FINALLY got around to testing C-7 light strings with my Home Depot tester http://www.homedepot.com/p/LightKeeper-Pro-Light-Tester-1203-CD/100686478
It was actually quite easy to use , following instructions . Fixed about 70% of my box o' 1/2 - not-working strings of lights. The light/sockets I COULD get out mostly a wire on a bulb was corroded off or broken. Using this tool made it surprisingly easy to segregate WHICH bulb(s) were causing the problem. I used bulbs from other strings squirrels had lunched on as spares.
So only buy strings WITH removable light sockets (some aren't), and this handy tester that will also test individual bulbs, i.e. Noma's, as well as the wires themselves . Even managed to cut out and reconnect some chewed up sections of wires by carefully separating and re-splicing individual wires !
Bought a bunch of fuses, replaced some, but wasn't any help, problem, again, was usually corroded off bulb wires .