A DAC overflow means the control asked the axis to move and did not see a response. In other worked you have a bad position loop. This usually means a bad encoder. But it could also be a bad cable or a bad connection somewhere and/or a bad BMDC (but you eliminated this).
FYI: A encoder can be good and count but a machine can still not home if the marker pulse has stopped working in the encoder. The machine looks for the switch and then the marker pulse. If you start the homing process with the machine on the home switch, it will back off and come back on the switch. If it never reverses direction after coming off the switch, the switch or wiring is defective.
If an encoder shorts out or a cable shorts out on a pre-cutler hammer control machine, it will fry the L1 on the BMDC. Starting with the cutler hammer control, there is a circuit breaker on the 5VDC so this should not happen.
If you know how to get to DOS, with a full size keyboard, go to the root directory, type in PFM.exe and return (enter), cursor down to tuner, hit enter again, and you will be in a tech screen that is very helpful in diagnosing problems.
The other machine needs the CMOS battery replaced. It it has the half size mother board, it may have a Dallas Real Time Clock chip instead. That also can be replaced. Google it.
George