There a couple of defects that could prove to be fatal to the ZW. The transformer coil is tapped by running spring copper arms with carbon rollers across the windings. If the carbon rollers wore out and were not replaced, the copper arm would drag across the windings. In time this situation can result in the windings being cut with the copper arm being dragged across them. I know on only one person to ever rewind the ZW transformer, and that was a long time ago.
There is no over current protection for shorts between A, B, C, or D circuits. I have seen where the control arms and windings have been annealed because of high current shorts that were not interrupted. There is really nothing that can be done once the windings are overheated like this.
Your description of the transformer not shutting off, could be caused by worn out rollers allowing the control arms hitting each other. A quick look in will provide a lot of information.
