It's begun....

[Washout]

Gents,
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I have a problem I could do with some guidance on:
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Yesterday I cut one half of the job in the video above and all went well.
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Then I flipped the part and as I am now using a centre hole/bore to zero the part off, I stuck my probe into the collet and jogged over to the hole to zero off of. AT this point my VFD was off, so the only thing running was the machine and my PC. As I have both a probe and touch plate wired from the probe pin I flicked the switch which isolates the touch plate and went into MACH to change the probe to "enable low", which the probe needs to operate.
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I then started the probing wizard screen in MACH 3 and MACH crashed with an error. OK I thought I've had this before, so restarted the PC loaded MACH and at which point there was a loud hum/fizz and a loud pop. Figuring something electrical had gone I turned everything off and went to diagnose and immediately found that the Optoboard V3 I recently installed looks to have a blown capacitor on it (marked C1) - at least there is a light scorch radiating out of the corner there on the PCB. I also checked all fuses etc. and my wiring on the drivers, just in case something is loose which shouldn't be. Fuses on the toroidal transformer appear to be OK also.
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Last night therefore, I chucked my old ZP5A-INT board into the control box and this morning started it up and whilst the driver LEDs are green, I have power to the stepper motors, the board doesn't appear to be active and MACH (which I reconfigured for the replacement board) is reporting an External E-stop and try as I might I can't seem to get it to cancel.
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Things I have tried are:
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Checked all e-stop/probe wiring, XLR connector (my probe and e-stop share the same 4 core CY
Removed the e-stop xlr connection and switch to take the hardware out of the equation
Tried to disable the e-stop pin in MACH3's input screen - weirdly this saves/applies but then doing a MACH reset trips the e-stop and the pin has been mysteriously re-enabled
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I'm now stuck, as I can't get MACH3 out of "external e-stop".
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Any ideas gents? (fully expecting it to be a MACH3 thing I have missed).
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Cheers
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Chris