After reading a research paper where they were looking at using one of these unit and the mention of bad drive belts in the wavelength drive I popped the monochromator off the table and pulled off the cover. Pretty neat design:
The wavelength is selected bu moving a delrin nut aiming a ground thread rod, that moves a bracket that holds a glass plat which a cam follower runs against and rotates the gratings, the two gratings are coupled with tensioned steel cables for zero backlash. At the bottom of the threaded shaft is the encoder which sends the position signal back to the wavelength controller and next to that is a 10 turn pot on a gear.
That took me a bit to figure out, it's what controls the order sorting filter. I had known the filter was operated separately from the wavelength controller, I had figured there were just maybe some opto interrupters or something that controlled the position. Nope, they used a 10 turn pot which feeds into the board at the bottom. That board has a quad comparator and measure the voltage of the wiper and as it passes each set point set by the grey trimmers it selects the next filter position. There is a little stepper motor that belt drives the filter wheel which just keeps rotating until a magnet in the wheel travels over a reed switch for whichever position was selected. Simple!
The drive is accomplished by the Vexta stepper at the top, it is driven by the board at the bottom by what looks like a very simple unipolar stepper design. The motor has two o-rings on an aluminum pulley with a cam mechanism that engages it against a knurled wheel when you want to drive it with the control. Good thing I did pop it open, there was not much left of the o-rings.
So I replaced the rings and took some Krytox 240AC and greased the gears and popped it back together, it does run a little more smoothly now.