APC, aka Step Tone Generator. I was fiddling around with one I made today. Put together a really crappy housing, but when I add to the circuit I'll make it more sturdy and appealing. Hope yah enjoy the square wave bleeps and boops. One day I'll be making analog chiptunes. Notice the incorrectly placed holes and bendy sheet metal.
My WTB thread (Sega CD/Saturn games)
Also looking to buy: Ys III (TG-16 CD), Shadowrun (Genesis) Hori N64 mini pad and Slayer (3DO) in long box/just the long box
Ctrix(Gatari) does some amazing software stuff. He's programs into actual carts.
Right now it's just a noise maker. I'll be adding other circuits to make it more musical. Adding a "Baby 10 sequencer" soon that I can control with a 555 timer or some CV. It's another pretty normal mod that people do, but it's good for getting used to audio circuits and how they interact.