What is this connection?

[KLund1]

The 3 RCA connections behind a 520ST say practical solutions A B C. What dose it do. I do not want to kill a 1224 by connecting it wrong.

thanks

[+CyranoJ]

Possibly a Monster Sound of some sort?

[leech]

From some quick poking around the net, it sounds like it's the Practical Solutions Video Key, though that thing is clearly an external device that connects to the video port, though seems one article says it's for Recording, but another says it's for Composite output.

 

I bet if you could show what it's attached to on the inside, we could tell more what it is. But my guess would be it's a composite output device, though usually even if it was input or output, it'd be colored and also the ST wasn't in stereo so it could be adding that in as well (or just having a dummy audio split to fake it.)

[atarian1]

That's for the Practical Solutions Tweety Board upgrade. The three RCA jacks are for each of the three sound channels of the Yamaha sound chip.

 

Video Key was an external box that connected to the monitor port.