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My venerable self-installed Panasonic KX-T30810 has worked fine at home for almost 30 years. But recently one of the three CO lines won't generate a dial tone, regardless of which CO port on the KX-T it is plugged into. The other two CO lines work fine on all three KX-T ports.

I suspect that AT&T recently changed my physical copper pair, or what it is connected to in their CO. The killer is that the new line works fine with a modern touchtone phone, so of course the AT&T tech resolves my trouble report with "no trouble found" and says the problem is with my equipment.

When I use the KX-T to dial on the line without a dial tone, I get CO error messages like "Your call did not go through", or "It is not necessary to dial a 1 when dialing this number". If I just leave it off hook without dialing anything, it says "Your call did not go through" rather than the usual "If you'd like to make a call...". So the CO line is seeing something, but not liking what it sees.

I measure the on-hook voltage at the DMARC as 51.9V, and the off-hook voltage as 6.8V.

Any idea what's going on? Is there some modification I can make to the KX-T to make it compatible with the new CO line? Is there some way I can describe to the AT&T tech how the CO line needs to be different, or what's wrong with it?

Len, an engineer end-user

PS: Sorry for posting in "General", but I can't figure out how to post in the Panasonic forum.

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Did you check the cross connect or what ever you are using from the AT&T demark to CO port

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When I use the KX-T to dial on the line without a dial tone, I get CO error messages like "Your call did not go through", or "It is not necessary to dial a 1 when dialing this number". If I just leave it off hook without dialing anything, it says "Your call did not go through" rather than the usual "If you'd like to make a call...". So the CO line is seeing something, but not liking what it sees.

Obviously then the line IS going off hook. It sounds to me like either your system is seizing the line without your lifting a handset or there is something else on that line like an alarm dialer that is stuck off hook. Could also be bad wiring, a corroded jack, etc.

What happens when you call that line? Do you get a busy?

-Hal


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You said that it works fine with a touch tone phone. Where are you plugging in the touch tone phone?


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(Ah, I can reply now -- thanks, hitechcomm!)

- I don't think it's bad wiring, because when I swap lines at the DMARC, the problem moves to a different CO line on the KX-T.

- When I call the bad line from outside, it rings normally through the KX-T and I can answer it and talk on any of the eight KX-T7020 feature phones.

- The only other thing that I know was on the CO line was a "Whooz Calling" caller ID box, but I've removed it.

- When I test with a touch tone phone, I'm plugging the cable that normally plugs into the KX-T switch into the phone. So all the same house wiring is being used. (Of course it also works when I plug the phone directly into the DMARC.)

- The house wiring *is* the usual 50-year-old messy kludge, so maybe I'll try running a new clean test line from the DMARC to the KX-T and see if that changes anything.

Last edited by LenShustek; 09/21/20 10:44 AM.
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- When I test with a touch tone phone, I'm plugging the cable that normally plugs into the KX-T switch into the phone. So all the same house wiring is being used. (Of course it also works when I plug the phone directly into the DMARC.)

You say that you can move that line to other CO ports on your KSU and still have the problem? Or is it only on one port?

-Hal

Last edited by hbiss; 09/21/20 12:11 PM.

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The problem (no dial tone) happens on whichever of the CO ports on the KSU I plug that central office line into.

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Originally Posted by LenShustek
- The house wiring *is* the usual 50-year-old messy kludge, so maybe I'll try running a new clean test line from the DMARC to the KX-T and see if that changes anything.

Ok, I did that experiment, with a fresh pair directly from the "bad" line in the DMARC box to the CO inputs of the KX-T30810. On any of the three CO inputs, the behavior is the same: no dial tone on pickup, but calls are received ok. Plugging that fresh pair into a touchtone phone works fine.

The other CO lines in the DMARC work fine on all three CO inputs to the KX-T30810.

There's something about that CO line that is incompatible with the KSU, but that's not a complaint that AT&T will listen to since it works with a "regular" phone.

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Lenshustek

Your welcome.

There's something were missing here.
Let me think. (That's not easy)

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It needs to be asked Is the KSU set to tone dialing?

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Interesting question. Tone dialing is set for each CO in the programming, which I did 30 years ago. But I just broke open the old installation manual, set the KSU to program mode, and verified that all three CO lines are set for tone dialing
(Besides, having them set to pulse dialing might have interfered with dialing, but it seems like it wouldn't prevent me from getting a dial tone.).

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Originally Posted by hitechcomm
There's something were missing here.

I'm really thinking that the line goes off hook when you plug it into the system for some reason. I wish I was there with a butt set to determine that. The other possibility is the system pulsing or dialing a digit when you go off hook. The digit could be out of spec for the other lines but not for the CO equipment on that line. Again, a butt set would tell.

But the question is why is whatever it is happening? Even if you can see what's happening how do you fix it? It's not something that you can complain to Verizon about because it's definitely not normal for a properly operating system. I suspect the age has everything to do with it. Those KSUs are cheap and if it were me I would just get a refurb (not used) and replace it. If the problem goes away you've fixed it. If not, return the KSU and keep scratching your head.

-Hal


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Originally Posted by hbiss
I'm really thinking that the line goes off hook when you plug it into the system for some reason.
But if that were true, wouldn't I not receive calls on that line? Incoming calls work fine.

It seems like the line goes off hook when I pick up the receiver, but then the CO immediately interprets something as an auto-dialed "1". After I dial 7 digits, I get the message "We're sorry. It's not necessary to dial a 1 when calling this number".

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Let me think for 1/2 second. The 30810 is MD'd since 1990 something and unavailable for sale since Kari's law. It's hard to believe that a line doesn't work in position 1 2 or 3 and it's in the KSU, especially since the 3rd line always doesn't work.

I'd have a chat with the provider and beg/demand new hardware.

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Originally Posted by LenShustek
Originally Posted by hbiss
I'm really thinking that the line goes off hook when you plug it into the system for some reason.
But if that were true, wouldn't I not receive calls on that line? Incoming calls work fine.

Yes, I have to agree.

Quote
It seems like the line goes off hook when I pick up the receiver, but then the CO immediately interprets something as an auto-dialed "1". After I dial 7 digits, I get the message "We're sorry. It's not necessary to dial a 1 when calling this number".

That's what I speculated above. Possibly the other lines are able to ignore it but not the third line. Different equipment? The system might cause a noise pulse that's interpreted as a "1" when it goes off hook. I believe there are relays for each line and they have to be at least 30 years old.

Again, I would want to listen in on the CO lines as the system goes off hook. That would give you the answer.

-Hal


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I borrowed a friend's Harris TS-21 test set, and recorded what the bad CO line sounds like going off-hook on the KX-T30810, and what the good CO line sounds like. I'll try to attach them here. I can't tell anything from those sounds, but maybe you can.

I also hooked up my oscilloscope in A-B probe mode and recorded the waveforms, but given noise and grounding issues I don't have a lot of confidence in what I'm seeing.

Attached Images
CO1_PBX_no_dialtone.mp3 (22.86 KB, 14 downloads)
SHA1: c22679ba113c49cf243e05b8f6c35046c391346e
CO3_PBX_dialtone.mp3 (25.71 KB, 19 downloads)
SHA1: bf656d3dcb58f7a3553796205b1c9806261043b6
CO1_PBX_no_dialtone.jpg CO3_PBX_dialtone.jpg
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The start of a call sounds exactly the same. I have no clue.

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Have you checked the loop current? there should be at least 18 ma and no more than 28ma.

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I hadn't checked the loop current, so I just did. It's about 23 ma when off hook on both the good and bad CO lines.

Incidentally, in the process of making that measurement I discovered that there is an inconsistency on the polarity of my three CO lines. The two good ones are different from each other. So I reversed polarity on the bad line, but it's still bad in the same way. I then reversed polarity on one of the good lines, and it still worked. So polarity evidently doesn't matter for the KX-T.30810.

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Your scope shows the voltage between your test points isnt dropping as close to 0 on the bad pair as it does on your good one. This indicates a high resistance connection past your probes and closer to the unit trying to go off hook.

It's still an odd condition since the co is still breaking dialtone for you.

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Good observation, but maybe the difference is the presence of the dialtone on the good line.

Being a digital electrical engineer who likes to design complicated solutions to simple problems, I'm going to experiment with a circuit that does the following: when the KX-T takes the line off-hook, quickly (in 50 msec or less) break the connection to the CO line and simultaneously put a 500 ohm shunt across the CO line to maintain the off-hook. Wait 600 msec or so for the KX-T to do whatever it does that the CO is interpreting as a dialed digit '1', then reconnect to the CO line and remove the shunt.

Any chance that will work? It will take me a week or so to cobble it together. I'll report back on my success or failure.

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Originally Posted by Carl Navarro
The start of a call sounds exactly the same. I have no clue.

No. Definitely the "no dial tone" has a third pulse, "dialtone" has two.

I don't know how your scope is connected or what it shows. I would want to see a capture of the line as it is taken off hook by the system for the good line and the bad line. Only one channel is needed directly across the line. You should see the line drop from 51.9V to 6.8V. Trigger off that. Maybe 1 sec/div horizontal. You should see those pulses and noise right after then the audio dialtone.

-Hal


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Originally Posted by hbiss
... Only one channel is needed directly across the line.

Ok, to do that I'll need to power the scope with an isolation transformer, otherwise the probe ground is AC ground. I'm currently out of town for a few days and will try it when I get back.

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Originally Posted by LenShustek
Originally Posted by hbiss
... Only one channel is needed directly across the line.

Ok, to do that I'll need to power the scope with an isolation transformer...

Well, that would be best. But there is always that little grounding adapter that is used to plug grounded plugs into two prong outlets. Accomplishes the same thing in this case.


-Hal


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Originally Posted by hbiss
...I would want to see a capture of the line as it is taken off hook by the system for the good line and the bad line...

Ok, I did that. I can't see a big difference, except that for the "bad" line, as soon as I put the (isolated) scope ground clip on Tip, I get a buzz, which you can see in the trace. I don't know why that happens, and it doesn't happen for the "good" line.

Here's good news on another front: My crazy kludge circuit works! I detect off-hook, quickly isolate the KX-T from the CO for 600 msec, then reconnect them. Dial tone, then dialing/talking works perfectly. A photo of the prototype board is attached. The next step, assuming it keeps working for a while, is to make a little printed circuit board and put it in a small box with two RJ11 connectors.

That may not be as efficient a solution as demanding/begging AT&T for a new line that is different in some way I can't describe, but it sure is a lot more fun for a retired engineer who is in pandemic isolation.

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CO3_PBX_dialtone_02.jpg CO1_PBX_no_dialtone_02.jpg prototype.jpg
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as soon as I put the (isolated) scope ground clip on Tip, I get a buzz, which you can see in the trace. I don't know why that happens, and it doesn't happen for the "good" line.

Are you using an isolation transformer to power the scope? It occurred to me that the ground is not isolated on most, just the line. That's why I suggested the adapter as a ground lift. But no matter.

On the bad line I can see garbage from 450-700ms that happens earlier and ends after about 425ms on the good line. Betcha that's your problem and why the 600ms delay stops it.

-Hal


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Originally Posted by hbiss
Are you using an isolation transformer to power the scope? It occurred to me that the ground is not isolated on most, just the line. That's why I suggested the adapter as a ground lift.
I had used a power adapter without a ground for the scope, which caused the noise. I just tried an isolation transformer (see attached), and the noise is gone.

I still don't see a significant difference between the good and bad lines, except for a little positive spike at about 450 msec on the bad line. Incidentally I had earlier tried putting a "snubber" (resistor and capacitor in series) on the bad line to suppress inductive spikes, but it didn't help.

My kludge circuit continues to work. If you're curious, I posted the hardware and software design as one of my repositories on Github: https://github.com/LenShustek/phoneline_fixer. I'm awaiting receipt of the printed circuit board (5 boards in 2 days for $2 from China) to make the final version.

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CO1_PBX_no_dialtone_02.jpg
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My printed circuit board is back, assembled, and installed. My problem, I think, is solved -- although in a rather unconventional way.

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No offense, but that is the stereotypical engineer's solution to what probably could be fixed simply by replacing the KSU for $150.

-Hal


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I don't believe that would have worked, unless the design of the KSU changed, because the new CO line fails on all three inputs to the KSU. And the other two CO lines work on all three inputs to the KSU. That implies that it's neither a fault in the line-specific circuits nor the common circuitry. I may buy a spare KSU just to test my theory, and anyway it's probably not a bad idea to have a spare around after 30 years of reliable operation.

No offense taken. It was a fun project, and probably the sign of an engineer with too much time on his hands.

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Yes, nice job.

You might take a look at the KX-T61610 which is the "next one up" 6x16. I believe it was more popular which is why it can be had for less than the 30810 since there are more of them around.

There might also be design differences.

-Hal


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