Quote:
Originally Posted by Radio Wrangler
The ability to understand signals and circuits and to work out faults and fixes as if they've never been seen before is more necessary nowadays where there aren't loads of workshops doing repairs in industrial quantities.
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David,
Yes this is my preferred method.
But having said that there are least two scenarios where it can fall down.
One is when there is poor or no documentation. Often a lot of modern equipment; there are no schematics, as often there are none for computer cards and motherboards. Then swap out (board jockey) has to occur which is very unsatisfying. I repaired a faulty mobo recently, only because the failed IC was heat sensitive and it was found with freeze spray, but I don't like it.
The other type of fault, which is even more diabolical and it is becoming more and more frequent, with the abolition of lead based solder, is tin whisker shorts.
The philosophical system of understanding a circuit and therefore which faulty part might be responsible for the signs of a fault, get tipped on its head. This is because the tin whisker can create links between sub-circuits to create entirely new circuit configurations that are not possible from the schematic and therefore produce weird and inexplicable faults.
So it is possible to have a faulty board in front of you on the bench, but not a single component on it is actually faulty. I had this in one of my 2465B scopes and it took me a week to find and fix it, one of those ones where you wonder if you are losing your mind as nothing is making sense:
http://worldphaco.com/uploads/TEKTRO...K_LEAKAGE..pdf
(For the OP: fortunately this sort of thing is very uncommon in old transistor radios)
Electrolyte from leaked caps on a pcb with high impedance circuitry can also do this sort of thing too with multiple conduction paths between tracks.
So there are always faults that cause extreme difficulty and frustration, no matter how experienced the technician is, or what fault finding techniques they practice.