[sdiy] Another SID-related question

Antti Pitkämäki antti.s.pitkamaki at gmail.com
Wed Jul 30 21:42:35 CEST 2025 

Thanks, Hugh! I might need to get myself the C128DCR finally to be able to
proceed with any mounting plans.

BTW I found an old bit of information on sid noise-reduction by utilizing
74LS245 buffers between the buses and the SID. So somebody else also had
the idea of buffering the address/data lines for the SID. Let's see
whether I need to do that for my mod - although I wouldn't do it for
noise-reduction but for reliable operation. My noise-reduction technique
has always been to short the SID-input to ground by using a shorting plug
at the audio input connector. Especially with the later 8580 SID chip there
is very little background noise with this trick, it's really not a problem,
not at least with a single SID chip. Of course 4 SIDs is four times the
noise, but I'm not terribly worried about this.

Some people seem to have a serious problem with the SID background noise.
Perhaps some C64s are noisier or they have connected their C64 in some
less-than-optimal way or they have poor equipment, dunno... I've read about
so many noise-reduction solutions while all I've ever needed is that
input-grounding trick.

Cheers,
Antti

On Wed, Jul 30, 2025 at 3:21 AM Hugh Blemings <hugh at blemings.org> wrote:

> Hi Antti, All,
>
> I'll offer some thoughts on this, but with the important caveats that:
>
> a) I'm working from first principles only - I've not done such mods myself
> b) it's been a while since I've done hardware - but when I did it as a
> younger person it was very much in the C64/C128 era... :)
> c) there are -many- more experienced folk than I on this list who's advice
> I'd heed ahead of my own :)
>
> That said then...
>
>    - Yes, as you allude to, I'd be trying to keep the interposer cable
>    from the socket to your board as short as reasonably possible - more like
>    10-15cm max if you can
>    - Using a 40 way IDE cable seems fair enough, but probably only gains
>    you the advantage of being able to buy them pre-made and/or finding 40w IDC
>    crimp plugs/sockets will be easy
>       - Extra wires on the 40W could allow you to put some GND signals
>       between active signals or at least keep the analog signals a bit further
>       from data ones.
>    - A small PCB that slots into the SID socket and is a breakout to the
>    40W connector - most of it would be to the right and forward of the SID
>    chip socket (as viewed in the photo) to clear the PSU
>    - It looks like there's room behind the front panel - either
>    flat/horizontally above the PCB or vertically behind the panel - perhaps
>    put the board there instead - shorter cable runs.
>       - With some care I suspect you could use double sided adhesive PCB
>       mounts to stick to either the PCB or behind the front panel.  Probably want
>       a couple cm of clearance for airflow for chips underneat
>       - You could then lay out the board so it was landscape orientation
>       - SID chips in a row so the signals that are duplicated can just flow from
>       one to the next ?  Think ala EPROM layouts that kind of thing ?
>       - I found this photo and associated discussion useful
>       https://retrorepairsandrefurbs.com/2021/05/26/commodore-c128dcr-restoration/#jp-carousel-657
>    - I don't know if it would be prudent to add some buffering to the
>    Clock, R/W, Address and Data lines off the main PCB given you're adding
>    three more devices. I may be overthinking it (shock!) but be worth looking
>    at the fan out on the CPU etc. for those signals
>       - If you did decide to buffer, perhaps this could be done with SMT
>       parts on the interposer PCB that goes on the socket.  Would have the
>       helpful side effect of reducing the impact of the cable length
>    - On a quick bit of research I note there are a few 4 and 8 channel
>    SID based designs over the years, I wonder if anything can be learnt from
>    them layout wise ?
>
> Hope that helps and/or inspires others more experienced than I to comment
> :)
>
> Cheers,
> Hugh
>
>
>
> On 29/7/2025 21:40, Antti Pitkämäki via Synth-diy wrote:
>
> Hi,
>
> I have another question regarding my project of adding 3 additional SID
> sound chips to a Commodore 128DCR computer. It concerns mounting the extra
> SID board inside the computer.
>
> The basic idea for the mod is to connect an extra circuit board to the
> original SID chip socket, which provides most of the required connections.
> The SID only "listens" to the five least significant address lines. Hence,
> in a stock C64 separate glue logic is utilized to provide the chip select
> signal for the SID when memory locations allocated to it are accessed. In
> my mod I'll use a 74LS138 decoder, enabled by the original chip select
> signal. Two additional address lines will be connected to its inputs and
> its outputs will be connected to the SIDs' chip selects, placing each SID
> in its own dedicated memory space. With this system it would be very easy
> to add up to 8 SIDs if I wanted to go totally crazy. But four SIDs for now
> :)
>
> Anyways, it would be awesome if I could mount the extra SID board to the
> back wall of the C128DCR's case and connect the board to the SID socket
> from there. However, I've been advised to mount the board either directly
> on the SID socket or to use as short cables as possible. This is due to
> potential "parasitic effects" caused by long cables. If I mounted the extra
> board to the back of the case, I would need to use cables of considerable
> length to connect it to the SID socket - I don't have a C128DCR yet, so I
> don't know the exact length, but perhaps 30-40cm.
>
> What do you think, am I asking for trouble with my mounting approach? How
> would you mount the extra SID board? Somebody recommended that I should use
> an IDE-cable for connecting to mitigate any problems with my approach, but
> I'm not sure if that is enough.
>
> It would be a bit inconvenient to mount the extra board directly to the
> SID socket. The power supply is right next to it, and its frame could be
> used to install some L-brackets for mounting, but it seems a bit fiddly and
> I'd like to avoid it if it is possible. Unfortunately my vintage computer
> modding skills are limited, hence I'm asking for guidance...
>
> Below is a picture of an opened C128DCR for reference. It has the original
> PSU. The copyright holder of the image forbids altering the picture, so I
> can't add any markings, but the SID is the chip to the right of the power
> supply frame near the lower right corner of the PSU, under the bundle of
> cables. My planned mounting spot for the extra SID board would be on the
> back wall, above the module (?) port on the right.
>
> I admit that my question concerns less synthesizer DIY and more vintage
> computer hacking, but I wouldn't be surprised if I got valuable information
> here even on this topic :)
>
> Regards,
> Antti
>
> [image: Commodore-128D-006-Motherboard.jpg]
>
>
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