I have two assembly microscopes: A mediocre Chinese AmScope (which is nonetheless convenient, small, and a pleasure to use) and a monster 1980's Olympus SZH for when I need resolving power, like high-resolution photos.
Aside from size, the SZH has a big annoyance: The zoom setting won't stay put at low magnification. It's a cam and roller mechanism with real bushings and bearings throughout, and it's so low-friction that the stage springs constantly pull the zoom out of the low magnification range. I'm guessing this is part of the reason for the click-detent system in its successor.
I picked up another SZH body recently for parts, just in case, and it didn't have the same problem. It needed a serious cleaning, so I opened it up. And HUH.
What do you know. Olympus added a counter-balance spring at some point. Completely eliminates the problem.