Modding a Commodore 64 or a Commodore 128DCR for extra sound chips - Page 1
Forum section: electronics / general

author: uneksija
author_rank: Newbie
timestamp: June 19, 2025, 06:23:27 pm
content: Hi, I'm thinking of modding a Commodore 64 or a Commodore 128DCR for extra sound chips. By default, these computers have a single SID 6581 or 8580 ("Sound Interface Device") installed, providing 3 channels of wonderful beepy sounds. There is music software that runs on these computers and that supports up to four SIDs (SID-Wizard), and it would be very nice to install 3 additional SIDs on a 64/128 with separate outputs for each SID. That would be a neat little 8-bit music studio with up to 12 channels of sound!I've already got a grasp of the basic idea for this mod. In a stock C64 the SID is connected to the address lines A0-A4 and the data lines D0-D7. The SID is enabled by memory decoding circuitry, placing it at $D400-$D41C in the memory. To add further SIDs, the original chip select signal would be used to enable something like a 74LS139, which would have the address lines A5 and A6 at its inputs and the chip select of each SID at its four outputs. This would place the SIDs at $D400, $D420, $D440 and $D460 in the memory. Naturally all the address lines would be connected to each SID and the same goes for the data lines and everything else except the chip selects and audio input and output pins of the SIDs. The original SID socket provides all the required connections except the A5 and A6 lines and the audio ouputs (when keeping them separate).That's the simple part. What is more challenging is to actually install such a mod physically inside a C64/128. One has to take into account the required physical space, how an extra circuit board is attached and secured, how heat build-up is prevented etc.I've seen pictures of such a mod inside a C64. I've asked if the pictures can be shared - we'll see. Anyways, the mod involves an extra PCB that has been plugged in directly to the original SID socket. Based on the pictures, I can't see anything else securing the board on its place. The mod has been performed in a very neat way, but I'd like to take my modded C64/128 to gigs, C64 demoparties etc. - hence I'd really like some additional way of securing the board to prevent it getting loose during transit. It very well could be that there really is no need to worry that the board gets loose, but it is difficult to say just from a few pictures!So what I'm thinking now is installing the mod to a Commodore 128DCR. These have quite a bit of space inside of them, and they use the later 8580-SID, which I prefer (it has slightly different voltage requirements compared to the older 6581). There's also a replacement power supply available for these, providing double the wattage of the original - this would likely be useful for the extra SIDs.There are two options for installing the circuit board with all the SIDs:1) It could installed as in the mod I mentioned previously, plugging it straight to the original SID socket, resting above it. The board could possibly be secured in place by using L brackets screwed to the power supply frame, which is right next to the SID socket2) Another option would be to drill holes to the back or front wall of the case, mount the extra board there and connecting it to the SID socket with a cable.I'd really prefer the latter option, but I have been warned that longish cables could cause "parasitic effects" and some problems and that it would be better to install the 4SID board on the original SID socket.So my question is: what do you think, would option 2) for mounting (the extra board mounted away from the SID socket connected to it with a cable) be problematic and if so, what can I do the prevent any "parasitic effects" or other such problems? Or should I forget it and go for option 1) (mounting the extra board directly to the SID socket) instead?Any help would be much appreciated! As you can see, I'm not very experienced with modding digital equipment. I can fix old electronic organs and analog synths, but digital stuff is a mystery to me.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 19, 2025, 09:27:05 pm
content: I would consider mounting the extension via the extension port. There may still be a few extra signals (like an upper adress or selct signal for the $Dxxxx range).

author: uneksija
author_rank: Newbie
timestamp: June 20, 2025, 07:44:41 am
content: Thanks for the idea. However, I need the extension port for a device that emulates the disk drive (Ultimate 1541-II). Also, I have no idea how to make it work through the extension port, whilst the way I proposed is pretty clear to me apart from mounting.

author: darkspr1te
author_rank: Frequent Contributor
timestamp: June 20, 2025, 10:21:44 am
content: When i added dual sid's to my 64 breadbin i used SIDFX fromhttps://www.sidfx.dk/works a treat. plug and play, no soldering. but that was for dual sid, not quad.darkspr1te

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 23, 2025, 07:14:55 pm
content: Quite a project, SIDs are expensive and relatively easy to damage, of course naturally make sure you don't plug 9V SIDs into 12V, and try to avoid using the SID's external audio input. I don't know why but it's easy to fry the SID by using this feature.