Can anyone provide useful experience based comment on the potential for or cost of repair of the "zoom bearing" mechanism in this lens specifically or with lenses liable to be similar? My question is mainly a "is this a known problem with this lens or family of lenses and if so, is there often a known cause that is easily enough remedied".
I'm asking about the possibility of my personally repairing a specific possibly minor problem on a specific lens. I'm competent mechanically and comfortable "wandering around" in the outer layers of lenses in those areas where this does not affect alignment and require specialist equipment. I'm aware of the usual expert-territory / realignment / hard / what is my time worth ... issues which usually apply to general DIY attempts at lens repair. I've had some past successes with repairs which need only basic outer layer lens dismantling.
What I really want to know is, what are the chances that if I buy it and pull it apart that I'll find that I can restore it to "within the range of normal" operation with a few hours of pottering, some suitable lubricant and perhaps a bit of bending. Such an answer MAY be able to be given by people who have met this sort of problem with this family of lenses, IF this is a typical fault in this lens series.
I've been offered a "somewhat wounded" Sigma 24-70mm f2.8 EX DG Macro (non HSM) Nikon mount lens at a price slightly around 33% - 50% of what I've seen them for on international sites (ebay and other).
The main defect is that the zoom action becomes stiff in the centre of the travel range. So much so that it can be almost impossible to move through this centre point when the lens is horizontal. Tilt the front upwards close to vertical and the stiffness vanishes. ie it appears that the "bearing or bearing surfaces are binding due to the lens weight but are free enough when no downward (ie sideways relative to long axis) is applied. The binding is NOT harsh - it does not grate or "skip" - just stiffens. No noise is made when moving against the binding force and the action when the lens is vertical feels acceptable.
A look for lens teardowns for this lens found some very simple procedures to access the "zoom bearing" (their term) for the HSM version. This plus prior experience with other zoom lens dismantling suggests it is probably possible to dismantle the basic lens outer without causing major issues requiring recalibration and realignment.
It seems likely (but not certain) that dismantling and careful & sensible [tm] lubrication of the zoom bearing surfaces may effect an adequate improvement. Knowing if this was a common problem with this or similar lenses and what is typically required for repair would be interesting.
Dismantling and lubrication would be acceptable. Anything requiring spare parts or expert involvement would be liable to make the purchase non viable.