1. attached images of the board with visible values of R135, R101 and R143
2. unfortunately, I don't have schematic nor I have reliable way to determine value of zener diode, so can't help here.
3. it is easiest to replace it than to fiddle with oscilloscopes. programming is not needed, just replace it


I've fixed 5 of 6 PR1500ELCDRT2U (230V/European, rack version) by replacing many capacitors on the board, one on hot side, other on cold.

6th UPS that I have has an additional problem, with output voltage (BAT+/BAT-) being too high (58.5V), while others...