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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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
CALIFORNIA PROPOSITION 65 WARNING: Some comments made by me are known to the State of California to cause irreversible brain damage and serious mental disorders leading to confinement.
... 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.
... 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
CALIFORNIA PROPOSITION 65 WARNING: Some comments made by me are known to the State of California to cause irreversible brain damage and serious mental disorders leading to confinement.
...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.
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
CALIFORNIA PROPOSITION 65 WARNING: Some comments made by me are known to the State of California to cause irreversible brain damage and serious mental disorders leading to confinement.
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.
No offense, but that is the stereotypical engineer's solution to what probably could be fixed simply by replacing the KSU for $150.
-Hal
CALIFORNIA PROPOSITION 65 WARNING: Some comments made by me are known to the State of California to cause irreversible brain damage and serious mental disorders leading to confinement.
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.