I went on the web and found that model. It has a rubber pulley, and a rubber belt, however I didn't see how it is hooked up to the dial. If the rubber belt isn't tensioned right that could be the problem, since I have no experience with that setup this would be my suggestion. Get a rubber belt made out of neoprene from Ace Hardware. They are somewhat stretchable and don't deteriorate. Most however are round rather than flat, but you may luck out.
This next suggestion would be to make an idler arm with spring.
You could also find rubber grip liquid that may soften the rubber.(also called rubber dressing.)
Another Idea would be to use some sandpaper to rough the wheel and belt surfaces because with age they do get hard and loose grip.
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Heres a clip from a typewriter page on the web:
The platen is the printing surface of a typewriter -- normally, a rubber-covered cylinder. The rubber on an old platen may get hard and slick, so that it doesn't grip paper properly and the type hits it with a harsh, loud impact. What to do? •Vigorous scrubbing with Soft Scrub will remove the dirty and slick exterior layer of the rubber, and improve the grip.
•You can also try roughening the platen by scrubbing it with sandpaper, but I like the results of Soft Scrub better.
•Brake fluid (DOT 3) reacts chemically with rubber and breaks it down. It will soften rubber unacceptably when exposed to it for the long term. A little exposure, however, can add a little flexibility and grip to the outermost layer of a platen. You can wipe a thin layer of brake fluid on with a paper towel, leave it on for about an hour, then wipe off any residue. Avoid skin and eye contact. Allow several hours of drying after this procedure, because at first the platen exterior will be too soft and should not be handled or used.
•Use one or two sheets of backing paper for cushioning if your platen is hard.
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