A flyback diode selection problem?
The supply is applied between CONTROL WIRE #1 and CONTROL WIRE #3? It looks like the diode it there only to freewheel the current through the field coil (which is usually highly inductive) when power is removed. It shouldn't be taking current continuously, only momentarily on disconnection, so the continuous rating of the diode isn't the issue.  The forward drop of the rectifier plus the resistance of the "RESISTIVE LEAD" would serve to decay the current quickly.

Selenium diodes are used since they have no reverse recovery issues, silicon diodes have to absorb a certain amount of charge to establish the depletion region, still allowing a short voltage spike to be generated. A Schottky diode would work in place of the selenium device if you need to replace it.