Troubleshooting a N* Advantage

My Advantage decided to stop booting from floppy disks (the only media it is capable of booting). At first it threw random disk errors, then progressed to the point where I get an immediate "Hardware Failure" diagnostic on screen at power up. If I reset and hit 'Enter' it starts the disk drive comes right back with a "?" prompt and LOAD SYSTEM. I've checked the obvious: - Power supply voltages are good - Pulled, cleaned leads and re-seated 52 very tarnished 4116s - Cleaned and reseated all other socketed parts. A lot does seem to be working. Display is fine. I can enter the mini-monitor and poke around. Keyboard is being read properly. I accidentally found an undocumented diagnostic (enter mini-monitor with Ctrl-C and enter 'A'). This displays the disk controller status registers. The only activity is the sector mark pulse, so something is responding to disk data. Any advice from other Advantage owners or those familiar with the beast would be appreciated. Interesting data point: I replaced the 8035 slave processor with a newer 8035 from the parts bin and none of the I/O operations it manages worked at all - no keyboard no disk activity. Is there perhaps something special about the older part? I'm not aware of any major differences in this part through the years, but you never know. --