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I recently switched to the comcast phone service from ATT. I have a structured wire set up. I also kept an ATT land line so I'd have a telco line in an emergency--like an earthquake. On every phone in the house there is a buzzing on the comcast line but not on the att line. Suspecting the structure wire panel, I use a punch down panel to bridge around the panel. Same buzz. The phone guys I've had in said it sound like a power hum but can't explain why it would buzz on comcast but not on ATT. A phone plugged directly into the voip modem yields a pristine signal--no hum. I have disconnected all the phones and then reconnected them one by one. Buzz persists. I have disconnected the fax--buzz persists. So what's the difference between the signal that comcast puts into the phone lines and the one that ATT uses?

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  • fixya793 Apr 27, 2010

    Already tried switching the pairs and making comcast line 2 and att line 1. No change, the Comcast line had static, the ATT line was quiet.

    Actually, the panel is now totally out of the loop. The RJ45 jacks were cut off and the lines punched down onto a block. This was done to eliminate the panel and see if that was the problem. No luck--the buzz exists. Seems to me that there is some difference between what ATT puts on their line (dc voltage and ringer voltage) and what Comcast puts on their line. Something is different between Comcast's output from the voip modem and what comes in on the ATT phone line.

    I'd be happy to post a picture--how do I do that?

  • fixya793 Apr 27, 2010

    Brobin, thanks for the help. I'll check that out. But, before I got comcast, I had three ATT lines all routed in the same way, through the same wires to the same jacks to the same phones. No noise. No static. Just blissful dial tone. :-) Comcast comes along, I discontinue one att line, dump dsl (it's disconnected), keep an ATT line for disaster and get Comcast for the main number. Static. So the question, it seems to me is: if the wiring was fine for ATT but not for Comcast what's the difference between the two? And thanks again for the responses.

  • fixya793 Apr 27, 2010

    Several things:
    How do I check the voltage of the line coming out of the Comcast box?

    Using what brobin said about telco wires, I went ahunting. There's a wire that runs from the DSL box in the garage to the wiring closet where it's punched down to connect it to a rj13 plug that used to be connedted to the dsl modem. (At least I think that's the wire coming from the garage.) Anyway I disconnected the wire from the DSL box--there are now just four wires waving in the air in the garage. In the wiring closet this wire is still punched down and connected to the rj13 plug. The buzz persistes. :-( Could this wire, that's now connected to virtually nothing actually induce the buzz? If so, is there a way to take it out of the loop. It's impracticable to remove the wire entirely. Thanks. Bob

  • fixya793 Apr 27, 2010

    Again using brobin logic that the buzz on the comcast line has something to do with grounding issues on the ATT line I disconnected the ATT line from the house. I now have a delightfully quiet comcast line. Given the welter of wires in the garage, where the ATT line is connected in the garage it's going to be a while I figure out who the culprit is. There are connections there between boxes that were for old, disconnected ATT lines and the one surviving line. Got to be it--yes?



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Assuming you have 2 line phones, what happens if you swap lines? If your structured wiring is the 568B standard, which is the most typical, the second line must be punched down to the white/green pair instead of the orange/white pair for 568A wiring. It sounds like the second line may be daisy chained in the panel so you have to see how it's wired and troubleshoot from there. Sometimes a pair is connected to a power supply to provide power for a phone or light but that's not typical any more. Sorry I can't be more specific but if you have more info or photos of the panel and jacks I'd be happy to help further.

Posted on Apr 27, 2010

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  • Bruce Robin Apr 27, 2010

    So if you plug the phone directly into the modem it's OK but when you plug it into a jack elsewhere you get the hum, correct? If that's right, check the jack in the wall outlet to see how it's wired and be sure the same pair are used at the other end. If you still have the hum then then somewhere along the line you've got a ground on the line or induced hum from a power line running adjacent to the wiring. Where are the lines for all the jacks daisy chained for the house? Probably at the block and the daisy chain wire should be removed so that each station can be tested independently to find the culprit. As far as the line voltage goes there may be a difference but if the phone is OK at the modem it should be fine at the jack if all the wiring is OK. I have 3 lines in my home, one POTS line from the telco, one from Time-Warner cable and a third that T-Mobile provides through a router plugged into my main router. I have a KSU-less 4 line phone system and all three lines sound crystal clear - in fact the VOIP lines are cleaner than the telco line. The task at this point is to break the daisy chain link so each individual line can be tested.

  • Bruce Robin Apr 27, 2010

    As a retired telecom engineer my curiosity about this issue has been piqued. I measured the on and off hook voltages for both my telco POTS line and my Time-Warner VOIP line to see what differences there might be. The on-hook voltages were -50.2 vs. -40.1 and the off-hooks were -6.3 vs. -5.6 (all DC). Both are acceptable (-48 on and -6 off are nominal). I didn't measure line current but have in the past and both were similar. Of course, your readings will be somewhat different and if there is an overcurrent condition, that could cause other problems but not typically a ground hum.
    If the hum could be heard when the phone is connected directly to the modem, that would lead me to believe there was a problem with the modem output or it's power supply but apparently that is not the case.
    When you test the Comcast line by itself through the house wiring do you lift both wires of the AT&T pair from the block? If there were a wiring issue where AT&T ground was present on the pairs you would get the hum because the ground would be floating - IOW not tied to the pair from the modem.
    !!! IMPORTANT!!! Be absolutely sure that the incoming telco pairs from the 2 old lines have been physically disconnected from the house cabling. Although AT&T turned off dial tone to those pairs they would still be be connected to the outside plant and grounded. That would explain why you have the hum when using either pair while the POTS line is fine. In thinking about it, this is the most likely cause of your problem.
    You can also try reversing the VOIP pair although I doubt that is the solution - easy enough to try though and it could work.
    BTW, what brand/model phones are you using?
    Unless there is something going on with the phones themselves or a power influence from power supplies, I really think your issue is tied (pun intended) to unwanted grounding somewhere. That ground would most likely be coming from AT&T since the AT&T lines were not affected by it.
    Please keep me advised of your testing progress (and hopefully a solution!) as I am very curious about this and want to see if we can find the answer.

  • Bruce Robin Apr 27, 2010

    To measure the voltage place the leads of a DMM (Digital Multimeter) across the pair from the modem - most easily done at the wiring block. Disconnect the old AT&T wires at the block too.

  • Bruce Robin Apr 27, 2010

    Bingo! You've got it. I would have been very surprised if that hadn't solved it. Now, just connect the one remaining AT&T line to the one pair on your block that will carry that line and leave the other AT&T wires disconnected. Unless there is some other problem in the wiring, which is unlikely, you should be hum free. Let me know if you still have any problems. Glad I could help!



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