No one seems to know anything on this, so here's what I discovered:
Despite name, address & technical helpline tel. no. on label, 'it's not on the computer' at B&Q (it's their helpline), so no clues there, even though they're the UK distributors. (Bear this in mind when making future purchase decisions?!)
Similar Bosch tools have exploded drawings / parts lists available on the net, so one get undertsand what normal / standard condition should be. With this, there's nothing but a GMC PBSM operating manual (12 pages to tell you how to turn it on & off etc.!)
Someone in our local tool hire shop told me that the sort of belts on these machines don't stretch: failure mode is usually losing a couple of teeth if someone gets cord caught in the sanding belt; maybe snapping.
I stripped the sander down to find that one of the bearings on the shaft driving the belt cog was rough. In use, it had got warm, softened the surrounding plastic, allowing it the shaft to get pulled over by the tension in the drive belt. Plastic had migrated from one side of the shaft's hole to the other, then cooled and set. Shaft was at a sufficient angle to lose tension on the belt, but still be driven at the other end.
Don't know why bearing failed - dust ingress is a common cause (e.g. on Dyson Vacs). Replaced with new bearing c£2 (these are faily common; used in roller skates / boards too!) which I set in some 'plastic metal'-type bodge in the right place.
All working, although bodge might soften if it gets too warm ...and I'm still unimpressed with the on/off switch!
Steve Milner, November 2009
