A 1.4 A shunt DC motor, with brushes, in a home made pottery wheel from 1970s runs slightly erratically.
In general I am well impressed with it as the motor choice is appropriate for the application, being low speed and even running under varying load. The speed controller circuit board is also home made but I think the fault may be simple to correct (not sure as I am amateur.)
In the attached circuit diagram there are two bridge rectifiers with connections "x and xx" to field windings and "A and AA" to armature.
The field winding rectifier is about 10mm diameter "S1-60" and the armature rectifier is grey square with 400 V printed on it. So this looks as if the circuit controls armature current, but would it be better if the winding current was controlled? If so can I swap the motor wires between rectifiers? Would the rectifiers still be appropriate?
33 kΩ resistor may be 22 kΩ, 0.1 μF components look like Mullard capacitors - I have not used symbol and value may be wrong.