Mycenter 2, Fanuc 6M. Tacho gone?

[FuddMcDee]

We have a 1981 (roundabouts) Mycenter 2, running a Fanuc System 6M. While an operator was running the machine (just doing simple drilling operations), the machine went into an E-stop mode, and the only warning displayed on the screen was "Not Ready". Did a little playing, had to hard reset the machine to clear the alarm, and went to start it up again. The Not Ready display popped up again the second the spindle start command was read. After another restart, i attempted to start the spindle using the manual "CW" button, and the second it was pressed (as suspected), Not Ready popped up on the screen.
On the spindle control board, the OVERSPEED LED lit up the second the alarm popped.
We're assuming this is a tach issue, as the spindle doesnt even move 1 deg before alarming, but is it possible there is more to this?
Has anyone had a similar problem?
I know the old 3 and 6 series are pretty much dust, but there is a few still around

[FuddMcDee]

UPDATE:
I wish i had been around on first shift when they were further diagnosing this issue. Apparently they got the spindle to start temporarily, and the brushes were so worn that the rivets were touching. I missed a pretty good light show.
Anyhow, Fanuc doesn't make parts for this perticular drive anymore, so we have to send it out to get custom made. At least i'm not paying for it, as we're probably doubling the machines value by replacing the brushes.
Anopther concern arose, however, when the first shift operator put the machine in edit mode and powered down, as the memory was cleared and all programs were lost. Guess we'll cross that issue when the spindle turns.

[FuddMcDee]

And final update:
Motor came back from being rebuilt, got installed, and all is well. Took some time figuring out the MDI mode on a 6M, but in the end, we got it working.
Still no idea on what happened to our original programming, and since we've entered new programming, the problem hasn't reared its head again. Time will tell though.