I seem to remember replacing a lot of tachs on both 6010 as well as the Miprom 21 with analog AB drives. Check them all for ripple with a scope or even an old Simpson or Tripplett works like a champ on output. I use to stick a 5k resistor across the tach to clean these up a bit. Put a bit of a load on the thing and it works better. I just went through a similar scenario on an Epoch One job. Montgomery's also start messing up when the drive fans don't work. On the 6010 you have the infamous sail switch to shut you down but the later stuff didn't have that. Good find anyhow. The bad news is that you'll be stuck with that stuff awhile longer.
