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Old 06-08-2021, 01:45 PM   #7
Badcaps Veteran
Join Date: Dec 2010
City & State: Alberta
My Country: Canada
Posts: 2,164
Default Re: APC Smart UPS 620 not working (overload light on)

For your time and money, just outright replace all the electrolytic capacitors.
These UPS are powered 24/7 and are warmer running over 35C so the caps just dry out. The batteries are worth more than the entire unit lol.

The circuit is centered around a big iron-core transformer, it puts out about 18VDC when mains are present, and during a power fail the mosfets start switching battery voltage to the transformer at 50/60Hz. So the transformer is being used in reverse then.
There is also a (relay) stepped autotransformer to boost or drop mains to deal with sags and surges.

I think the battery charger is LM2575 with two 330uF 25V output capacitors C41, C44.
Did you replace these with some small crappy tiny parts? The picture shows something real small compared to the silkscreen.
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