These Revox B77 Reel to Reel Mains suppression safety filters suppress the spark from the main switch and protect it from shorting due to arcing.
 
Highest possible safety regarding flammability due to the metalised construction.
See the photos for the location of the original filter inside the machine.
 
These often burn out, removing this protection and sometimes shorting out the on/off switch.
Easy to replace with a soldering iron.
 
Brand new.
Free postage.