PZL vario zero adjustment.

Hello.

I have a mechanical PZL vario that indicates -0.5 kts while sitting
still and I would like to zero it. Does anyone know the proper
adjustment procedure?
When I take the glass off I cannot see a place where I could put a
small drop of paint or wax to act as counterweight. Is there a "zero"
screw that I am missing? I haven't been too ambitious with it since it
is an extremely delicate mechanism and I don't want to mess it up. Any
help would be appreciated.

Thanks,

-Gonzo (EE)