Back in 2012, I purchased an APC RS1500 UPS for $20. The guy I got it from could not get it to work and said APC tech support told him that his house wiring was to blame. This was right up my alley!
![](https://komodo-dragon.net/wp-content/uploads/2022/08/APC-RS1500.jpg)
After running a few tests, it was apparent that it would not stay on-line, switching to battery mode randomly. I could see why tech support suspected a noisy line supply. However, putting my scope on the input, it was clear that the line supply was very clean: the first clue.