3000 VA UPS Service Manaul of Fiskars Power Systems Make

Oh, I used 2N2907A to replace the drivers Q29-Q32 and STP40NF10L to
replace the ouput transistors.

If you look at the 20 output transistors, they're all the same. Mapping
it out a little shows its a full bridge driver with two totem-pole type
ouput columns, the primary of the transformer is strung between their
middles. Further back in the circuit there are two PWM controller chips,
so I guessed they would be used to switch the stages on and off to
shape a sine wave - perhaps also using that magnetic sensor thing to
help, but not sure how.

In my case, output rows 1 and 2 had blown, indicating they were both
being driven ON, which should never happen. Over 200 amps straight from
the battery would fry them in short order. The driver for row 2 (Q29)
was also fried, and if that leaves row 2 ON, it will short out 50 times a
second. This is what I think happened, but there are alternative
scenarious.

Oh, I used 2N2907A to replace the drivers Q29-Q32 and STP40NF10L to replace the ouput transistors.
If you look at the 20 output transistors, they're all the same. Mapping it out a little shows its a full bridge driver with two totem-pole type ouput columns, the primary of the transformer is strung between their middles. Further back in the circuit there are two PWM controller chips, so I guessed they would be used to switch the stages on and off to shape a sine wave - perhaps also using that magnetic sensor thing to help, but not sure how.
In my case, output rows 1 and 2 had blown, indicating they were both being driven ON, which should never happen. Over 200 amps straight from the battery would fry them in short order. The driver for row 2 (Q29) was also fried, and if that leaves row 2 ON, it will short out 50 times a second. This is what I think happened, but there are alternative scenarious.