SMT1500 buzzing after 1 hour

[hkpolice_apc]

Just acquired an SMT1500 120v and everything works normally except that if I leave it plugged in for more than an hour, it starts buzzing.

I narrowed it down to something in the battery charging circuitry because if I do a self test or unplug the battery and plug it back in, it stops buzzing for 1 hour then it comes back. The noise is a very noticeable low-mid frequency electrical buzz. I have a spare near new condition genuine APC RBC7 pack lying around and tried that with the same results. Have tried 2 outlets on different circuits with no difference and the UPS has NO devices plugged in to it so it was completely unloaded.

There is a tick sound as if something inside is switching circuits right when the buzzing starts, and it happens every hour. Could it be switching to a different charging circuit?

[BillP]

I would suggest going through E-Support via apc.com/support or phone support as I cannot do every single RMA. You can request next day delivery via tech support.

[hkpolice_apc]

Just acquired an SMT1500 120v and everything works normally except that if I leave it plugged in for more than an hour, it starts buzzing.

I narrowed it down to something in the battery charging circuitry because if I do a self test or unplug the battery and plug it back in, it stops buzzing for 1 hour then it comes back. The noise is a very noticeable low-mid frequency electrical buzz. I have a spare near new condition genuine APC RBC7 pack lying around and tried that with the same results. Have tried 2 outlets on different circuits with no difference and the UPS has NO devices plugged in to it so it was completely unloaded.

There is a tick sound as if something inside is switching circuits right when the buzzing starts, and it happens every hour. Could it be switching to a different charging circuit?

[BillP]

This UPS has a single charging circuit with multiple modes. If the hum is loud and other units do not have the same hum, then I'd have to think there is a problem with it. It's tough to judge if its normal or not because depending how loud or quiet it is, it may seem loud. Or perhaps the fan is also making noise and making it louder, not sure.

[hkpolice_apc]

I hooked up the UPS to a power meter with a small load of modems/routers on the UPS itself. During the first hour, the UPS was pulling 39-40w from the wall socket. When the buzzing starts after an hour, the power usage dropped to ~23w. So the sound is caused by this "lower power usage" mode of the charging circuitry. Is there any way to disable this mode via firmware or console cable?

[BillP · Accepted solution]

No, unfortunately, nothing related to the charging circuit is modifiable by the user.