Charging,?.

[Reckless Rat]

It's the one with a sliding mount on the left hand side of the dynamo as you look at it end on. The other two brushes are diametrically opposed and their carbons are twice as thick, so it's the thin one that's the third brush. If I remember correctly, rotating the brush anticlockwise increases the output. There is no need to undo anything (except removing the dust cover if it's still there) the brush should slide with finger pressure. Give the commutator a clean while you're at it with a cloth.