A few things I can think of:
*Be sure you are using the correct traditional tungsten filament #51 lamp. Any other lamp has different electrical properties that are not matched for the thermostatic switch.
*Adjust the adjustment screw so that it just ever so lightly makes contact. (If it is too tight, it won't ever break the circuit causing it to always heat)
*Test for continuity thorough the thermostatic switch - if there is none, replace.
*Re assemble the 193 and test the thermostatic switch under normal operation.
Service Documents are here and mention an adjusting screw: