OK Einstein!
Firstly, he said he's no use at diy so....
Secondly, I would hazard a guess that most faults with equipment is from bad/corroded connections.
Basic basic fault finding as you put it, may seem basic for one person and like rocket science for another. If everyone is making as much as they claim on here then why bother wasting your time on a diy box which doesn't save you much in the long run.