Tim:
I have a UPS sitting next to me, right now, in pieces, which (half) died in a most peculiar manner:
A burning smell was eventually traced to it. There's no visible signs of burning, and no schematic available for the model, that I can find. A rather acrid smell, not one I'm used to with component failure, I'm beginning to suspect a large AC transformer.
Mauricio Tavares:
Did you check the impedance of the transformer?
Not yet, the lack of schematics put me off, I'd really have to delve into reverse-engineering it to fix it (assuming I could get any needed parts), it's a very old model (SOLA 310). If the transformer had cooked one of its windings, I doubt I'd find replacement (there are multiple windings), and the board is chock full of ICs (very few discrete components).
I never really bothered with UPSs at home before, but we've been getting lots of little power cuts the last year. Previously things were very good here. You might have gone a year or more without any interruptions.
The other thing is that this only has a few minutes of supply capability, allowing you to ride through blips on the mains, and cleanly shutdown during longer power failures, but not carry on working for a prolonged period. Though my modern low-power PC gives much more runtime than the UPS's prediction in its manual.
And it was quite noisy in stand-by mode, while AC powered, it had quite a hum to it. Bad enough that I'd run it in the adjacent room, with long leads running to and from it.
Much as I dislike waste, and I repair a lot of equipment, I've been considering something newer with more capacity.