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APC Back-UPS XS 900

Discussion in 'Troubleshooting and Repair' started by virtual1, Mar 29, 2012.

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  1. virtual1


    Mar 29, 2012
    Hello, I'm new here, found this group while looking for information to repair my UPS. I see a few other posts here concerning repair so hoping someone has experience with this issue as the symptoms are not quite the same as I'm seeing around.

    This is an older unit and has worked fine and gone through at least one change of batteries, it's running a smallish load for its size, (<50w) several external hard drives attached to a server. Its size is used for extended runtime, not capacity.

    Server got grumpy with me the other day and I find the UPS is not providing power and is flashing the Overload light. It flashes four times, pause, and repeats. I assume the alarm would be going off, but I have disconnected it along with all the other UPSs around here, you can imagine the noise when there's a power failure at 2am and the whole house sounds like a smoke detector. I wasn't very successful at googling for the four flashes indication. No other lights come on at the front, specifically not the Replace Battery.

    When I power it off and turn it back on it does its self test and then provides power normally as though there's no problem, even when attached to a 100w light for load. It also has no problem cutting in and out if I pull line power while it's running the HDDs or the lamp. Then after a random amount of time (seen 10 minutes and seen 4 hours) it will fall back to this overload state. There is no apparent problem with any of the attached hard drives that would cause an overload. I have tried changing the batteries for new ones as one of the pair was a little low, but that did not cure the problem.

    I suspect it is doing its periodic battery self-test and is failing at that point, but I don't know why its self-test at power-on doesn't have any issue. I've had these apart before and they're sufficiently complicated to make troubleshooting non-trivial, and I can't reproduce the fault condition on demand which makes it a good deal more tricky to troubleshoot.

    I'd place myself near the high end of amateur electronics tech and have useful tools at my disposal (dvm,scope,varactor,etc) as well as spare parts. I do not have a schematic of this UPS. I have had very similar problems with a couple other smaller, less important UPSs and have simply replaced them, but this one I would like to fix if possible. TIA
    Last edited: Mar 29, 2012
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