Hi Mick
You are certainly having some bad luck.
As you know, both Craig and I are converts to ac servos, and without doubt that would be the ideal solution if money needs to be spent. But I appreciate that this would involve greater expense and more work, both electrical and mechanical, than simply replacing the analogue drive.
As to the suggested analogue alternatives, I'm not sure that the lack of tacho feedback would matter greatly. The tacho integrates motor torque to provide velocity feedback which effectively makes the motor speed proportional to the voltage applied at the command input. An equivalent function could be provided within the new drive, for example by taking feedback from the armature voltage. The same function can be provided by the Kd term within CSMIO. Crudely speaking, all these options boil down to the same thing in the end. In practice, you will probably need some re-tuning of the PID loop but I cannot see why you should not expect an acceptable performance if the drives have a similar specification.
Allan