I've got a device similar to thishttp://patentimages.storage.googleapis.com/US6400995B1/US06400995-20020604-D00002.png
or this: http://patentimages.storage.googleapis.com/US6249417B1/US06249417-20010619-D00000.png
It has a coil in it switched by a triac.
I opened it up and found a 47ohm NFR was blown, so I replaced it with a 47ohm resistor.
I'm not sure if it's working (switching coil on /off), I'm looking for a test to discover that.
If I were to meter the mains for frequency, I wouldn't likely see the coil switching as it still is 50Hz (they claim it uses electromotive force to disrupt noise on the cable to deter pests).
I did feel the coil after around 10 mins of running and it was midly warm, which I think would happen if it was switching, but then.. what happens if you have a coil with active on one side and open for a long period?
I checked the MOV inside the device, it's got very high resistance across it (normal).
I checked the IC (a 14 bit counter) for frequency output to the triac, and this was 1.5Hz.
I checked resistance across coil at 4.7kOhm.
Any thoughts on how you'd test it was working?