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I'd noticed a minor problem a while back: One light on a 3-head home signal never showed Clear (green), only Caution and Stop. I sat down (under the layout) last night with the diagram to diagnose it. I'd used female spade terminals for connections to the Circuitron SD-3, rather than solder, so first step was pull the Y wire off. It stayed yellow, so the problem was in the SD-3. I had another on the layout whose signal hasn't been built, so I swapped them. Now both work. Maybe a bit of cut-off wire had fallen into it and moving it cleared it out, but a clear case of Problem Disappeared In Process of Diagnosis. 
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