When mine (different make but looks fairly similar) had problems with the belt it was because one of the excitor bearings was knackered. I vaguely recall that it was hard to tell because the other bearing (there's one on each end of the excitor shaft) and the oil seals held the shaft more or less in place while it wasn't running - there wasn't the wobble on the pulley you'd expect to find. It wasn't too much of a job to strip the excitor and subsequently replace the bearings - it was mainly held together by circlips.