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The SC unit uses either a TRIAC or RELAY...for power switching. Likely a triac.
There also is a device called an optocoupler--that drives the triac and isolates it from the AC line. NOT like a modern one--this will look like a tubular or possible a little squarish "blob", with a pair of wires on each end. Sometimes..they short--and THEN...the chassis cannot be turned off. Other times...they will only partially open, and "clip" the signal top the triac..causing an impure output...and the power tranny will NOT like it..
There also may be a disc cap on the triac...if it shorts...the same result--no turn off.
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