POTS Problem

[J.B. Wood]

Hello, all. In addition to cellular service I still maintain a
copper-back-to-central office phone line at my residence. I've had this
for about 40 years and never had any problems - until now. Here are the
symptoms (same behavior on any one of 3 extension phones):

1. Phones ring as expected in response to calling party
2. Called party (me) can hear calling party
3. Calling party can't hear me
4. DTMF (Touchtone) phone keypads produce audio tones but won't break
dial tone.

I've got a call into Verizon on the above but am scratching my head as
to what might be causing all of this. Your thoughts would be most
appreciated. Sincerely,


--
J. B. Wood e-mail: arl_1...@hotmail.com

[mike]

Neighbor had a similar problem.
Turned out to be a busted ground wire half a mile away.
The local ground connection was enough to make the audio
work, but the broken wire made the other stuff fail.

[Jeff Liebermann]

At both ends of the 2 wire POTS line is a 2 wire to 4 wire hybrid
circuit, which deals with seperating the audio in both directions
while maintaining full duplex operation. My guess(tm) is that the
receive part of the hybrid circuit at the Verizon end is broken. Not
much you can do to solve the problem at your end.

--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[J.B. Wood]

On 07/06/2016 12:29 PM, mike wrote:

> Neighbor had a similar problem.
> Turned out to be a busted ground wire half a mile away.
> The local ground connection was enough to make the audio
> work, but the broken wire made the other stuff fail.

Hello, and interesting. By "ground" do you mean earthed? In the olden
all-analog days IIRC the only connection to earth on a 2 wire phone line
was at the CO where the positive terminal of the 48 volt common battery
was solidly connected to earth. (I'm not including party-line
ringing-to-ground configurations that are decades obsolete.) Sincerely,

[J.B. Wood]

On 07/06/2016 12:51 PM, Jeff Liebermann wrote:

> At both ends of the 2 wire POTS line is a 2 wire to 4 wire hybrid
> circuit, which deals with seperating the audio in both directions
> while maintaining full duplex operation. My guess(tm) is that the
> receive part of the hybrid circuit at the Verizon end is broken. Not
> much you can do to solve the problem at your end.
>

Hello, and that certainly makes sense, assuming we treat the DC current
loop functionally separate from the AC (voice) part. That means voice
frequencies (voice and DTMF tones) generated at my subscriber location
are of insufficient amplitude by the time they reach the CO. But voice
signals from the caller are getting by OK in the other direction. I
believe those phone hybrids used on line repeaters function the same way
as the 3-dB power splitters/combiners that I've used in RF circuits.
Sincerely, and thanks for replying

[mike]

There are two wires. One is grounded, the other has -48 volts.
If you cut the grounded wire and use a voltmeter to measure
the voltage to ground at the customer end, you measure -48 volts to ground,
but the signal is trying to use one wire and the local ground
path that it could find.
That didn't work for phone, but it did work for DSL,
although I didn't measure the speed or error rate.
I was gonna TDR the line, but was afraid the phone might ring
and blow up my instrument.
The phone guy took a quick look and drove away.
Came back in half an hour and said the ground wire had
been severed down the street.

[Wayne Chirnside]

I experienced precisely the same thing due to corrosion on the
connections at entry causing it to behave as a rectifier.

Cleaned up the connection and all was well.
Took me a week to find it.
YMMV

[JJ]

If you reverse the pair you get audio but no touchtone action.

Also if you use a wonky wall to set cord you get strange behavior.
Tried the cord on other phone and it was good there but not on a
different set. This while AT&T tech stood over me so I know it was not me.

Been there done both.

[Pat]

On Wed, 06 Jul 2016 13:09:35 -0700, mike <ham...@netzero.net> wrote:


>There are two wires. One is grounded, the other has -48 volts.

Not true. POTS lines are balanced twisted pairs. Neither side is
supposed to be grounded (except for old party line ringing schemes).

[Pat]

On Thu, 7 Jul 2016 07:00:05 -0700, JJ <J...@AIOESPAM.COM> wrote:

>If you reverse the pair you get audio but no touchtone action.

Not necessarily true. Old Western Electric touch tone pad required
the correct polarity to generate tones, but most modern equipment has
some sort of bridge rectifier to power the electronics so polarity
doesn't matter.

[J.B. Wood]

Hello, and it's the voice frequency (~3 Hz to 3 kHz) part of a 2-wire
common battery telephone line that's balanced to (earth) ground to
minimize the introduction of common-mode noise. Repeating coils and/or
chokes allow the common battery to supply loop current to subscriber
sets while not short-circuiting the voice frequencies on the wire pair.
Party-line ringing systems aside, it's been common practice in the U.S.
to connect the positive terminal of the central office battery to earth
(I'm told the reason for this was originally as an anti-corrosion
measure). From a DC perspective one of the twisted pairs is earthed
(but only at the CO). Measurement with a DC voltmeter at a subscriber
set location from the Ring (red) side of the line to ground (something
leading back to earth such as a metal water pipe or ground prong input
of an AC utility receptacle) will provide confirmation. Just don't
leave the meter hooked up this way very long as it will most likely
upset the voice-frequency line balance and cause considerable hum to be
introduced. Sincerely,