I recently open up brother's APC sine wave UPS to fix and trying to repair some damage that usually happen over time due to age by replacing components I suspect one by one.
When I start doing AC output LC filter, I notice the UPS triggering surge light on surge protectors connected to UPS output again and again but only when switch to battery operation.
Currently, UPS have 6 AC output in parallel and 3 surge protectors from vendor A, B and C hooking to them and 3 directly to other device.
Vendor A very well known in U.S. and B and C only midgrade from Costco.
Previously, before I start fix, only surge protector from vendor A trip some time, but now all surge protector trip the moment I cut UPS input power making it switch to battery operation!
I not be explain this besides that some change I did is making output sine wave peak perhaps swing above (1.7 * 120)V AC?
But I only change component with expensive high quality parts and as per original datasheet spec, so UPS is actually better than new (but brother not willing to believe as he sees opposite and he paid for all expensive high quality parts himself after I request him with confidence).
Old circuit pics still visible: Old circuit pics