Years ago, when I actually worked in an arcade, I got to know the guy who fixed the machines. He was a nice guy and suffered my teenage geek pestering well. I eventually got up the nerve to ask him for the schematics for my favorite game, Asteroids. I thought for sure he'd flip out on me, telling me it was top secret corporate information and that Atari would throw him in jail if he released it. Instead, he reached into an Asteroids cabinet, grabbed the documents pouch, and said, "Sure, here you go."
I pored over those prints at home that night. Most of it I didn't understand at the time, and I knew there was no way I was going to be able to buy the parts needed to build my own machine from scratch. But one area of the schematic interested me - the audio section. In it I saw parts that I recognized, stuff that I had used in my lame little experiments or had seen in my Forrest Mims books. It turns out the sounds are completely generated in hardware - no samples, no synthesizers, just cheapo 555s and the like. That I figured I could build, so I filed away the schematics to await the day that I could afford to buy the parts.
Forty years later, those schematics are long gone. But they live on in the Internet, and I found them again the other day. I'm starting to capture the schematics for the audio section in KiCad, mainly so I can design PCBs for this when it gets a little further along. I'll breadboard it all first, of course, but I figure getting the schematic off paper is probably a good first step.
I've also started tracking down some of the obsolete and harder-to-find parts, like the TDA2002 amplifiers for the finals and the LM566 VCO chip. Those were both eBay purchases and are on the slow boat from China. The rest of the BOM - 555s, 74LSxx, 40xx, etc. - should be pretty easily sourced from Digi-Key or Mouser. I've only made one substitutions so far - the original calls for a 9316 synchronous 4-bit counter, but the 74LS161 is supposed to be a pin-for-pin replacement, so I went with that rather than try to hunt down something that might not exist. Plus KiCad already had a symbol for the modern device.
I'm shooting for all through-hole here, just for the old school look. I probably will include some kind of microcontroller, though, since the original sound hardware was controlled by IO from the 6502A microprocessor. Things like the rate of the thumping or the different tones of the explosions are controlled by setting four or five bits, and rather than try to do that with dip switches or some kind of encoders, I can have an Arduino twiddle those bits programatically to demo the sounds. Might be fun to have it "play" a random game with just the audio.
As for the audio amp, I could just use something off the shelf, but I'm trying for authenticity here. I want this to sound like I remember, so I'll make sure I use the original audio components. The power supply gives me pause, though - I might have to make a concession here and go with SMPS modules to get the 12V, 10.3V, and 5V rails that I need. Not really worried about 5 or 12, but that 10.3 supply is unregulated and powers the amp. It comes off a center-tapped secondary on a big multi-tapped power transformer, and just has a half-wave rectifier and a big electrolytic filter. The ripple that's coming across might be part of the charm of the audio stage, so I might have to build that power supply stage too.