The motor either has a broken or detatched spring, after relatively few turns the barrel rotates in the oposite direction and the spring loses tension.

DSCN1716 by RWPD, on Flickr
I feel confident to tackle this one myself. Is my assumption that removing the spring clip on the top of the spring barrel axle will enable the barrel to be slid out, and that undoing the screws holding the two parts of the barrel will reveal the springs? Is there anything else I should be aware of?
I intend to tackle the sound box by replacing it.

DSCN1710 by RWPD, on Flickr