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My good old UPS started behaving weirdly. On power loss it's output voltage went down and up so that the PC behind it booted and started bootup when the UPS was running on battery power. First I thought that the batteries were dead but it wasn't them. When I shut down the UPS, the LCD screen said: "DC bus fault".

This can be fixed by changing capacitors C10 and C14. C10 = 2200 µF 16V and C14 = 100 µF 63V. The capacitors can be located from the upper part of the main board near the left end of the big cooling rib.

WARNING

There might be dangerous electric charge stored in C40 and C42 which are the big capacitors on the lower area of the main board. It's not a good idea to touch any components near them either. Do not make this fix if you aren't sure that you really can do it. You break yourself and your equipment at your own risk.

Remember, this looks like design error, so be prepared to fix the UPS again and again.