i just repaired 4 of them,the shaft is seized,i had to heat the threaded area and carefully twist and then i cleaned the old grease out of the shafts and holes,then i have been putting type of grease simular.. (from sony for metal parts,it's yellow).
they seem to test fine now,moving freely..i have 2 more to do..
i can see what you say about the wiper,these are interesting controls..i wonder if they get scratchy..
thanks
terry