It was Yet Another Color Computer, designed by Don North in Apple's Education Research Group, the people who formed part of Apple ATG. It was mostly used by the ATG Graphics and Sound Group and eventually had a 68020 daughter card.
https://twitter.com/DosFox1/status/1616767033076756489
What an amazing little machine - and they managed to build it using (a lot of) standard 74-series TTL chips and a few more PALs than the original Mac uses. The board picture on bitsavers shows ICs with date codes between 1980 and 1984, so it's a bit hard to say when it was designed/built.
It seems that the CPU still runs at the original 7.8336 MHz (oscillator U285) and it doesn't seem to have separate video RAM. In addition to the 1 MB DRAM (32 x Fujitsu 81257 256kBit chips) there are only 2x2kB 45ns SRAM for the MMU and another 2x2kB, which I assume are used for the color LUT.
I wonder how they managed to squeeze the required bandwidth out of the design without significantly slowing down the 68010. The DRAMs feature a fast "nibble mode" allowing for high-speed access to four bits of data, but I'm not sure if this makes a lot of a difference. Do you know if the YACC used the original 512x342 classic Mac video resolution?
