Comcast cable modem problem

Let me start by saying I know very little about networking issues. Hopefully, at the end of this, someone can tell me what the next step is to diagnose my problem. I live in jacksonville, Florida, and have Comcast as my broadband provider, using a Motorola SB 5120 cable modem, provided by ComCast. I know that ComCast has been having some problems with their service lately and that may be part of my problem. I've had broadband for just over a year, and other than some problems when I first got it, it has been working without a hitch for a year. Over the last couple of weeks my broadband connection would go out, with the Receive and Send lights on the cable modem blinking, and the Online light being dark. I tried unplugging the power supply to the cable modem for a minute or two, and then plugging it back in, but the modem would not reconnect. I called ComCast, but other than saying their service was down for a while, or offering to send someone out to look at it, that was about it.

Here's what I did find out. If I disconnect my cable modem completely and take the modem and the power supply (AC) cord into my attic, I can plug the cable that supplies my house directly into the modem and plug in the power supply cord, the modem will connect and all four lights (Power, Receive,Send Online) will be on (not flashing). I takes about

2 or 3 minutes to disconnect the modem, carry it into the attic, and plug it in, which seems to "reset" the modem. I can then disconnect it, take it back to my computer room, hook it up, and it will work for fine for a few days, or until the next service interruption by ComCast.

One thing to note about the way the modem is connected to the cable supply in the attic. The cable comes into my house into the attic, and then goes into a cable amplifier. It was an Electroline model 2400 4 port amplifier. When I first got broadband, the cable modem would sometimes lose the connection, (the send and receive lights would flash, and the online light would be dark). So I hooked the cable going to the cable modem directly into one of the Out ports on the Electroline model 2400 4 port amplifier. The cable that goes from the amplifier to my cable model is 50 feet long. I've had no problems for a year. However last week when I went to check the connection I noticed that the Electroline amplifier was really warm, almost hot to the touch, too hot to really hold in your hand comfortably. It was about 75 degrees F. outside, and the attic was a little hotter, but not nearly as hot as the amplifier was. After a couple of times of connecting the modem in the attic (to "reset" it), I bought another 4 port modem off of eBay. It was a Scientific Atlanta "new in the box" amplifier. (Note: both the Electroline and the Scientific Atlanta amps were 1 GHz amplifiers, and amplify 7dB). I replaced the old one with the Scientific Atlanta amplifier, and this worked fine for about 4 or 5 days. Then there was some kind of interruption, and I had to go back into the attic, and hook the modem up directly to the cable coming into my house and the modem "reset" itself.

OK. After all that explanation (sorry it was a so long), can anyone answer the following for me, (please)?

1). What's going on here and how do I fix it? Any insight would be greatly appreciated. 2). Can a cable amplifier go "bad". It was only two years old. 3). If a cable amplifier is really warm, borderline hot, is that operating normally? 4). Do I need to get another amplifier and put it right in front of my cable modem? 5). Can you put more than one amplifier on a line to your cable modem? Is that OK? 6). Is 50 foot to long a run for cable when used for a broadband connection. 7). Maybe best question of all. Is the reason that the cable modem will connect when hooked up directly to the cable coming into my house because the signal is stronger there than after it comes out of the cable amplifier and goes down a 50 foot length of cable to my computer room? Is there something I can use, or buy, that will test the strength of the signal? Something that doesn't cost a fortune, and an average guy can use? 8). Is there something that others are aware of that is happening with ComCast that may be causing this?



Reply to
Loading thread data ...

Wes, I'll bet the cable "amplifier" is only for the TV side of your cable and you cannot use it on the Internet side. Somewhere in your wireing should be a "filter" (sometimes blue) that is on the TV side. polesplittermodemcomputer \==>filter==>amplifier==>TV

Reply to
Rick Merrill


It depends on the design of the product, but IMHO an amplifier that runs that hot is either a poor design or is operating improperly. The last thing you need is a fire hazard in the attic, where it can burn for awhile before you're even aware of it.

Please, no more amplifiers. I would remove the amplifier, especially if it was added by someone who doesn't work for the cable company, and replace it with a cableco-supplied splitter. There will be a drop in signal levels after this change, but it's up to the cable folks to provide the proper signal levels to your house. It's not a homeowner's duty to amplify it because amps don't always play nice with cable Internet signals, especially the upstream levels.

Same as #4.


Does the 5120 cable modem have a user interface at ? If so, check your signal levels there and tell us what you see listed for Downstream Power Level and Signal to Noise Ratio, and Upstream Power Level.

Reply to
Bill M.

For starters the modem is designed to work at a certain signal level (input +8 to -15) and will transmit upto 60db out.

Your problem is not the AMP but the way it is wired - you need to hook it up as follows -

cable in from street --- 2way splitter - 1 leg to your amp for the TV's and the other leg dedicated to the modem - the amp you have will work for return but may not have the 5-50MHz fully functional

Any Questions feel free to email me


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Reply to

Rick, Bill, CableGod,

Hey, thanks a lot for the responses and good advice. I really appreciate it. After reading your replies, and in response to, I gathered a little more information. Incidentally, my broadband seems to be working fine since last Sunday noon. But that doesn't mean anything much.

I did not see anything that resembles a filter on any of the cables in the attic. I think there is a box on the outside of the house, I'll check that for a filter of some sorts. The cable comes from the street about 300 feet down the driveway to the back of the house, into a box outside on the back of the house, and then into the attic, where it gets split up. We put a room on the back of the house two years ago. The subcontractor who installed the phone and cable wires is the guy who put in the drop amp. It was my bright idea to hook up the cable going to my cable modem to the cable amp when I first got broadband and had problems with it staying connected. I assumed this was a good idea. (You know what happens when you "assume"). From reading your responses, I take it this cable amplifier is not such a good thing after all. Here are are the specs on the two cable amplifiers, (FWIW):

Electroline EDA 2400 5-42,54 1000 Mhz 15V 270 mvA + 7dB. (This one got hot. I replaced it). Scientific Atlanta Surge Gap Drop Amplifier 5-40, 51-1000 Mhz 12-16V

150 mA +7dB.

I typed in the address ( into a browser and got a web page with a bunch of information. (I did this about 10:30 last night). One of the items it displayed was the "Downstream Power Level" which was "4 dBmV" with a comment that "The Downstream Power Level reading is a snapshot taken at the time this page was requested. Please Reload/Refresh this Page for a new reading." I hit refresh 3 or

4 times, and it always said the Downstream Power Level was 4 dBmV. Maybe a better time to take this reading would be when the modem has lost it's connection? What's an acceptable power level anyway? The Signal to Noise ratio was 34dB. And, the "Upstream Power Level" was "61 dBmV". That's OK I suppose?

I also saw 2 buttons on the page. (I didn't try either one). One was "Reset All Defaults" . The other was "Restart Cable Modem". What does "Restart Cable Modem" do? Is it the same thing as unplugging the power supply?

There was also an event log web page. It contained a lot of messages that had a priority of "3 - Critical" and some text messages that I did not understand. But my broadband was working OK.

So, is this the recommended way to go? Get a two way splitter, with one side going directly to the cable modem? Will just any two way splitter work? Does the splitter need to state that it operates through the 1000MHz range? Or have any other specs?

Again, Many thanks for your help,


Reply to
fj250520 wrote: ....

For most modems and routers "restart" is Not the same thing as a power off cycle.


Correct. Get a 2.4Ghz Splitter. (Yes, data signals are high freq.!)

Lower range splitters will only appear to work, but they will actually fail "occasionally".

Reply to
Rick Merrill

An example of this from my SB 3100 (yep, I've had her a few years now...) Configuration manager for reference (for the thread originator):


Freq: 705000000 Hz Locked

Signal to Noise Ratio: 35dB

QAM: 64

Net Access Control Obj: On

Power Level: 0 dBmV The Downstream Power Level reading is a snapshot taken at the time this page was requested. Please Reload/Refresh this Page for a new reading


Channel ID: 3

Frequency: 23800000 Hz Ranged

Ranging Service ID: 1373

Symbol Rate: 2.560 Msym/s

Power Level: 59 dBm V

Reply to
L Alpert

FYI, I have had cable service since the original role out of @home back in '98 or '99 with no issues. I have a line in that is split, then direct to the modem, the other goes to a distribution amp (just as described). It is the recommended way to do it.

See my earlier post that goes over the output from my modem (an older Motorola SB 3100).

Equates to a three finer salute to your PC, or a soft reset.....

Yes, it was recommended to me from the beginning. The splitter you mentioned should do the trick. Good luck.

Reply to
L Alpert

The recommended power range for the downstream direction is -15 to

+15dBmV, so your system is well within those limits. Downstream Signal to Noise ratio also looks good at 34dB. The upstream power level, on the other hand, looks high to me and could be the cause of your connectivity problems. Check your power levels again after adding the splitter mentioned below.

In my experience, the best splitters are the ones they give away free down at the local cable office. Cheap splitters from Wal-Mart or Radio Shack probably won't be of the same quality.

Reply to
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