On 7/7/19 9:43 PM, Jim Brain wrote: > Memory in a 250466 PCB went flaky, so I tried my hand at repair. The > PCB is the 3 ROM version, but with 4164 DRAMs. After desoldering the > DRAMs, socketing, and installing some new DRAMs (all working now), I > decided to see if I could switch the unit over to SRAM. It appears I > and some other folks had similar ideas, as I see another project on > Facebook to convert as well. I went a different direction and leveraged > a small CPLD to do the conversion. That's a bit Overkill, you need just 2 TTLs to make it work. A 74HCT573 and a 74LS32 or 74LS04 (*) will do the trick. (*) You need to either OR /RAS and /CAS to create the /CS for the RAM or use both CS-Lines on a 128 KB SRAM and invert one signal since one of them is high active. > I decided to play around with the unit and map out the address pins. > Interestingly, I find that MA6 and MA0 appear to be swapped on the PCB. > I verified that MA6 indeed goes to MA0 on the 6567, though without a > schematic, it's hard to trace MA6 (it goes through a '257, etc.). > Anyone have a schematic? (Zimmers does not appear to have one) Sure it does: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/252278-1.gif http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/252278-2.gif GerritReceived on 2020-05-29 22:35:51
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