Well I put the scope on the 0-10v rail last night and immediately the RPMS shot up from the 23.6K to the full 24K RPM. I assume the scope is acting like a parallel resistor and effectively reducing the resistance across the vfd. Could be extra inductive resistance on the wires going from the BOB to the VFD? I'll replace that with quality shielded cable later and add a ferrite bead for good measure.

The scope didn't give me any further clues except the signal looked a little bit noisy. Might try a smoothing capacitor if the shielded cable doesn't help.

Cheers

Dave.