Axis overrun

Hi Guys
This is my very first post here so hope you will be both gentle and helpful :)
I am a virtual CNC virgin when it comes to repairs and faults!

I have a problem with my Bridgeport CNC Miller series 2.- very ancient I know!!
For a while now either 1 or 2 transistors have been blowing on both X and Y axis
I seem to have got both working except the axis movements are over running the points I have programmed them to move to.
I put in a simple program, just to move both axis , very, very basic but although the digital readout looks fine the actual distance travelled is longer
e.g. If I ask it to move, (on feed) X -200.00mm it is actually travelling around 230.00 and the same happens on the Y movement but it returns to the X0Y0 position at the end of the test program.
I have changed the stepper motors to send the originals for testing (on advice) but I have put a different motor on in the past and didn't have the same problem. (The motors are from previous defunct Bridgeport and are the same type of motor,( Sigmas I believe))

PS, I don't know if I need to change to a different thread but I am looking for diagrams for the Sigma stepper motors for the guy who is testing my original motors.
If anyone can point me in the right direction to a website etc that would be helpfull

Any help or advice would be greatly appreciated