Mantis microscope friction clamp

[nigeb123]

I managed to pick up two Mantis (original) microscopes which were being thrown out as the business I work for is closing.
One has no friction clamp lever, the other has one but no matter how hard I turn it, it doesn't have much effect. (Tried the lever on both, same problem). I suspect it's been dragged about with the clamp closed and caused some wear - but I can't figure out what to do about it. Anyone with any experience of this ?