OK - just a thought! Don't blow any more fuses.
My thought was that which terminal 3 (or 9) linked to (or whether it did at all) might determine the voltage level required, akin to different taps onto a potentiometer.
If you are running from Mach to get the voltage level, you can reduce the step signals, thus reducing the converted voltage from your speed board, by 'de-tuning the motor' in the configuration set up.
I did this with my 160 to keep the max speed within range, because my speed board gave out 10V and was only adjustable within 10% of that.
I don't have the exact figures to hand, but my steppers were 200 steps per revolution so I started with that figure for the spindle motor tuning.
If for instance you tell Mach that it only takes 100 steps per revolution, the output at any given dialed up speed should be halved.
There is probably a point at which this does not work, but it worked fine to trim my max speed to 3000 rpm from an exciting 3800 rpm when I started.
The other alternative is a resistor to drop the voltage itself.
Melee