The question is showing us the following scenario: Nelson's hobby is tinkering with small appliances. Tinkering with implies that Nelson is unskilled at his hobby. You want to change the flavor of this sentence to show that Nelson is, in fact, quite skilled at his hobby. What we need to do, is to find a similar, but a positive word to substitute for "tinkering" - and the word will be "repairing" because it shows us that he is actually successful at what he's doing - his actions improve the small appliances.
