For a particular role of the VC10K3, it was necessary to do a background radiation check, which included the flight deck. All clicking away normally, until the boffin moved the detector to the nav's instrument panel, when it squealed like a stuck pig.... The detector, that is.
It seems that the DF indicator, which had previously done time in the Victor (and probably in a Neptune or something before then), had luminous markings which were still quite active...