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[Weaver]

During Tuesday night my line 2 was very unstable, with several loss-of-sync events, loss of downstream sync speed and hollow curve disease phenomenon seen in modem #2. This modem was swapped out on Tuesday evening for the modem that had been in use on line 3. This however did not help with the hollow curve disease or loss of sync rate. One theory is that potential four modems were in some way damaged in January/February 2020 in a lightning surge event and that this brought about the long-term period of HCD (hollow curve disease) during the whole of the two years to date. Recently modem cold restarts after prolonged off time have been a medium term cure for HCD, with a return to full performance for an unknown length of time.

Since a modem swapout hasn’t helped, the current thinking is that I used a second bad modem, the line 3 modem, one of the four damaged two years ago. So the next thing to do is to pull a completely by-me unused ‘brand new’ modem (but maybe not literally, since very many in stock are second-hand) from stock.

I informed the ISP Andrews and Arnold (AA) and support’s David is looking into it, with repeated tests to be run overnight.

This is the latest SNR-vs-tones graph from Thursday early morning :




Here is the graph of SNRM-vs-time over night last night:

[burakkucat]

Yuck!    I think this is the very first time you have shown the SNRM v time plot corresponding to an HCD event.

In all cases where I have witnessed a bad SNRM v time plot, similar to that you have shown, the fault has been a bad joint. HR or semi-conductive. The cure has always been to identify the joint and re-make it.

I really wish that there was some way to apply 95V AC ringing current on your three lines. In their normal state they just have the constant exchange battery DC voltage present. I wonder if that constant DC whetting voltage without the occasional overlay of the AC ringing current actually accelerates joint faults?

I remember reading about how, in the early years of the 20th century, before there was the National Grid, some local electricity supply companies actually provided a DC mains supply. One artefact of a DC mains supply was that light bulbs would fail far sooner than the equivalent bulbs used with an AC mains supply. One trick to extend the working life of the bulbs with a DC supply was to turn the bulbs around in their sockets once a week (or was is once a month). Hence, by analogy, my suspicion is that the occasional application of ringing current to each of your lines may minimise the bad-joint effects. Something for A&A to consider, perhaps? 

[Weaver]

I sent the SNRM-vs-time plot to AA.

[Weaver]

I have now swapped out two modems for brand new ones. Were boxed, never used and the hollow curve disease is still around in line 4; can’t tell in line 2 as it won’t even sync, it’s so ill.

Here’s the first stage of line #4’s notch development:



And here’s the second stage:

[Weaver]

This morning, Sat 2022-03-19, after an engineers visit to sort out line #2, we now have the following picture of beauty restored:

[burakkucat]

Much better.

Does A&A have an opinion on my thoughts that the lack of AC ringing current being applied to the lines may be behind these regular "defective joint" problems? With just the -52 to -48 VDC being present, I wonder if "iffy joints" are slowly being conditioned into semi-conductive states.

[Weaver]

Could we write something together that AA will understand?

[burakkucat]

Paragraph three, Reply #1 and paragraph two, Reply #5. I've probably typed similar in some of your other threads on the "HCD" phenomenon.

Perhaps you would like to merge them all together and then edit the result to use the sort of words that A&A will understand. 

[Weaver]

Have written to AA explaining Burakkucat’s good theory.

Update on line 4. Down to 1kbps downstream at 6.6 dB (should be ~3Mbps).





Notice the packet loss (‘dripping blood’ - red from the top down) building after 13:00:

[burakkucat]

Update on line 4. Down to 1kbps downstream at 6.6 dB (should be ~3Mbps).

Yuck.     

The SNRM plot looks like just what has been seen with a HR (and/or semi-conductive) joint. A good "blast" of AC (~95V) would probably clear it.

A static -50V DC plus 95V AC would result in a potential swing from -145V to +45V at the iffy joint.

[jelv]

@burakkucat

One thing I've never been sure about is whether, when ringing a line to clear dodgy joints, it needs or is more effective if there is an actual handset on the end of the line?

[Weaver]

Do we know if it is possible to ‘ring’ a line with no active voice service if you are an ordinary human or maybe you have to be a BT engineer with a magic wand? How might engineers be clearing faults? They always do - for a long time anyway.

Now this is the first time that we have definitively proven that it’s not the modems, because we have new modems that have never been used before and so we’re not exposed to the surge event of jan/feb 2020. So that’s good to know. But what puzzles me is that at times turning the modem off for a good while seems to help. - why? That made me think about guilt of modems.

Because of the dripping blood, I have now turned off modem 4 and it’s staying off unless AA need it. I sent them an extremely polite nudge today not to forget about me. AA must be so fed up with this going on and on. Do you think they’re getting charged by OR? I assume AA is supposed to moan at BTW to make it all good. Not entirely sure about the sequence of actions.

Do we think that an OR engineer should be able to find out where the HR or semi-conductive joint is?

[burakkucat]

@burakkucat

One thing I've never been sure about is whether, when ringing a line to clear dodgy joints, it needs or is more effective if there is an actual handset on the end of the line?

Ideally if the zap has been organised from a remote location (relative to the end-user), say the serving telephony exchange, then the best effect would be obtained with the pair looped (i.e. a hard short-circuit) at the subscriber's end. Without having the pair looped, the standard series connected capacitive-resistive shunt across the pair, provided by all master sockets (be it an LJU or an NTE5), will provide an attenuated AC path.

If the end-user owned an ex-GPO magneto connected to a standard BT plug, just plug it into the test-socket and crank the handle. That would (probably) temporarily clear the fault.

[Weaver]

(FYI I think I was still adding to that last post, we may have crossed over slightly? )

I was asking about whether or not OR should be able to find the fault.

@Black_Sheep If you’re about, my friend, I’d love to hear your thoughts.

[burakkucat]

Do we know if it is possible to ‘ring’ a line with no active voice service . . .

I have tried that for you on three (or maybe four) occasions. The most recent occurrence being just moments ago --

Drop cable 1, Pair 1,  01471 nnnn70  engaged tone returned
Drop cable 1, Pair 2,  01471 nnnn14  (abandoned 2021-04) number unobtainable tone returned
Drop cable 2, Pair 1,  01471 nnnn44  engaged tone returned
Drop cable 2, Pair 2,  01471 nnnn75  engaged tone returned

. . . maybe you have to be a BT engineer with a magic wand? How might engineers be clearing faults? They always do - for a long time anyway.

Via the TAM, maybe? Black Sheep might be able to assist our search for knowledge.

Now this is the first time that we have definitively proven that it’s not the modems, because we have new modems that have never been used before and so we’re not exposed to the surge event of jan/feb 2020. So that’s good to know. But what puzzles me is that at times turning the modem off for a good while seems to help. - why? That made me think about guilt of modems.

Agreed. Twice over.

Because of the dripping blood, I have now turned off modem 4 and it’s staying off unless AA need it. I sent them an extremely polite nudge today not to forget about me. AA must be so fed up with this going on and on. Do you think they’re getting charged by OR? I assume AA is supposed to moan at BTW to make it all good. Not entirely sure about the sequence of actions.

Your guess is as good as mine, if not better!

Do we think that an OR engineer should be able to find out where the HR or semi-conductive joint is?

With a lot of effort, yes. I'll guess it is somewhere out on the high moor, in a joint-closure that is half-buried in mud or peat at the side of a water-course.