This is a simple two-SID-chip plug-in board for most Commodore 64's and 128's, including the plastic-case C128-D.

The second SID can be placed in any I/O page the machine supports, and it can be moved off of the base of the I/O page you put it in and placed at any other $xx20/40/60/... section therein, if desired. You can also put it in the $D400 page just above the primary SID.

This board has provision for the "background noise silencing" hack also, to reduce the digital bus noise the 6581 tends to pick up from its external input. This feature is optional, and is perhaps best used with a DPST/DPDT switch.

For more details on wiring/installing, see the README, attached below (it's also in the git repository).

Note: you may need to trim the pin header that goes under one of the sockets, depending on how long the top ends of the pins are and how tall the affected socket sits above the board.

If you want the C128-DCR version, you can find it here:

https://www.pcbway.com/project/shareproject/C128_DCR_Internal_Stereo_SID_f1f15587.html

Part substitutions are fine as long as the bottom-facing pin headers are the round machined kind (there's also a fairly rare kind that have thin, flat pins -- those are fine too). I strongly favor Mill-Max machined-pin sockets for the SIDs, but plain old "double-wipe" style will do also.

This project is a by-hand full redraw of a working design I originally made with gEDA, but now re-done with KiCAD. It has been built and tested, and seems to work as-intended.