Mk1 Collar Problem

[sjy 16]

Got a mk1 grey knocker box with 2 collars 1 collar works perfect but cant get other 1 to work tried swopping cap of good 1 onto dodgy1 still wont work,is there anythink eles i can try,cheers

[Leeview 791]

try cleaning the copper stud lightly wire wool or emery paper

Y.I.S Leeview

[3175darren 1,100]

try cleaning the copper stud lightly wire wool or emery paper

Y.I.S Leeview

and the cap threads mate inside and out,