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[sheddyian]

A few weeks ago a friend asked me to look at their phone/broadband.

Symptoms : Phone plugged into master socket doesn't work, phone in extension socket works ok but crackles sometimes. 
ADSL generally unaffected (stays up for many many days) but synch speed a little lower than estimate.  Kitz (BT) ADSL checker indicates estimate of 12,000, synch speed was actually 10,104 when I went to look at it.

Main fault that got them to ask me to look at it was that the phone would only ring once, and then the call would be "answered" by a very crackly but otherwise silent line.  If the recipient of the call picks the handset up before the caller hangs up, the call can continue as normal, and the crackles stop.

A faulty answering machine was ruled out by unplugging it from the extension phone socket.

What I found was interesting - there'd evidently been problems in the past.  An old master socket fitted flush in the wall had been disconnected.  Jelly crimped to the incoming BT line was a short run to a surface mount BT master socket.  Coming off the same jelly crimps was the wiring to the extension socket, it wasn't daisy-chained from the BT socket.

Via a long set of extension leads from the extension socket was the ADSL modem and a phone.

Both master sockets didn't work, both had signs of green corrosion inside.  The wall they are fixed to is very damp.

I disconnected it all, put a new master socket in with an SSFP, moved the modem to this, connected the extension socket wiring to the filtered output of the SSFP.  I disconnected all the extension leads plugged into the extension socket.

This didn't fix the "answer after 1 ring" problem.  Disconnecting the extension socket at the SSFP did.  Friend has since purchased a DECT phone with extra handsets, so the extension socket is no longer needed.

So, although it's all fixed, I just wondered if this is a common sort of fault, and what might cause it?  I can understand crackles being down to poor/corroded connections, but why did the phone appear to be "answered" after 1 ring?

Slightly disappointingly, after fixing the crackles, removing the star-wiring like arrangement and long extension leads feeding the ADSL modem, the line synch speed actually dropped very slightly, from 10104 to 10072    The SNR margin went from 6.0 to 6.5 up and 26.5 to 27.0 down.

Before and after bitloading plots attached, for interest. 

Ian

[burakkucat]

Ooh . . . Phantom ring-trip. 

Logically, the only origin for that event must be low insulation between the A- & B-wires. In the normal, quiescent state the wires have an approximate difference of 50V DC. In that state the insulation resistance is sufficiently high to inhibit the electronics at the exchange recognising that the pair is looped. To that state, when a call is incoming on that circuit, is added the ringing voltage of 80 - 120V AC. The extra alternating voltage is sufficient to cause a breakdown in the insulation between the A- & B-wires. Hence, to the exchange electronics, the pair has been looped. The exchange then ceases the supply of the ringing voltage and connects the caller through to the circuit. At this point, the insulation resistance is at a dubious level -- hence hissing and crackling is heard by the caller. If the local instrument is brought into operation before either the caller "clears down" or the pair slowly regains sufficient insulation (now that the ringing voltage is no longer being applied) a true loop is provided to the exchange and the call can proceed as normal.

You have already identified that the problem is associated with the extension wiring / extension socket. Only close physical examination of the entire extension wiring and the socket may show the site of the defect. Replacing both items with new would be the only sure cure.

Perhaps your friend should address the problem with damp? 

[Black Sheep]

Exactly as Mr Clever-clogs above has mentioned  , but in layman's terms it's what we call 'Ring trip'.

Like Mr Cat has pointed out, the likelihood is that damp is causing a resistive loop at the Master Socket, (ie: not a full short circuit of A-B, but a partial one). Under normal conditions, when the phone rings and you answer it, a similar resistive loop is put across A-B. Ergo, with the fault being present each time somebody rings your friend the call gets 'answered' by the damp socket. Hope this makes sense ?

Word of warning, Openreach charge for faults caused by damp, but we also have to move the Master Socket to a new dry position in the premises. If this avenue is to be explored, make sure the engineer doesn't just replace the socket like-for-like, and still charge the EU. 

[sheddyian]

Thanks for the replies, whilst I assumed it was caused by the damp, it's nice to have an explanation as to why 

There's unlikely to be any call to Openreach, as I fixed the problem by replacing the small surface mount master socket with an NTE5 and an SSFP, and disconnecting the extension socket entirely.  (Only by doing this last step did the "ring once then answer" fault get resolved. 

But there is the underlying problem with damp, which is not easily rectified in the building as it's damp walls    At the moment, I've left the socket laying on it's side on a shelf beside the wall it should be fastened to.  I was wondering how to improve it's chances against damp ingress, perhaps by putting some of those silica gel sachets inside it, and insulating it from the damp wall with some plastic sheet of some sort. 

Ian

[renluop]

Ian! As a bear of little brain, I hesitated before posting this!

Won't silica gel need periodical  renewal or drying out?  It seems to me that it is a temporary  fix only of the symptoms.

FWIW I thinkyour friend needs to tackle the cause of the damp. I found this interesting article. A forum that is good on building matter is at diynot.com

[sheddyian]

Ian! As a bear of little brain, I hesitated before posting this!

Won't silica gel need periodical  renewal or drying out?  It seems to me that it is a temporary  fix only of the symptoms.

FWIW I thinkyour friend needs to tackle the cause of the damp. I found this interesting article. A forum that is good on building matter is at diynot.com

You've got a good point there that I hadn't properly considered, although the damp does seem to be worse during long spells of rain (ie this winter) so there could be opportunity for it to dry out a little in the summer!

Agreed damp needs to be resolved properly, but I was just asked to have a look at the phone and see why it wouldn't ring for very long   I think there are some long-standing issues with the original builder in the 1980's.

Ian

[Ezzer]

The first time I fitted a new line with an NTE in a socket which was surrounded with wet plaster (house renovation) I almost got out of the front door when the phones started ringing continously. It was due to the humidity carrying enough plaster inside the socket in the air to cause the weird ring trip. Took me a while to figure out what I did wrong.

Damp can cause issues even when there is no obvious visible sign. Over time I learned how gypsum can cause some interesting electrical effects.