Thanks so much for your quick reply. The tension adjustment made no difference, but following your advice I removed the drive ring. I actually had to loosen all six screws on the back to give enough play to remove the ring, but it came off with no parts
making a dash for freedom.
The problems I can see are: first, the inside of the unit is quite dirty, with a layer of grime over everything; second, the metal liner inside the drive ring is scored as if something has been scraping against it. Either of these issues could account for the harshness in the operation of the unit. The clutch
plate rollers look likely to be the cause of the scoring. The ends of the bolts holding the nylon rollers are rather sharp, and could be making contact with the ring, although they don't stick out very much.
So the first step is a good cleaning
of the interior
, then I will try filing the ends of the roller bolts a little. I'm tempted to put a drop of light oil
on the idlers and the clutch plate as well, although I'll have to be careful not to get any on the belt. I'll report when I'm done.
Thanks again for your invaluable advice.