In reply to a post by eckiedoo:

Most solid crystals are poor/non-conductors, so gradually the connection/s deteriorate, so should be opened/closed occasionally, say once per year.

Opening and closing breaks down the dendrites, effectively smearing them back to conducting gold.

Many years back, I was taught one of the first things to consider when fault finding on installed systems was to power down, then pull every board out of the backplane connector re-insert and repeat or wiggle it around left/right/up/down, . Power up and see if the fault was cleared!

As you say, it cleans up the contacts.

Well, I've just installed a new NTE5a master socket and telephone extension at my mother's house.

On looking inside the master socket I discovered that the A and B wires were reversed. This was confirmed by plugging a test telephone into the master socket and using its polarity checker.

This was from a new build back in about 1980. It's never caused a problem with voice, dialup or ADSL.

Everything's sorted now and with the bell wire disconnected and no 26A surge arrestor (which Openreach recently had removed from NTE5a master sockets) the maximum attainable downstream sync has jumped by about 1MBps and upstream has improved slightly. I'm sure the polarity change had nothing to do with this.