MVS RAM chips often fry due to overvoltage from a badly adjusted or failing cab power supply. It appears that the most common failure mode is the inability to write new data, resulting in errors during system self-test at the beginning of address ranges.
Basically, all NeoGeo RAM chips with "256" or "257" in their markings are 8-bit 32kB RAMs.
Pinout and cross-refence
The number after the - indicates the speed (max time between /RD going low and stable data):
- 15: 150ns
- 12: 120ns
- 10: 100ns
- 70: 70ns
- 45: 45ns
Speed grade must absolutely be equal or less to replaced RAM chip.
Look for 5 volt, 32kB (32K x 8bit) / 256kbit SRAM (Static RAM) with the right speed grade (most today are 45ns, so any will do) in 28 pin SOIC package. Cypress, ISSI and Lyontek still make them, they go for about $2.50.