Troubleshooting F06 Error on APC UPS BR1500G - Part 1

Trying to repair this APC BR1500G UPS. It triggers an F06 error after starting up and performing its self-test. According to the manual, F06 = relay contact welding. I've tested all 7 relays out of circuit and none of the contacts stick (I've also replaced them all with brand new ones).

interestingly, the unit will work if supply voltage is increased enough to put the unit into Automatic Voltage Regulation (AVR) mode. I did this by increasing supply voltage to ~130V. It will run fine at this voltage. I can simulate a power failure and it will run on battery power fine as well. I can also put a load on the unit while in AVR and it will handle it fine, too.


However, there's definitely some significant ripple (~10V p-p) that appears/disappears at regular intervals. You can clearly see it on the oscilloscope. It does not appear when the unit is running on battery power or mains only.

I'm beginning to suspect the transformer (maybe the 110V tap is bad?), but in my previous video I HI-POT tested this transformer up to 1000V and it passed. Anyway, any troubleshooting tips/suggestions/help would be much appreciated!!!
Part 2 here    • Troubleshooting F06 Error on APC UPS ...