06/03/2026
APC BE###G UPS Repair Note
* Symptom: UPS alarms and/or fails to properly transfer to battery during a utility power interruption.
* Battery: Tested good.
* Root Cause: Degraded electrolytic capacitors on the control power supply circuitry.
* Failed Components Found:
* C14 — 100 µF, 16 V (low-voltage filter capacitor supplying IC1/main control circuitry)
* C13 — 4.7 µF, 450 V (weak capacitance/likely elevated ESR)
* C36 — 4.7 µF, 450 V (weak capacitance; slight bulging observed)
* Repair: Replace C14, C13, and C36 with quality 105°C replacements.
* Result: UPS returned to normal operation and successfully transferred to battery power during AC outages.
One thing worth noting for anyone troubleshooting similar units: a capacitor can be electrically bad without being visibly swollen. In your case, C36 showed physical signs, but C14 turned out to be the primary culprit. Measuring capacitance and especially ESR is often more reliable than visual inspection alone.
