cable modem signal issues, and how they were resolved

I feel the need to share the iterations that woway took to resolve a signal issue with my cable modem. I'm pleased with their effort.

It's a Scientific Atlanta WebStar dpx100 series cable modem P/N

749874. The symptoms I was having were periodic loss of connectivity, less than advertised downstream bandwitdh (4.5mbps vs 6), and in some instance, failure of the upstream channel to sync.

My cable modem is at the end of the line in a 2 story home with 4 outlets upstairs, and 4 on the first floor and 1 in the basement. Cable drop comes into the basement level.

Installation to truck roll 1 4.5mbps downstream, 1mbps upstream. Borderline downstream signal level, upstream power level pegged at max 45 or 50dB or whatever it was.

Truck roll 1 came after failure to sync was rectified by me moving the cable modem to teh basement before the house distribution of the interior wiring. The tech replaced at least one splitter with a directional coupler and reterminated some of the cable ends.

Truck roll 2 came after a similar issue cropped up again. This time, another DC was added to sacrifice some TV signal for the cable modem. An amplifier was added at the ingress of the house. This seemed to help bring downstream bandwidth up to 6mbps and the upstream stayed at


3 weeks pass, everything is ducky, weather warms up a bit.

Truck roll 3 was today due to lack of upstream sync on the 2nd floor, rectified by moving cable modem to the basement. They added a DC in the attic and removed one in the wall of the 2nd floor bedroom, disabling an unused jack in that bedroom. They pulled the amplifier out of the signal path to the cable modem, tech stating that the noise it adds was causing upstream sync problems now. Not sure what changed in the weather to have it become an issue again.

So now, the system looks like: /-through -> cable modem /-through->to attic->--DC< / \\-tap->AMP->split to 3 tv's 2nd floor

-feed--DC< \\-tap-->AMP-->split to 3 tv's first floor/basement

If that's munged, looking back upstream, the cable modem sees the through legs of 2 DCs and a long cable run to the attic. No amplifier is seen by the cable modem in this arrangment.

The first floor and basement tv's see an amplifier and the tap leg of a DC upstream of them.

2nd floor tv's see a amplifier in the attic, and then the tap leg of 2 DC's before they get out of the house.

Seems to work. 33dB snr on the downstream, -8.54dBmV downstream power, pushing 45dBmV upstream.

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Cable (the actual stuff outside) works "better" when the tempature drops. The difference can be dramatic. Where I live, the tempature can swing from 75-80F in the summer to -30 or lower in the winter. Amplifiers have automatic controls to compensate for the changes, but they can only do so much, so we spend a lot of time in the fall and spring re-adjusting them.

-8db is close enough for downstream signal levels (and 33db SNR is great). You should be OK until things warm up again.

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