Warped Sifam meters

[artag]

I'm repairing an old Farnell power supply in which neither of the large Sifam meters on the front panel were working.

On investigating, I found that the black cover over the coil mechanism was warped in such a way that it was stopping the needle moving. Both meters were warped in almost exactly the same way.

This would suggest overheating, but the rest of the meter casing looks perfect. However, those parts could be different plastics : I think a phenol plastic for the main casing and acrylic for the cover 'glass', while the black cosmetic piece is probably polystyrene.

Has anyone else experienced this problem ? Is it likely to be due to temperature, or age ?