I received this heavy UPS APC SUA3000RMI2U in order to fix a strange behavior during switching operation between battery / main power. Value of output voltage was more than 220V for a short period during battery operation startup. After that the UPS locks to a fault state (all LEDs flashing). Output voltage was around 400-500V and load connected has damaged by the voltage peak. The malfunction can be recognized by a high frequency sound generated inside of transformers. The oscilloscope reveals abnormal high frequency, around of 1 KHz, instead of 50Hz wave. All electrolytic capacitors were replaced without any improvement. The malfunction appears due improper compensation of feedback loop. After many measurements and experiments, C108 and C110 smd ceramic capacitors were removed. These capacitors are located near to U19 IC. C108A can be modified for best overshoot and stability during battery operation startup. After that APC UPS was tested several times with good results.

Here you can find useful information regarding operation of APC UPS: Russian page Mirpu.ru.

I made this page in order to bring some information, necessary for your troubleshooting. Schematic diagrams of SUA/SU series are almost same and are hard to be found...

APC SUA3000RMIU2U cabinet
Feedback capacitors location

Feedback capacitors schematic
APC SUA1000 schematic (sorry for poor quality)
APC SU1000 schematic
APC SU2200/3000 schematic
APC Smart UPS 1400 schematic




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