Another lesson this evening!
I decided to take a supply to the driver board from my 5 v PSU rather than regularly falling over a Usb lead from the computer.
And I took the opportunity to make some of the other wiring a bit more compact. So I had the power switched off , and unplugged for (I think) more than 20 mins when the 24 v supply cable touched the chassis giving me a hefty flash and lightly welding itself . Maybe need to be careful of big capacitors .
I think it has taken out the driver board as the traverses dont seem to be working. Spindle controller still OK though .

Is there a way to quietly discharge the capacitor when I switch off??