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Hi Bob, have you checked the latency of the Ram? Couldn't tell the original Ram against the replacement was there a typo?. The latency (access time its measured in NS) of the upd5101 seems very old school slow on the data sheet. It says 650ns-450ns. Not sure what the original should be? I'm sure the mpu4 Ram will be around 100ns so much to fast. I've used 6264 Ram both too fast and to slow and found the memory doesn't hold but it's more like Ram corruption that's why it will Ram clear with each switch off. Maybe it's different from the original and replacement?

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This one is a bit of a head scratcher. Some of the CPU's have battery backup and use a UPD5101 as the RAM chip. These are very rare now and a replacement is the UPD5101 which is supposed to be a pin for pin replacement. The problem is it doesn't save the credits on power down however the games run absolutely fine. I've also tried making up an apadptor board with a more modern chip (MPU4 RAM) and this doesn't save either! It's the same on three different CPU cards but they all had bad RAM in. One issue is I don't have a spare UPD5101 to prove it so has anyone out there had any experience with these chips regards compatibility.