I remove the CPU PCB this morning to check the wiring from plug X43.
The plug is connected to the same Peripheral Interface Adaptor chip (PIA) that feeds the stepper motors.
I could not see any broken or burnt tracks.
I re assembled everything and checked all the leads with an oscilloscope and there is no activity on any of them.
At the moment I am wondering if a section of the PIA maybe failed.
They are not expensive but it would be a bit of a toil to change as they are 40 pin chips directly soldered into the PCB.
Before I do that I think I will wait and see if anyone has a later manual with the floppy drive instructions.
MFI works well with my C5 so I can up/download to my PC OK.
I found the service and manual for the floppy drive but unfortunately nothing for the interface PCB.
I'm sure there must be many C5 MK4 with floppy drives out there along with the manuals so I hope someone has one, fingers crossed.