I recently picked up an old Marrutt drying cabinet. While the unit is fully functional, it was apparent that the filter was original....
So after much vacuuming and cleaning the unit is back to its old self. I replaced the original fiberglass filter with a modern G4 pleated filter. While having my head in the bottom of the cabinet, I noticed that the unit has two cavities which bypass the filter. I suspect this was an intentional design by Marrutt to help prevent the fan unit burning out from a clogged filter. This evening I made some airflow tests, and while the strongest airflow is through the filter, there is still the potential for non filtered air to make its way in to the unit. I think the next time I open it up I'll fully seal those sections with foil tape.
The exterior now has a light coat of Renaissance Wax. I don't ever see myself setting the temperature high enough for the wax coat to melt. Hopefully, this wax coating will preserve the existing patina and prevent further surface rust.
Still on the to-do list are to make some new hanger bars out of 316 steel, and to remove the rust from the bottom plate and seal the metal from further attack. The years of high heat and humidity have corroded the air intake grate, so that may be replaced down the line too. There is a bit of glue left from where various notices were taped to the unit. After some testing, I was able to remove the glue, but the gloss finish on the paint was removed, too. Due to this, I've paused on removing the rest of the glue residue, until I decide how to proceed.
While tinkering with the "thermostat", I discovered that it doesn't have much bearing on reality. It's essentially a resistor/timer working on intervals for triggering the heating element. I suspect it controls a variable resistor to the heating element. The higher the temperature setting the less resistance to the element. I'm getting into PID controllers at the moment, so when I finally become compelled to replace the current thermostat, that'll be the way I go.