Finally, after all this time, I've stumbled onto the solution...
So, the problem turns out to be in the power supply for the SC1200 console. I never considered the power supply as a possibility as all other console functions have been working normally (I have been using my cell phone, which the console dials upon a tripped sensor, as an "alarm") and I'd assumed the console power supply was just that - a simple power supply, and that all other functionality was embedded in the console. Apparently however the power line signalling is part of the power supply itself (other members may have more info on this aspect).
p.s. I happened to find this after I obtained several other alarms from a friend and none of them worked either. I then happened to try my console with their power supply and voila - problem solved. I have since opened my (old) power supply and did not see anything grossly amiss but will look more closely at it when I'm sure it's safe (I am not expert at this and wanted to be sure any capacitors it might contain have time to discharge). I will report any further findings. Thanks to all!