The connection needs only two wires acting as a balanced transmission line. If the �good� cables both have only 2 gold contact pads, and the �bad" ones all have 4, a possibility is that the extra wires somehow unbalance things. In particular, look for connections at the sockets to the additional wires.

Getting a bit beyond my capabilities now... However, all the cables I've bought are sold as replacements to the supplied ones. I'm told that other people don't have this problem using the same cables, one supplier commented that they had sold thousands of these and had never heard of a drop in speed like this.

Once again, I'll point out that I've had 4 different cables from 4 different suppliers and a bad cable does not seem to be the diagnosis, unless all 4 are bad and that's one heck of a bout of bad luck !

The speed you get with a DSL connection is in part determined by the signal to noise ratio, as I am sure you know. Therefore it is a fair bet that line noise is somehow increased by the �bad" cables. Common mode noise (induced equally on both conductors of a balanced pair) should be largely rejected by the modem input stage.

You may therefore be looking for something that unbalances the configuration. This might be capacitance to an additional wire in the cable which is connected to ground, or to some other wiring. You could make up a longer 2 wire cable to try using a RJ11 coupler, and indeed try different cable pairings. You could also try a common mode filter core.

Also check if your master socket has an earth connected, and if so try it disconnected.

Also check if your master socket has an earth connected, and if so try it disconnected.

.... because you may not have noticed the humming noise on the line ???

.... because your installation has remained unchanged since lines needed an earth connection way back when ???

It would be more meaningful & fairer to be comparing sync speeds with the various cables & routers & noting the attenuations & NM that go along with them.

It is the sync speed that these RJ111 cables are "carrying" not the throughput. Why introduce the variability of speedtests & their remote servers into a purely local problem?

In reply to a post by Zarjaz:

Also check if your master socket has an earth connected, and if so try it disconnected.

.... because you may not have noticed the humming noise on the line ???

.... because your installation has remained unchanged since lines needed an earth connection way back when ???

No phone connected normally, unplugged. Checked, no hum or crackle.using a wired handset.

New install June 2017 by Openreach engineer. No 'earth' connected

In reply to a post by XRaySpeX:

It would be more meaningful & fairer to be comparing sync speeds with the various cables & routers & noting the attenuations & NM that go along with them.

It is the sync speed that these RJ111 cables are "carrying" not the throughput. Why introduce the variability of speedtests & their remote servers into a purely local problem?

OK, I did tests with synch speeds noted but got lost with finding other info on new Smarthub. If you want more than this I'll try, I've added the info for the final/current connection now I know what else was required.. Having swapped out several times this evening I am loathe to do any more in case the system decides I have a fault...

Old brightbox2

Upstream 9820 (Kbps)
Downstream 48292 (Kbps)
Noise Margin 6.2 (dB) 6.3 (dB)
Line Attenuation 0.0 (dB) 19.4 (dB)
95.146.235.nnn (5m black wire)

Upstream 9865 (Kbps)
Downstream 48292 (Kbps)
Noise Margin 6.2 (dB) 6.3 (dB)
Line Attenuation 0.0 (dB) 19.3 (dB)
95.144.53.nnn (5m black wire)

Upstream 9928 (Kbps)
Downstream 59460 (Kbps)
Noise Margin 6.2 (dB) 6.3 (dB)
Line Attenuation 0.0 (dB) 19.2 (dB)
2.30.226.nnn (original wire)

Upstream 9995 (Kbps)
Downstream 64350 (Kbps)
Noise Margin 6.2 (dB) 6.3 (dB)
Line Attenuation 0.0 (dB) 19.2 (dB)
2.28.227.nnn (original wire)

new Smarthub

Downstream sync speed: 65.06Mbps
Upstream sync speed: 8.99Mbps
95.145.0.43 (original wire)

Downstream sync speed: 55.95Mbps
Upstream sync speed: 9.06Mbps
2.26.82.nnn (10m wire)

Downstream sync speed: 58.97Mbps
Upstream sync speed: 9.23Mbps
2.26.82.nnn (5m CAt5e)

Downstream sync speed: 56.34Mbps
Upstream sync speed: 9.34Mbps
Data rate:9.34 Mbps / 56.34 Mbps
Maximum data rate:9.53 Mbps / 56.71 Mbps
Noise margin (up/down):6.2 / 6.1
Line attenuation (up/down):27.4 / 19.1
Signal attenuation (up/down):27.2 / 17.7
2.27.229.nnn (5m white)

In reply to a post by TMCR:

In reply to a post by Zarjaz:

Also check if your master socket has an earth connected, and if so try it disconnected.

.... because you may not have noticed the humming noise on the line ???

.... because your installation has remained unchanged since lines needed an earth connection way back when ???

No phone connected normally, unplugged. Checked, no hum or crackle.using a wired handset.

New install June 2017 by Openreach engineer. No 'earth' connected

Yes, I was being sarcastic about an ill informed post by �Tripslaw�

I have read most of the posts and they are all making sense.
But the problem might not be in the RJ11 cable as there is also the internet adapter to consider and also the device settings [PC or other device].
The adapter usually has a settings part that might be causing some bottlenecks.?

What do you mean by internet adapter? The latest posts show varying modem/router sync speeds with different cables which would not be affected by device settings or the mysterious internet adapter.