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Failing MSAN Ports

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siofjofj:
I have a TalkTalk ADSL service and in November 2017 I had a fault where I would lose sync both randomly and when the phone was picked up or rung. There was also data noise audible on the phone when the router was connected. I assumed this was a high resistance fault on the line, and indeed the remote fast tests did sometimes (but not always) state the presence of a CIDT fault. However after 1 PSTN and 2 SFI Openreach engineer visits the fault was found to be with the port on TalkTalk’s MSAN in the exchange. I was changed to a different port and all was well.

However in April 2018, after my router had been in sync for 160 days, the same symptoms reoccurred. A fault was reported and I gave the attending PSTN engineer the name of the SFI engineer who ultimately found the fault last time. The SFI engineer told the PSTN engineer what to test for, and it was once again found to be the MSAN port. Once again this was changed and all is now well.

The engineer said it is very rare for a port to go bad, so two going bad suggests something else is causing it, perhaps some other undetectable fault on the line. The engineer left and returned to his van, and I reconnected my internal wiring and router (I had been using the TalkTalk provided HG633 router and a basic corded phone plugged into the test socket via a dangly filter for diagnostic purposes, but my normal set-up has a data extension from a MK3 SSFP and a TP-Link VR600 router).

From the van the engineer initiated a line test (presumably this is standard procedure when closing a job) and got back a CIDT fault. He tried this 5 times and got the same result, so returned to the house and asked me to remove everything again. On initiating a final line test, no fault was found so he closed the job, but commented that perhaps the house wiring and/or the VR600 is bad and is causing the MSAN ports to go bad.

The line seems perfectly stable now, and I did try swapping to a different pair on the data extension which made no difference to anything. But this has got me wondering, can bad wiring / the VR600 in the house cause MSAN ports to go bad over time (excluding things like shoving 240V down the phone line which would presumably blow up the MSAN port in an instant)?

burakkucat:
That is an interesting situation.

As you have checked your data extension wiring and found no obvious fault, then there are just two other items to be ruled out. The SSFP which, as far as the circuit is concerned, is a passive device and the TP-Link VR600.

All modems will connect via a high-pass filter which, at the very basic level, can be regarded as a capacitor in each leg of the pair. I am at a loss as to what sort of fault a modem might possess that results in the associated MSAN port to be zapped.

If I was in your situation, I would test by having the VR600 powered up but disconnected from the circuit and then checking for any spurious voltages across the pair, also between each leg of the pair & a good earth. (Prerequisites: a DMM and a good earthing point.)

Weaver:
Has there been any thundery weather, dark looming clouds?

How long is the phone line?

siofjofj:
Before the fault occurred in November I was actually using a MK2 SSFP. I changed this to the MK3 just after the fault was fixed, so I think this rules out it being this. Good thought regarding the check for spurious voltages appearing from the DSL input to the VR600. I'll check tonight.

The phone line is about 1.5km long and has a FTTC cabinet just outside the exchange and an AIO FTTC cabinet around 900m from the exchange. The BT wholesale ADSL checker predicts an ADSL sync speed of 14mbps. In reality I'm syncing at 17.7mbps, and DSLstats now shows an arrow straight and flat SNRM, so I'm inclined to think the whole metallic path from the exchange to the router is in perfect condition.

There weren't any notable external conditions in November when the port went bad, but in April the day the problems started we had around 3 short duration (say 2 seconds) power cuts.

siofjofj:
I can now confirm there are no significant* voltages, AC or DC, present across the pair or from each side of the pair to earth.

* 1V RMS @ 50Hz was detected between each side of the pair to earth, but my DVM detects this if one probe is grounded and the other is left disconnected, so this is almost certainly not real.

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