Finally got it fixed on Sunday morning! I was hoping to get it done in ten minutes, but it took more than an hour. I loosened the screws holding the motor so I could swing it around and get the belt on, only to find they hand nothing holding the other side, so they just went round and round and I couldn't get the nuts back on
. In the end I found the motor had been mounted (at least 30 years ago) on a timber frame that was attached to four bolts that allowed it to slide up and down to tension the belt. It was these bolts I should have loosened
. The original motor had been replaced before it made it way to the PMG!
Instead of the clunk (as the worn section goes around the pulley), clunk (as it goes around the other pulley), flap (as the loose bits of belt hit various parts of the lathe)....clunk, clunk, flap, clunk, clunk, flap! It now goes "whirrrrr"
. Just need to do a bit of machining to check the belt tension is ok.