The brown stuff is very likely left over flux. If you use IPA and a toothbrush, it should come off eventually.
In a computer where the modulator can be switched between two channels (e.g. the TS1000), there should not be any solder linking the switch solder points. You will get continuity through the switch contacts depending on which position the switch is in. The centre pin of the switch goes to the modulator. One outside pin of the switch goes to 0V/GND. The other outside pin of the switch goes to +5V. So, on one position, the switch sends +5V to the modulator, in the other position it sends 0V to the modulator. If the modulator is not being used, the switch can be desoldered and removed.