Have you continuity-checked the cable from right where it leaves the SSC all the way to the DB9 serial port on the PC?
I'm also a big proponent of proper grounding - having had my shares of mishaps, blowups, and battles with interference. Also, if I don't understand what a connection does, and it's supposed to be there, then it gets put there.
There's also the possibility of testing ADT on your PC side with AppleWin and a com port emulator. It should catch everything right up to the com port driver. Yup, you can bootstrap AppleWin if you so desire.
Edited by Keatah, Tue Oct 31, 2017 12:11 PM.