It should charge at a lot less than 2000 rpm... but without knowing what you've got on it for a pulley... I say 2K to be certain. Some will charge at 800. Some less if they've been setup with a low cut in.
If all has been done and it still won't charge, replace it.
Just the same... I'm not a big believer in the idea that 2 have now burned out for no reason... Make sure there isn't something grounding out and making smoke somewhere else in the system. That WILL fry the reg... sometimes.