Computers & Electronics

can an backup-UPS APC screwup power to a cable modem?

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  • Aug 5th, 2021 2:06 am
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[OP]
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Jul 19, 2019
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can an backup-UPS APC screwup power to a cable modem?

since yesterday I have issue with my cable modem (TC4400) and a tech of my ISP told me to try to plug it directly to the wall instead of the UPS, I finally saw that it was rebooting itself after the call (a few time)

I tried to plug it in the wall and so far it look like it worked, I have no idea why ... i don't understand

I had that setup for years now and somehow now I would have an issue? I have other device connected to the UPS and they behave properly, a quick test (from the ups) show no issue at all.

the only other thing that I can think of is since it was kind of colder outside, I turned off the AC and opened the windows, where the modem is was a little bit warmer than normal but not that much (maybe +2-4c versus ac ON)

I also closed the windows and turned the AC on at the same time that I switched the power to the wall

I will try to plug it in again in the ups and see if the issue show up again, maybe it was overheating? that modem get pretty hot

anyway, this is weird / bizarre ... is it a sign that the modem could be dying?
modem.jpg
Last edited by Spirch on Aug 2nd, 2021 9:16 pm, edited 1 time in total.
15 replies
Deal Addict
Jun 8, 2005
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It's possible there's something failing with the UPS electronics, esp if you plug it in and the reboot behaviour continues. I've had those cheap Back-UPS units die in the past, but never act flakey, just plain not work.

Was the modem ever directly in the sun? Like in front of a window facing south/west?
[OP]
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trane0 wrote: It's possible there's something failing with the UPS electronics, esp if you plug it in and the reboot behaviour continues. I've had those cheap Back-UPS units die in the past, but never act flakey, just plain not work.

Was the modem ever directly in the sun? Like in front of a window facing south/west?
hopefully it's not the ups, i got my NAS and my router plugged in but if it is.

also, how would I know if the ups is dying? model is Back-ups UPS 1500 from APC (bough from costco)

and no, never sat under the sun
Deal Fanatic
Sep 29, 2005
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I had the same unit until about 2 years ago when it just died. It would restart itself randomly for no reason.
At first I thought it was time to change the batteries but that didn't help.

My backups was close to 10 years old when it died.
I suspect yours is probably of a similar age.
I think trane0 is probably on to something.
Phils
[OP]
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Phils wrote: I had the same unit until about 2 years ago when it just died. It would restart itself randomly for no reason.
At first I thought it was time to change the batteries but that didn't help.

My backups was close to 10 years old when it died.
I suspect yours is probably of a similar age.
I think trane0 is probably on to something.
my unit is not even a year old and seem to work nicely and the only problem so far, it might not even be related to the UPS, is the modem
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Apr 10, 2011
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Some electronics require a pure sine wave to function properly. This UPS generates a modified sine wave. I don't know why it would have worked fine in the past but if the modem works right in a wall plug... It's hard to deny evidence: doesn't work with the UPS, works plugged in the wall.

If you think the modem is overheating, try putting a small computer fan on the vent holes rigged/plugged via a USB port from the modem if possible.
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plugging modem into UPS through coaxial would cause signal degradation. I tried it with my cable modem and saw that in the stats. The chances of a lightning strike that takes out the modem is fairly rare. I havent bothered to plug it into the UPS for many years for that reason.
[OP]
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Temporel wrote: Some electronics require a pure sine wave to function properly. This UPS generates a modified sine wave. I don't know why it would have worked fine in the past but if the modem works right in a wall plug... It's hard to deny evidence: doesn't work with the UPS, works plugged in the wall.

If you think the modem is overheating, try putting a small computer fan on the vent holes rigged/plugged via a USB port from the modem if possible.
i have put a fan on it yesterday, way cooler now, it range ~35c instead of ~52c, i also did a modem hardware reset yesterday just in case

tomorrow i will put the modem back to the ups, just monitoring for a day to see if i have any issue

i have been using ups with my modem for over 10 years (different unit / battery) so I can't explain why it would go bad, hopefully it was just a fluke or overheating or "something" and everything will go back to normal tomorrow
[OP]
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evilYoda wrote: plugging modem into UPS through coaxial would cause signal degradation. I tried it with my cable modem and saw that in the stats. The chances of a lightning strike that takes out the modem is fairly rare. I havent bothered to plug it into the UPS for many years for that reason.
i use it for power only, i don't put coaxial through these thing, i only had bad experience a long time ago and i decided to never do that again
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Sep 16, 2013
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Temporel wrote: This UPS generates a modified sine wave.
UPS's output approximated sine waves only when they run on battery. When they are plugged in and there is no power failure you get passed through whatever is in your power outlet. And approximated sine waves only cause issues in inductive loads like AC motors. Modern switched-mode power supplies in electronics don't care about the shape of wave. They will work even with irregular waves or plain DC. That's why most UPS's don't even bother to output pure sine wave.
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OP, I was going to say that when batteries in UPS age and go bad many APC UPS's tend to cut power randomly even if there is no power failure. People think that their UPS has died but they just need to replace batteries and it will be like new. But your UPS is relatively new, and you run NAS off it that doesn't reboot I suppose. So, I firmly believe that it was just a coincidence and UPS has nothing to do with your modem. Does your modem keep any logs? Maybe there is some info there.
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Have you tried powering the modem off an SMPS based Power unit, like an old computer power supply, plugged into the UPS? I would guess the issue is with the power brick that the modem comes with.
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Spirch wrote: since yesterday I have issue with my cable modem (TC4400) and a tech of my ISP told me to try to plug it directly to the wall instead of the UPS, I finally saw that it was rebooting itself after the call (a few time)
It's not clear when your modem reboots. Here you wrote it rebooted after you plugged it into the wall (after the call with the suggestion to do so). The problem has been only "since yesterday". Maybe there was some work done in your area and it's normal for modems to reboot in such cases. Another theory - modem's power supply may be failing. If it keeps rebooting, try to find another power supply, maybe from some other piece of electronics with the same voltage and the same or higher power rating (Amp).
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Mar 8, 2013
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Spirch wrote: i have put a fan on it yesterday, way cooler now, it range ~35c instead of ~52c, i also did a modem hardware reset yesterday just in case

tomorrow i will put the modem back to the ups, just monitoring for a day to see if i have any issue

i have been using ups with my modem for over 10 years (different unit / battery) so I can't explain why it would go bad, hopefully it was just a fluke or overheating or "something" and everything will go back to normal tomorrow
Probably just a fluke / coincidence.

The "plug modem directly into a wall outlet" suggestion is a common tech support troubleshooting step because they're trying to remove other variables. Sometimes this gets incorrectly conflated with "never use a surge protector or UPS with a modem" by some techs.

If the modem hardware is flaky, when you unplugged it and then plugged it back to a different power source, you essentially did a power cycle, which some hardware treats differently than a forced reboot with continuous power.

You can try plugging the modem back into the UPS and note how long it runs without rebooting itself. If it does start rebooting, plug it into a wall outlet and note how long it lasts in this scenario.
[OP]
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so i had about 24h without problem on the wall Face Screaming In Fear

i moved it back to the ups and so far so good Smiling Face With Sunglasses

time to monitor and see how it goes :twisted:
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Spirch wrote: so i had about 24h without problem on the wall Face Screaming In Fear

i moved it back to the ups and so far so good Smiling Face With Sunglasses

time to monitor and see how it goes :twisted:

if happens again, try connecting to a different socket on the UPS
or connect a power bar to UPS and then modem to power bar
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