efx 500 echo gets stuck

my echo effect gets stuck when i toggle the switch up or down.  all other effects work perfect, it's just the echo that is experiencing issues.  i don't know what to do.  any ideas on how to fix it?

the echo literally gets stuck and when i toggle the switch back to off it keeps echoing until i press another effect button.  if i hit 1/4, 1/2, 1/1, 2/1 you can hear the echo effects one on top of the other and it won't stop unless i hit another effects button.  any ideas would be a great help.  i have turned it off, unplugged it, left it unhooked for days and it still does it. 

thanks in advance