Update:
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Sometimes I am a plank ;-)
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I had missed the Active Low setting for the estop - so that's one thing sorted.
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I also ran over the control box with a decent torch in case I couldn't see previously and had missed a 1Amp fuse on the PSU that had blown - I'll need to order a replacement (sod's law I have every other rating than this one).
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Of course that doesn't explain why it went pop in the first place, but I have a theory - I noticed that the stepper motors when in holding torque mode normally make a humming noise, which was altering tone when the probe circuit was completed/broken and this was after:
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a. installing the Optoboard v3 and running a 68V feed from the PSU to power it (my only other voltage feeds in the box without resorting to resistors etc. are 12V and 5V and the Optoboard runs on 15-70V)
b. installing my probe, which should not be shielded at the probe, but as I assumed that and earthed it at the star point in my case
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I am thinking that somehow either 1. via an earth loop involving the probe shielding (if it is connected to the probe body then my entire machine would also be earthed at that point as well as through the spindle to VFD earth, which is normally the only earthing point for my machine remembering that I have good conductivity from spindle to the bed as everything is metal).
or 2. the 68V feed for the Optoboard from the PSU on top of the 3x68V stepper circuit overloaded it (I'm hoping that the inputs voltage on that board wasn't 68V or I have likely had a lucky escape using the touch probe and plate on that voltage).
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Of course it could have been something failing on the PSU and that is worrying as it may go again when I replace the fuse.
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Hopefully I can get this sorted soon as the race car the parts are going into needs to be running by the end of Feb.
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Later
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Chris