No problem soldering to that pad.
AC on sys 11 is not tied to ground. You must measure with one lead on supply and the other lead on the return and your meter set to AC (I know.... duh but I’ve tripped myself up with that one). Supply and return is each side of the coil on the transformer. Typically all the yellow wires are tied to one side of the transformer and the striped wires are all tied to the other.