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"If isolating the cable modem with a transformer would solve the problem then I would consider it."
Does the cable modem have a 3-prong plug? Most do not. I don't recall seeing one with a 3-prong plug in a long time.
However, it can still be grounded through the cable TV line and there are ground isolators that can be used to solve the problem... The Viewsonics VSIS-EU is one such device. I read a report that a user on Cox cable got this supplied for free from Cox due to a ground loop problem involving a plasma TV.
There another place that it could be causing a ground loop--the Ethernet cable. Some cable modems were or are supplied with a screened twisted pair network cable. This has a metal shield around the RJ45 connector. If you are using a cable like this deposit it in the nearest trash can and replace it with a normal one.
Finally, if you are using the USB port on the cable modem I suggest switching to the Ethernet port. The USB port is a definite source of a ground loop.
Twisted pair Ethernet is, unless it's that screened twisted pair variety, isolated and will not cause a ground loop.
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As a test I will try taking off the CATV and Ethernet cables. I can also unplug the cable modem from the AC because it has a battery back-up. These tests could provide some interesting data. Thanks for the suggestions.
I also just ordered three different 2-line phones from Circuit City - RCA, Panasonic and IntelliTech. CC offers free shipping so I can try them all and drive them back to the store for a refund.
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Joined: Apr 2008
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With both the AC and CATV cables pulled off of my cable modem the hum goes away. The modem was running off of batteries. If I just reattach the CATV cable the hum comes back. Same is true if I just connect the AC and leave the CATV off.
Not sure where this leaves me. I will have 3 more phones delivered from Circuit City in about a week.
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Originally posted by ScottW:
Anyone have experience with RCA or Aastra phones? I like all the Aastra phones I've come across.. Originally posted by ScottW:
With both the AC and CATV cables pulled off of my cable modem the hum goes away. The modem was running off of batteries. If I just reattach the CATV cable the hum comes back. Same is true if I just connect the AC and leave the CATV off.
Sounds like Brian might be on to something. Perhaps one of these would be worth trying: https://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=180-075%20 https://www.cs1.net/cables/products/jensen_transformers/VRD-1FF.htm
"There is one thing and only one thing in which it is granted to you to be free in life, all else being beyond your power: that is to recognize and profess the truth." - Leo Tolstoy
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Joined: Dec 2002
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I would check to make sure the cable tv is properly grounded outside.
Jeff Moss
Moss Communications
Computer Repair-Networking-Cabling
MBSWWYPBX, JGAE
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If pulling off the CATV cable and leaving on the AC cable results in hum then I am not sure how adding just a CATV filter is going to help me. One costs $11 and the other is $50 which seems a bit steep.
Keep those suggestions coming. I really hope one of the three phones that I just ordered will solve this problem.
Still wondering why the RF jackextenders dont work on the VoIP line. For me the VoIP line seems to be the source of the hum on my 2 line phone. I would consider isolating it if none of the new phones work.
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If you decide to buy a CATV isolator use the Viewsonics one, it is designed to work with 2-way services. (As a very temporary test you could connect two 75-ohm to 300-ohm matching transformers back to back; I don't know that this will work properly and allow your cable modem to sync, plus it will result in signal leakage; however as a temporary test it may be OK. The matching transformers should act as an isolator. But they're not shielded at all so....).
That in conjunction with an isolation transformer should solve the hum problem you are having.
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Do you have a link to the Viewsonic isolator? For $10 I bought the one from Dayton. Hopefully it wont interfere with my internet or voice service. The Jensen transfer was $50 which was too steep a price for this test. EDIT - Found it here: https://www.broadbandamps.com/Products/GroundIsolator.htm
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Another option would have been to plug the computer into an isolation transformer. These occasionally turn up on Ebay pretty cheap, intended to provide ground isolation to computers used for medical purposes. [/QB] I am looking at one of these too https://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=120244590618&_trksid=p3907.m32&_trkparms=tab%3DBidding If I can get one for under $30 than I would give it a try too. Between three different 2-line phones, a filter on the CATV cable and isolating the AC I better be able to hear a pin drop!!!
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