IC1 is an 74ACT108

[Ralph Barrett]

In the interests of science, I just removed the lid from my RiscPC, and
untangled my home-brew SPDIF interface, to reveal the sound circuitry.

And there was a 74ACT108 staring back at me :-)

So it looks like IC1 is indeed a 74ACT108 J-K type flippety flop.

FWIW R20 should be around 22 ohms.

So apart from the inductor value (L1), and also where to purchase all of
the bits, that's about it for the IBX1xx sound circuit.

I would expect raw sound to work without L1 btw. However, for 'best
quality' you really need the DAC to have a clean 5v rail.

Ralph