Need tips in fixing APC 1500VA UPS having buzzing noise

[Behemot]

So, you have changed the caps and you have changed the batteries, and it still remains, correct?

Just a wild tip, are all the power connections (cables, joints etc. from batteries to board/transformers) OK? Because dying batteries increase their internal resistance, and if there is some transitional resistance somewhere, maybe it has the same results.

[Cartel]

Sorry to bump an old thread but I have bought a new APC 1500 BX1500M and it came today.
It buzzed on the battery power mode, and not just a soft buzz.
It also had a horrible transition time from AC mains to battery backup.

My old XS1000 from around 2011 does not buzz on battery power and the transition time is fast, almost not noticeable with a light bulb going.

I boxed it up and returned it within 4 hours of having it.
I suspect a stage of the inverter is open, a bad mosfet and it still runs but its not happy.

Regardless, from my hands on findings on non pro apc backups is buzzing is not good and return it if you can right away.

a slight hum on charge is normal after a outage but its only there if you listen closely. the buzz I heard today is like when you turn your washer on but you forget the water.

[Behemot]

Some units do that, but they're cheaping it out constantly, waddaya expect.