I don't see this board documented in the normal places and the PCB does not come up in a search. The VIC-II is missing. The board was originally owned by CBM engineer David Diorio and was donated to Kennett Classic when he unfortunately passed to the great motherboard in the sky recently. My initial guess is that it was a dev board from when the Commodore 64C was being crafted, or David was experimenting or modifying the design somehow. I have asked around to see if anyone knows more.
More 250451 pics
Jim Brain says:
"... That's an odd one. Like a CR board, but with 8 DRAMs, though not in a 2x4 config like on the 466 board. .."
Gerrit Heitsch says:
The numbering suggests that it's older than the 250466 design. Same goes for the datecodes on the chips on it. But it shows a lot of details that later appeared on the 250469 boards.
It would be interesting to know which VIC goes onto that board, is Pin 13 of the VIC socket connected to +5V or something else? Where does the 6581 SID gets the +12V from?
My guess is this board was supposed to replace the 250425, but they ran into some kind of problem(s) and had to make the 250466 as a stopgap until the 250469 was ready. .."
Mario Kienspergher adds:
"..Indeed, looks like they were testing it but trashing it because of problems, as the 250466 came much later with a NMOS VIC and old glue logic.
IC-stampings are from mid/late '84. Maybe the HMOS-II-Version of the VIC did not perform very well or the yield was too poor, or known problems with other HMOS ICs (7501, 7360) forced postponing.
"...They did use the 8500 on 250425 boards. Problems with VIC are possible,
after all when the HMOS-II version came out, it was R2.
And you can see on the right side that there is a space for a regulator
(VR1), so the board might have been able to use an NMOS VIC. ..."
"...Wow, you found a prototype board!
This is obviously (by the part number) after the 250425, but curiously BEFORE 250466. Maybe this board was not stable or the new PLA could not be manufactured quickly or cheaply enough (it was not a Commodore chip- I thought it was a Sanyo) so they stuck with the old design for one more iteration?
"..The PLA also came from other makers. I have seen YAMAHA and SHARP as well.
Might have been the VIC that wasn't ready since that board needs the
HMOS-II VIC. ..."
"...Looking at the board - doesn't it have !CAS1 (pin 19 of the gate array) connected to DRAMs CAS line ? 251715-01 has these CAS0..CAS3 lines (pins 18..21) that are gone in 252535-01, I was always wondering what they could be for and if they could be abused... ..."
"... FWIW, contemporaneous with these machines, I repaired several of those boards in the mid 80's. (small boards, integrated PLA/Colour RAM, 8 x 1bit Dram, PMOS), so it's clear they made it into full production. Of course back then I didn't realise the significance of them. Most of them (and there were quite a few) came to me badly damaged by runaway brick power supplies..... I'm in Australia, if that helps. ..."