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Those rollers are the transformer coil taps. Put a piece of card stock under the A and D rollers and there should be no voltage on either. You could also verify the voltage on the individual square copper wires (transformer turns). The wire to U should go from about 6 to 16 volts.   If there is a connection between A and D that is not supposed to be there, it is shorting the windings between the two contact points in the transformer windings. One of the weak points of the ZW is that there is no circuit protection for A to B to C to D shorts, so this should be a point of concern.

Most of the ZW's that I work on have this phenomenon that you described above.  I use a Klein low-voltage meter and it will read zero on D when power is applied to A and vice-versa.  I have heard it referred to as "ghost voltage".  In one of our old apartments, the knob-and-tube wiring would read voltage (sometimes up to 60 volts) when the switch was turned off!  That was when I invested in the low-Z meter.

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