next! (less info)