[Arran]

  I can't speak for Atwater Kent Model 42s but in anything with a full wave dual diode tube rectifier, and a transformer, there has to be a center tap on the high voltage winding someplace, if there is not the power supply will NOT work as the center tap is the return path and the most negative point in the B+ circuitry. The only way it can work without a center tap is if the rectifier was wired in as a half wave rectifier, which A.K clearly did not do in this case. So either you haven't located the center tap or the connection has snapped off somehow.
Regards
Arran