I have read everything I can find about the Nikon NPL 332 total station and this instrument does not appear to have a horizontal compensator.
After I set up the instrument and get it level I lock the horizontal clamp. Rotating the telescope from 90 degrees (vertical) to 270 degrees (vertical) with the horizontal locked causes the horizontal angle to change usually around 8 seconds. Rotating the telescope back to its original position of 90 degrees (vertical) changes the horizontal angle back to its original value.
If there is no horizontal compensator then why is this happening? Is there a hardware or circuit-board defect?
If there is a horizontal compensator how in the heck can I turn it off?
This instrument has a tri-axis adjustment procedure that creates a software correction for the double centering and telescope plunge factor. This allows a software fine tuning value to be applied for the mechanical error of the telescope axis bearing alignment. The software is compensating the horizontal angle to correct for bearings mechanical position. The tolerance for bearing position is 20" @ 30 degree slope. You are well below that factor so no real need to be alarmed at this point.
332 has a single axis compensator. (vertical axis only)
the software calibration is found on "menu" then option "7".
this may put a correction on the horizontal angle based on the trunnion axis error measured during calibration.
if you want to switch off the compensator you can press the arrow keys when the electronic bubble is displayed ("0" key)