I hope someone can help!

We have a mid 90's conquest 42 with full C axis.

For the last 3 months we have not been able to use the C axis due to it having a fault, we have had the Hardinge service engineers out to it 5 times, with scopes etc, and still not working, they are not sure if it is the encoder, amplifier, or drive board.

The machine still screw cuts OK, and spindle revs are good, but when the c axis is selected and indexed, it continues to rotate trying to find its position, while the screen is not changing the current position.

any help would be much appreciated, by me and the service engineer.