Comcast cable modem problem

Comcast cable modem problem

Post by fj25052 » Wed, 20 Apr 2005 04:00:19

et 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

Comcast cable modem problem

Post by Rick Merri » Wed, 20 Apr 2005 04:35:09


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.


Comcast cable modem problem

Post by Bill M » Wed, 20 Apr 2005 06:54:55

n 18 Apr 2005 12:00:19 -0700, XXXX@XXXXX.COM wrote:



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.


Comcast cable modem problem

Post by ozz6 » Wed, 20 Apr 2005 16:42:02

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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Comcast cable modem problem

Post by fj25052 » Wed, 20 Apr 2005 23:57:04

ick, 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.

> pole<==>splitter<==>modem<==>cmputer
> \==>filter==>amplifier==>TV

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

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,



Comcast cable modem problem

Post by Rick Merri » Thu, 21 Apr 2005 03:00:19


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".

Comcast cable modem problem

Post by Bill M » Thu, 21 Apr 2005 07:00:20

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.


Comcast cable modem problem

Post by L Alper » Thu, 21 Apr 2005 13:49:18

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

Comcast cable modem problem

Post by L Alper » Thu, 21 Apr 2005 13:56:27

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.