Optical spectrum analyzer : MTBF

[JFP]

We have a lot (university) of OSA (HP, Ando...) and they are almost all dying with the same symptom : "slit failure". I have dismantled an HP 70950A and the cause is obvious : the axle of the slit motor, fixed with a circlip rotating on the surface of the optical box, is stuck and nothing can be done. The B model has another system and the wheel/motor rotating freely but, as the analyzer starts, the slit wheel is not always correctly positioned : after several power on cycles the wheel is correctly positioned but, as you decrease the span and resolution, the slit wheel takes from time to time a wrong angle (or rotate like a propeller !). I guess it is the angular encoder (made by Computer Optical Products Inc). Other OSA have the same failure and therefore my question : what is the MTBF or, better, the estimated life of an OSA ? Thank you.