I've got a 37971 culvert loader that was working just fine. The "Transformer" terminals are wired to my accessory transformer, and the "Switch" terminals are wired to a SPST toggle switch. I've also got an SC2 that is powered from the same accessory transformer, and configured for 6 accessories (no switches). When I added wires from my SPST toggle switch to the SC2 ACC1 position in parallel (so the culvert loader could be turned on by the SC2 or the toggle switch), bad things happened. First, the 10 amp fuse on my accessory transformer blew, so something was obviously shorted out. And now, even after completely disconnecting the "Switch" terminals on the culvert loader, it's as if the "Switch" terminals are always connected, so the loader is always "on" when a train car triggers the IR sensor. Otherwise the culvert loader still works (including the IR sensor).
I opened up the culvert loader and got that "burned electrical" smell, and indeed one of the traces on the small circuit board is burned (though not all the way through). I'm assuming there's a transistor in there somewhere that is now burned out, causing it to act as if the "Switch" terminals are always connected.
But I can't figure out what caused this. I seem remember there's something special about position 1 on the SC2 being connected to the SC2 power inputs. Could there have been some kind of short circuit through that path? Neither the SC2 nor the culvert loader specifically marks the "U" and "A" terminals from the accessory transformer, so I didn't take any particular care on which was which on either device.
Two questions: Any thoughts on what could have caused this? I'm kind of scared of position 1 on the SC2 now. And does anybody know anything about the circuit board in the culvert loader, such that I might be able to replace a burned out transistor? I can ordered a replacement circuit board from Lionel for $22.50, but it would be more satisfying (and cheaper) to find and replace just the bad transistor. :-)