More Cable Modem reboots

This is a continuation of my previous topic below on 6/8/2017.


I am having T3 and T4 timeouts with cable modem reboots after over a month of no reboots or log entries except where TWC pushed out a firmware update for my SB6121 around July 17th. A tech will becoming out Tuesday to check the lines again.


I used and there are two TV stations with a 48 and 49 under Real. Why would that cause ingress though the insulation on coax cables where they come into my house or through the wall and the coax cable insulation? There is no wireless signal between the access point between the access point near the street and the cable modem.


There has been another ISP doing work on my street and has fiber optic lines on my street. Could there underground lines have been causing ingress issues?


What other things should I ask the tech to check?

Spectrum Employee

Re: More Cable Modem reboots

Hi CableCustomer,


No, other fiber optic services can ever cause you ingress/egress. Spectrum uses Optical fiber to the node + Coax to the house. Optical fibers use EM fields in the optical light spectrum, while coax uses RF frequencies and the 2 do not mix so that's not related. Unless they cut your drop at least that is...


Aside from that, SB6121's are terrible and outdated modems, best to replace them right off the bat, no questions about it. SB6183's are the perferred device for standalone modems if you perfer to purchase your own. Aside from that if you are a Spectrum customer who's upgraded from TWC, you can get a new company modem w/ no fee for the modem now so I would take advantage of the upgrade.


As far as the T3/T4 timeouts, that's indicative of a signal issue.


Can you please go to and post all the modem diagnostics for us to see please, that will give us a bit better insight here for us to investigate further and help you determine a better set of questions as well.


Thank you!

I post on my off hours, away from work and any statements made are my opinions, and do not necessarily reflect the views and beliefs of Spectrum Communications.

Spectrum Employee

Re: More Cable Modem reboots

NVM I found your previous post, I hadn't read that far up and found your modem Diag page.


So you either have a bad modem (50/50 chance) and it needs replacement, or you have line damage somewhere between the tap and the house.


You have noise getting into your cable and the basics need to be touched. Fittings at the tap, at the GB, behind the wallplate in the house. Then the wierd stuff, is the drop damaged, water in the drop? is your drop aerial? 


This is the process that goes through my head, and by far the most critical things to look for are any issues regarding connections that are either not tightened or possibly damaged.


Also, another common and overlooked point of failure is the connector IN the wallplate itself. Replacing the Barrel between the wallplate connections is also advised if everything else has been addressed.

I post on my off hours, away from work and any statements made are my opinions, and do not necessarily reflect the views and beliefs of Spectrum Communications.


Re: More Cable Modem reboots

There is nothing wrong with either modem that you are throwing at coaxial issues.

You have broadcast TV ingress and need the coax cleaned up, something is corroded or loose or there's open splitter ports.

That is causing the 4 channel modem to be assigned channels outside of it's acceptance bandwidth by the CMTS at the head end.

4 ch acceptance bandwidth is only 36 Mhz and channels must be adjacent without high amounts of noise on any skipped channels.

6141's and cm 500's have a little better DSP but neither  and even the 16-32 ch modems can't handle ingress on assigned or skipped channels if it's 6-10dB stronger than the weaker qam modulation.

You needed TWC out a month ago , never heard anything back, we assumed you hhad it repaired or dropped TWC


Proven Sharer

Re: More Cable Modem reboots

If you are planning to replace the obsolete SB6121 you should also consider the Motorola MB7420 modem over any of the SurfBoard series.  It is a 16 x 4 full-band (108-1002 MHz) receiver driving a much newer DSP decoder.


Re: More Cable Modem reboots

I had a tech come out and the tech replaced the fittings on the ends of both cables that run from the outside box to behind the wall jack and on the cable running from the wall jack to the cable modem. The tech also replaced the barrel in the wall jack because the tech said the barrel was corroded and suggested that I replace the SB6121 with a SB6183 so I purchased a new SB 6183 from a local store.


After 15 days of up time the SB6183 has had no errors on the log until this past Friday I got a DHCP Renew - lease notice then a T3 time out today. There were zero total uncorrectables on the 16 downstream channels.


Why is the modem trying to renew the lease? Is this a TWC server software issue?


Mon Aug 28 00:19:24 2017

Critical (3)

Started Unicast Maintenance Ranging - No Response received - T3 time-out;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.0;


Fri Aug 25 19:36:30 2017

Notice (6)

DHCP Renew - lease parameters Time Protocol Servers modified;CM-MAC=;CMTS-MAC=;CM-QOS=1.1;CM-VER=3.0;


Re: More Cable Modem reboots

The once a week DHCP lease renew is normal...

Glad they cleaned up the wiring.

The 6183 supports wide channel acceptance bandwidth whereas the 4 ch modems off all manufacturers only allow 6 channel spreads, and the TWC CMTS firmware sticks them with channels 8-32 channels apart making operation intermittent depending on if there was excessive noiuse and ingress. The 16-32 modems also have better/ newer DSP