New Netgear CM1200 modem issues

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VoodooRufus

Weaksauce
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May 23, 2017
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I've been having issues with my Arris Surfboard SB6900AC lately and decided to upgrade to separate modem/wifi router. I got a nice Netgear Nighthawk XR500 (works great, very nice), and a CM1200 modem to handle modem duties for my Spectrum service.

My issue is that the CM1200 will lock 31/32 of the downstream channels, but only 1/8 upstream channels. The downstream and internet lights are solid green, but the upstream will continuously flash. No internet access. I had a Spectrum tech come out to inspect it, and no issues were found except a mildly leaking coax connection (replaced).

Any ideas?
 
Provisioning, signal, hardware.
If the modem isn't properly provisioned by your ISP, it won't work right. Sometimes the provisioning is so messed up, you have to replace your modem. Nothing wrong with it, it's all on your ISP's end.
Signal. The primary culprit for cable modem connectivity issues. Cable techs are notorious for doing the bare minimum to check these issues. Could be in the lines, could be back at the relay.
Hardware. It is possible you are the lottery winner in getting a modem that is malfunctioning out of the box. This is what the cable company usually blames first, but the reality is that it is usually not the case.
 
Both of my modems are personally owned. I had Spectrum swap the provisioning by giving them the MAC for the new modem. Old modem syncs up fine.

Signal is strong both ways, near zero leakage.

I suppose it's possible the modem is defective, although that should be a very low likelihood. I intend to contact Netgear to see if they have any ideas as well.

Thanks.
 
You could have some splitter or barrel connector somewhere that isn’t rated for the required frequency range. Or you have a crimp fitting instead of a compression somewhere.

All your barrels should have a blue center and the splitter should be rated at least 5-1218MHz.

If all that checks out I would be willing to bet that your downstream power on most channels is fine but your upstream power is high. This is usually caused by a cable drop that is too long or is defective. Any drop over 100Ft can’t be RG6 anymore, it needs to be RG11. If your drop is more than 100ft and is the same size as the wire in your house, insist that the tech checks it a little more closely.

can you post a screenshot of your modem’s signal level as well a pic of the main splitter?
 
Not using a splitter. The house main coax goes straight from the exterior comms to the modem direct.

Signal.png
 
Your downstream power is a bit too high yet your downstream SNR is lower than expected with that power level. I would expect to see SNR of 43-45dB with a 9-10dB power level. Upstream power is in spec.

Since I don’t see any OFDMA channels I’m assuming that’s your DOCSIS 3.0 modem not the CM1200.

Got a pic of the one that’s causing you issues?
 
No, I never screenshot the power levels for the CM1200. But they're about the same. The screenshot above is from my current Surfboard.
 
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