[Coco] Rebuild and/or modifications for Multi-pak?

Rick Ulland rickulland1 at gmail.com
Sun Jun 14 14:17:47 EDT 2020

Back in the day, CoNect would repack a CoCo using a 6 inch ribbon cable 
to remote mount the MPI. We soldered the CoCo side of that cable, which 
kept the number of inline connections down and preserved the original 
buss socket for 'diagnostics' (ROMpak games). The ribbon might even be a 
bonus since those ribbon to cardedge connectors fit TIGHT.

I did this on maybe a dozen CC3 of various ages and origins, worked 
every time. This is important to note because there is a vast tolerance 
in what any random CoCo3 will put up with - CoNect once prototyped a 
great buss Xpander... for 1 out of 6 Color Computer 3s. Never figured 
out how to ID which one. Korean, American, 86, 87, it was just random. 
The issue was my bad design, but the result might explain why there is 
such a range of advice out there.

Other than the CoCo3 PAL upgrade and optional OS9 'IRQ strap'  I can't 
think of any must do things. Silicon works until it doesn't, and there 
is such a thing as 'too fast' in a chain of slower chips. For that 
matter, computer capacitors from the 80s rated for dozens of volts seem 
to hold up much better than tube radio capacitors from the 40s rated for 
hundreds of volts. Who knew?

I'd say fix the MPI, adding good machine pin sockets under any chip you 
pull. If you have to buy chips, make the minimum order with a full extra 
set and tape them inside the case, because 74ls chips are going 
obsolete. Then ride the heck out of it.


On 6/14/20 9:14 AM, John E. Malmberg wrote:
> I have an original multi-pak that arrived DOA from E-bay.
> The schematic and circuit board look simple enough for me to 
> eventually repair.
> I have seem some discussions here in the past about weaknesses in the 
> design, particularly signal noise.
> So looking for advice on what to do for the repair, which would 
> include a PAL upgrade.
> A. Replace with original 74ls parts.
> B. Upgrade to a newer compatible parts like 74ALS family parts, etc.
> C. Additional circuit mods to improve reliability?
> I see reports of COCOs being blown from the 12V signal from the MPI 
> hitting the COCO if the MPI moved while both devices are powered up.
> Is there any reason to have the 12 power being fed back to the COCO?
> It seems like just cutting that trace would be of help.
> How long could I extend the 40 pin BUS to add additional slots and 
> what are the gotchas.  It appears that I have been able to get away 
> with adding 3 slots with just a ribbon cable in the past.
> Not sure what to use for ground connectors for the additional slots.
> Regards,
> -John

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