AT&T PC 6300 Plus Unix box

Hey all, Yesterday I scooped an AT&T PC 6300 Unix machine on its way to the dumpster. Got it home, powered it up, and now I know why. It displays "Parity error on system board" and locks up. Actually, it displayed a bit more: Resident Diagnostics Ver 2.05P CPU (i286) PASS ROM Module PASS DMA Timer PASS DMA Control PASS Interrupts PASS Parity error on system board It seems to have 1MB of RAM, half socketed (36 81256's). So I yanked the socketed chips, and tried again. Then it did almost the same thing, but displayed a running count up to 512K before that parity message. So, what's next? I'm guessing that at least one of the socketed chips was bad, because it got farther before barfing when they were removed. But there is some other problem too, maybe in the soldered-in RAM chips, or maybe in the parity-check circuit itself. Anybody got a service manual or schematics or a big pile of clues for this thing? Cheers, Bill.