This is how the drilling machine return spring looks when I opened the cover:
A previous owner did a not very successful repair and modified the internal attachment of the spring. As a result, spring was badly deformed and have a unusual feeling during the descent. I extracted it by pulling it out of the cup and then giving it a quarter turn to free the external end from its attachment (it has a rectangular shape as you will see next). These are the pictures of the empty cup, the spring, and the modified shaft. Originally the internal attachment was means of a pin in the shaft while now it uses a slitted shaft.
The decision was to make a new spring. It was comisioned to Metal Springs Valles according to this specifications. They made it in a week. This is a picture of new and old springs:
Finally, the new spring was mounted in the receiving cup: