I recently purchased a 1943 Vibroplex Original bug and upon receipt of the key I quickly made a couple minor repairs, then proceeded to put the key on the air.
All was well at first but I soon noticed that once in a while the Dah contact would occasionally not make good contact while I was sending. I had of course already cleaned all the contact surfaces with a pencil eraser so my first reaction was to simply pull a strip of paper through the Dah contacts to dislodge any particle of dirt. At times this seemed to fix the problem but the problem would always reoccur.
I finally found the problem, it seems the silver contact was not making good electrical connection to the Dah adjusting screw. I removed the contact from the adjusting screw, cleaned the contacts small mounting stem and afterwards lightly tinned the small stem with a thin layer of solder. I then pressed the contact back down into the Dah adjusting screw.
Results: Intermittent Dah contact problem is now 100% resolved.
Donnie / WA9TGT