Unfortunately I have seen this many times. You have found the source of your problem I think. I am not familiar with your conversion and how it interlaces with the original components.
Usually the power resistor fails on a saturable core reactor (I replace them when I see cracks on them, especially the underside surface with resistors of twice the wattage). This causes the SMS fuse to blow, the corresponding resistor on the SMS to fry, and if not caught fast, the corresponding transistor on the ACC has the solder melt out and the board fries from the heat. Thus check all these components.
A transistor blows when it is shut off. Especially when an axis is moving and you press Emergency stop or hit the end of travel/limit switch.
George
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(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
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