The “Ultimate” SX-64 project part 1: audio and video

I’ve had this SX-64 in my closet for years. It’s the victim of a poor repair attempt that left the CPU board unusable. I considered parting it out, but I’ve really been hoping to resurrect it one way or another. The Ultimate 64 gave me a way to bring this machine back to life and give it a major upgrade in the process.

My first step was to get video & audio working so I could see that everything else worked. According to the schematics, the SX-64 uses separated luma & chroma just like the regular 64. A socket on the CPU board carries those signals from the VIC-II chip to the SX’s built in 5″ monitor. The same connector carries audio from the SID chip to the SX’s speaker. Using header pins, a spare RCA audio/video cable, and a bit of solder, heat shrink tubes, and electrical tape, I was able to make an internal A/V connector.

The white connector normally goes to the SX-64’s CPU board. Here, I’ve used header pins to attach RCA plugs for luma, chroma, and audio.

After plugging the Ultimate 64 into this setup, I had a beautifully functioning computer again. I gave it a good workout with my son’s favorite demo and I’m excited to report the screen and audio are beautiful.

The Ultimate 64 menu on the SX-64 screen.

My next step will be to get the Ultimate 64’s “multibutton”, which acts as a power, reset, and feature switch, wired up to the “reset” button on the front panel. Initial experiments proved each of the three signals going across the multibutton need to be switched separately. So I’ve got some relays on the way. I’ll post an update when that’s ready to go!