The machine that you have described seems to sound like interferential therapy. It is the tingling feeling that is having the effect, which is stimulating the nerves.

The cartilage in the knee very rarely heals. Only the perimeters of the cartlage have a blood supply so would be able to heal, but again tend not too. As long as there is no locking/giving way of the knee you will be able to use strengthing exercises to resolve the problem.