From the side door of his home on Moffat Street, John Clark hands a laptop to a customer and begins explaining his work.

He replaced the battery, he says, and updated some software, and while it won’t be as good as new, it should run better than before.

A piece of his own multi-colored geometric stained glass hangs in the south-facing window, casting tiny rainbows across the floor as he speaks.

Clark became the “Mac Doctor,” the name of his computer repair business, “totally (by) happenstan...