I've never heard that, but back when telepehones uses carbon mikes, it was
popular to fix them by removing the cartridge inside, and banging the
cartridge on a table top.

Geoff.

Oh, gosh! I didn't know the cartridge was supposed to be removed before
banging. I thought that the fix was to just whack a carbon microphone
on the nearest indestructible object. The mikes that came with the BC-191
and/or BC-375 often had metal housings and could be used as hammers
in an emergency.

Old Chief Lynn, W7LTQ