[Arran]

I would post this on the ARF, there are people on there whom have worked on a number of magnetic pickup phonographs. Part of the problem with heat shrink is it isn't compliant enough, and the older horseshoe magnets tend to get weak with age, it's a common problem with horn and cone speakers of the 1920s. Usually the problem with magnetic pickups is that they don't generate enough voltage, crystal ones put out too much which is why they shunt them with resistors, which often need to be removed when they are replaced with a ceramic cartridge, 3 volts output verses 1 volt. I have heard of rejuvenating the magnet by wrapping a coil around it and applying a DC voltage, but I would look on the ARF for information on this, Doug Houston made some posts on this I think.
Regards
Arran