We have successfully used mylar/acetate tympan on our SP-15s for years, but when we tried one on the Uni III, it keeps slipping. There could be multiple causes: 1. The mechanism for holding the tympan on the Uni II is not suitable for acetate; 2. the acetate is thinner than we usually use ( I suspect our latest purchase is .005) and we need to bump up to a slightly thicker acetate; 3. There is something wrong with the tympan mechanism on our Uni III, although we did not have this issue when there was a paper sheet on top.
Thanks for any advice.