Ok, so the machine runs in 'Play' mode? but won't let you write new patterns? Unfortunately I don't think I have the schematic for the 99 but from memory I recall it is very similar to the 88 in the way it functions, with expanded memory for longer patterns? I've repaired them in the past but not seen one for a long time... the usual problem with these things is start/stop (note/rest) switches not advancing or registering properly with the ram counter(s) - probably 4520 but can't remember for sure - or faulty ram itself... you're familiar with the DR-55 so it's a similar troubleshooting process. I've found that some Memory Rhythm machines are quite picky when it comes to certain cmos and ram types and had to try a few. Also easy to overlook mechanical failure of the slide switches... will check to see if I can find the schematic...