Finally got around to taking the thing apart. There was a tiny almost
invisible green-colored fibre no more than the thickness of a hair lodged
between the '2' ferrite core and its coresponding capacitive film backing
acetate pad. Had to examine it under a stereoscope to spot it it was that
tiny. Brushed it away and the pad's working fine again. That 1/000"
obstacle was the issue! Hopefully working again for another 25yrs now.
Thanks, all.