A couple of thoughts.
First, be sure the fuse to the transformer is sized close to the transformer rating. 50 VA would be about 1/4 Amp at 210V. So, no more than maybe 1/2 Amp at most. Do this especially on the first turn-on (Try starting with a 1/4 Amp slow blow) because maybe there is a short circuit that opened the primary coil vaporizing the wire you can't find. The fine wire can get weak due to vibration and can go pfffft before the fuse. Who knows at this point though.
Second, some people have put a cable-connected counter-weight on the Z axis to take some of the weight off.
Here's a fancy brake-rotor-based one that came up when I googled "z axis mill counter weight".
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