I have a Thornton Pickard cloth roller blind which I have repaired. My concern is with the internal spring for the blind return. The fact that it is some 100years old I am worried that if I tighten it to get the original 1/90 sec. speed it may break. At present I am stuck with using only 1 speed - 1/15 - or open shutter and guesswork That is OK but I have trouble in bright light. IF the spring were to break is it a repairable job and if so how?