After months of off and on troubleshooting I've given up and wanted to see if anyone has any ideas on this.
Background:
I moved to a new house in February, it had a rats nest of cables, and a powered amplifier with 5 or more cables running around the house. The amplifier was outside the house, the external demarc box to the amplifier was all outside, and someone just drilled holes in the outside of the house and pushed the wires through (no sealant, great for the heating bills). We only use internet, so I switched to using just the cable from the Comcast demarc to my modem (no splitters) until spring. When the snow melted I traced the cable and it runs through shrubs, dirt, etc.., it has deep nicks, and might have roots growing around it.
The issue:
I want to replace this damaged cable, and use a brand new one (This cable: https://www.amazon.com/gp/product/B01K0ZA61C/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1) running from the demarc through the attic to an office area. However I have found that for my modem to function the old junk cable is the only cable that can be used! Any permutation that works uses the old cable some where.
Modem is an Arris 8200
"new comcast cable" is the brand new cable comcast sent with their "activation kit"
"new long cable" is a 100ft coax cable I ran from demarc, through my attic to my office.
Works:
Demarc -> old junk cable -> Cable modem.
Demarc ->new comcast cable->old junk cable->cable modem
Demarc -> Old junk cable -> new comcast cable -> cable modem
Demark ->old junk cable -> new long cable->cable modem
What does not work (yes some tests were done with a really long extension cord and a table next to the demarc):
Demarc -> new long cable -> cable modem
Demarc -> new comcast cable-> cable modem
Demark -> new long cable -> new comcast cable -> cable modem
I have tried factory resets of the modem, I have left it as-is for 30-60 minutes, when I call Comcast they say they see no modem and ask if I have connected it.
Attached are screenshots with the old cable and with the new one.
Any ideas? The only thing I could think of is if the power levels are too high maybe with the poor old cable vs new one.
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