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I have a Motorola SB6120 cable modem that I bought last November or December. It's been working great since I got it. I have the cable modem connected to an APC surge protector and turn the surge protector off at night to save a few pennies on electricity. About 2 weeks ago on a Thursday morning when my wife turned everything back on, there was no Internet. Stayed down all day, I played with it when I got home, and I ended up scheduling an service call for the following Saturday. Later that night, I got creative, reset the cable modem to the factory defaults, left it off for about half an hour, and when I turned it back on - Internet!

We were fine until the following Thursday (Thursday thing is weird too) then the exact same scenario. Didn't come up in the morning, stayed down all day, but this time even after trying similar tricks from previous outage, nothing got us back online. Waited to Friday morning to see what would happen over night, and after still nothing scheduled service call for following Monday. By the way, no internet all weekend - really sucks!

On Monday, tech comes out, says signal is weak, thinks cable is damaged where neighbor behind us put in fence last year. On Tuesday, repair crew comes out and does something out by fence, but still no internet. Tech comes out later that afternoon, is able to get cable company's modem online (Scientific American) but my modem won't.

I ended up calling Motorola support, and I learn the power levels the modem is reporting are too low. Upstream power is 0, signal to noise ratio in the low 20s. So he says the signal strength is too low. I start paying closer attention after that and begin to notice a pattern. When I first turn on the modem, it reports good signal strength: signal to noise of 37, downstream power of 3, upstream power of 49. But after 10-15 minutes, signal to noise drop to low 20s and upstream drops to 0.

So now I'm not sure what to do. Call the cable company and demand they fix the frickin' line? Or call Motorola and insist they RMA the modem? I don't have a way to test my modem elsewhere, so I can't really verify it's still good. Cable company says signal must be fine if their modem will connect. I've also verified they have the right mac address.

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up vote 3 down vote accepted

If the signal drops in a reproducible pattern after the modem has been turned on for N minutes, that suggests a problem with the modem, not the cable. The cable is essentially passive and can't tell if the modem is turned on or not.

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I know just enough about electronics to sound like an idiot, so bear with me. Does having multiple equipment pulling from the service line increase the electric load on the line? In other words, could something be aging or damaged with the cable company's equipment that can handle a certain load that then overheats or overloads and starts degrading the connection? – TheOtherTimDuncan Jul 2 '11 at 12:58
Nope. The power level in the main cable on the pole (or wherever) is much, much higher than the level delivered to your house. Each "tap" on the cable siphons off a tiny amount of that power, in a well-controlled fashion (the "tap" is designed such that it CAN'T pass more than a small amount of power). And that amount of power is siphoned off whether the receiving equipment (such as your cable modem) is turned on or not. Even if, for some reason, your cable (or a neighbor's) were completely shorted out, the additional load on the main cable would be barely measurable. – Daniel R Hicks Jul 3 '11 at 3:06
you were right. Bought a new modem today, and it works just fine. <SIGH> Really didn't want to have to spend the money to replace a modem just a few months old. Also, per your comment about my powering the modem off every night, why would you recommend against doing that? – TheOtherTimDuncan Jul 3 '11 at 19:40
The cable company sends commands to the modem to reconfigure itself under some circumstances. If it's powered off it may take several minutes to get things sorted out when it's first turned on, and the opportunities for a misconfiguration are increased. – Daniel R Hicks Jul 4 '11 at 1:11

Could you try with another modem from an friend or neighbor and check your internet if it works...and the definately your motorola modem is culprit. See if you can get it returned.

My friend had similar problem something like the modem periodically resets very often like every 3min, for him the problem was with DC Power adpater, when he replaced with new it all worked fine.

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I have a Scientific American cable modem from the cable company temporarily, and it's working fine. But that modem is plugged into a different surge protector, so you've given me an idea to try... – TheOtherTimDuncan Jul 2 '11 at 12:09
No dice on moving the power cord to a different surge protector. After about 15 minutes, went signal-to-noise dropped to 11, upstream dropped to 0 and downstream dropped to -11. – TheOtherTimDuncan Jul 2 '11 at 12:52

Before you start, some of the Motorola's have a sleep button right on tome. If it happens again, push that first to check if it is just sleeping.

That said, I strongly suspect your cable modem is bad, and just failing intermittently. You are probably not going to be able to test a friend's since your specific modem is probably being authenticated by your ISP (the exception is if it can have the MAC address changed). I would not even wait for the cable company to come out again...assuming that you rent the modem, take your modem to cable company's the nearest location, and swap it out. That will answer your question.

And for Pete's sake, just leave the thing running from now on. It uses pennies of electricity per month. I did a search, and they cost about $10-$15 per year to run. So what are you going to save, maybe $5-$7 per year? Not worth it.

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Slightly off-topic, but one of my pet peeves is vampire loads. I keep my PC on 24-7 because I use it as a DVR, but it at least goes to sleep. I have my router, cable modem, TV, receiver and other little stuff turned off via the surge protector every night. Individually, not much. Altogether - especially when times are tight - worthwhile. – TheOtherTimDuncan Jul 2 '11 at 12:55
That makes sense for some stuff, but I would advise against it for your modem. – Daniel R Hicks Jul 3 '11 at 3:10

As others have suggested, it looks like the modem isn't getting the job done. I'm leaning this way because the cable company's modem worked and your current modem starts out fine but shows weak signals after running a while. There's no downside to replacing this modem and replacing it actually helps you with this issue.

If you replace it and the problem goes away, you win.

If you replace it and the problem doesn't go away, you also win. Because you did as the cable company suggested, you have leverage to get them to come back out and revisit the power level issue.

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