I am restoring ‘vintage’ Soldner throwing wheel for Irina.

Soldner Pedal Schematics

Input AC: 110V
Output DC: variable

Motor (connects to PEDAL outlet): Dayton 2m168 permanent magnet 90VDC 1/2HP
Diode: Motorola 1N4722
Bridge rectifier: Motorola MDA980-4
Powerstat: Superior Electric 21

Superior Electric article on Powerstats: http://www.danaherspecialtyproducts.com/uploadedFiles/Siteroot/Superior_Electric/OneTransformerManyVoltages.pdf

The voltage on motor is limited amount of rotation allowed by the pedal: The powerstat shaft simply cannot rotate beyond approx. 100 degrees from zero.

But why an additional diode on top of the rectifier? Extra protection?

PS Looks like the motor armature shorted. The motor was rotating only when DC voltage went way above 90C, and even then it was rotating slowly and jerking on every full rotation.