Electronics > Beginners

Old telephone line strange problem

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miguel.rocha:
Hi,

I have a very strange problem in my parents home old telephone line that goes from the first floor to the second one.

It's a simple cable pair, as the telephone on the second floor stopped working I began troubleshooting with a multimeter thinking it could be some kind of short.

I disconnected the cable on the first floor and measure the continuity thinking i will get open circuit if it was not a short, i was getting some 20Mohms in resistance no matter the order of the leads and if i used the diode mode i got open circuit in one position and if i swapped the leads i got a "diode" which was super strange.

I though the telephone itself got some internal resistance/diode or other component so i disconnected the second floor too, having now, what i though as a disconnected pair of conductors but here i got the most strange measure i ever saw in my life.

If i connect the multimeter in diode mode, now i get as expected open circuit no matter the order of the leads, but in resistance mode in one position i get open circuit but in the other i get 1Mohm, i'm without words to explain this, has anybody have an idea!!!!

Thanks.

helius:
A long pair of wires has some capacitance and inductance, which can be mis-read as a resistance by some meters. If your meter would read 1 Mohm when connected to a 100 pF capacitor, for example, it could also do so on an unconnected twisted pair.

floobydust:
It sounds like you have water between the wires. Check the wiring is not wet somewhere- roof, bathub etc. You will get different ohmmeter readings depending on polarity, and the readings will move.
Sometimes it can look like a diode is there, depending on multimeter. DCV will give a strange value.



miguel.rocha:
I went ahead and disconnected all the telephone lines inside my parents home and all of them show open circuit as i expected, no matter the position of the multimeter leads, the only one that has such a strange behavior is the one from first floor to second, and by the way there are no water pipes or other sources of water near the path of the cable, at least from what i can tell.

So i can deduce there is some problem with this cable, probably insulation degradation with age or something, at least the phone was working until last week.

It will be a pain to remove it but i think i don't have an alternative.

rstofer:
Wireless phones?

If the base station is on the ground floor, maybe a wireless handset will work on the second floor.  Answering machine would be built in, of course.

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