A question came up in a previous thread about the SC-2 concerning how the SC-1 was wired internally. I took one out and popped the case to check. All the outputs share the same ground, and it is also common with the control circuitry. There is no isolation between the traics and the internal circuit. The control ground also goes to one side of the input power from the wall-wart if that is used. Obviously, if you have a switch on position one and it's powering the SC-1, it's common is also shared.
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