VFD interference problem

[cropwell]

You dorks :D

Well, a clue could have been that when the 5V PSU was off, its power LED was slightly on (not fully, but you could see some light), indicating that power was flowing from THE PC through to BOB to the 5V PSU (receiving -5V I guess).

Anyways, problem fixed, let's now move on to cutting !!!!

Had you originally connected V- on your 5v PSU to GND ?

On your BoB the terminal marked PC GND is actually the current return path and should not be connected to GND but only to V- on the power supply. The BoB itself has a GND plane connection next to the 12-24v terminal, which is the current return path AND GND for the BoB electronics.