APC Smart UPS 1250I turn off event hardware issue

[BillP]

Hello,

I received an old, defective APC Smart UPS 1250I unit (SU1250I / AP1250I). The seller told me that it had an issue of producing around 300-400VAC while running on inverter. I did not have the chance to verify this issue, as he had tried to repair the unit beforehand, and I received it disassembled. He replaced the ADC0838 IC and a TL064 IC. In addition to that, I replaced other ICs in connection to the inverter, eg. LM358, TL062, CD4011, MM74C14. Basically, only 2-3 ICs were not replaced (CD4011 and MM74C14 in control logic & DAC). I also replaced all of the mosfets on the heatsinks, mosfet gate transistors and diodes and 20 ohm resistors + all of the smaller electrolytic capactiors on the board.

After assembling the unit, I successfully started it on battery [cold start], and it produced 230V output as intended. After that I turned it off, connected it to mains, and turned it on again. The batteries started charging, the self test ran successfully (transition to and off battery worked as expected). So far everything went good. Then, with the mains still connected, I pressed the turn off button on the front panel, and the mosfets blew up and the unit died. These units use some relay trickery for turning on / off, so I suspect that the inverter output was somehow switched against the mains, causing a short circuit which blew the mosfets. The short circuit was limited to around 10 amps, since the input circuit breaker rated for 12A did not trip (the transformer in the unit probably limited the current). Could anyone with some service experience help me to identify the cause of this problem?