Old telephone line strange problem - Page 1
Forum section: electronics / beginners

author: miguel.rocha
author_rank: Newbie
timestamp: August 23, 2018, 05:22:20 pm
content: 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.

author: helius
author_rank: Super Contributor
timestamp: August 23, 2018, 05:33:28 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2018, 05:47:18 pm
content: 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.

author: miguel.rocha
author_rank: Newbie
timestamp: August 23, 2018, 07:14:39 pm
content: 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.

author: rstofer
author_rank: Super Contributor
timestamp: August 23, 2018, 07:42:31 pm
content: 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.

author: jmelson
author_rank: Super Contributor
timestamp: August 23, 2018, 08:12:33 pm
content: There may be a rusty staple that has pierced the insulation of the cable. These can be nearly impossible to detect.Jon

author: janoc
author_rank: Super Contributor
timestamp: August 23, 2018, 08:59:22 pm
content: Check for a leak (e.g. from a roof) or condensation in the conduit, together with broken/degraded insulation. It doesn't need to be only from an actual water pipe. Or you can be getting a stray/phantom voltage there - e.g. by coupling from a nearby mains wiring - and the meter is picking that up.It is probably easiest to just use a battery with a lightbulb to check the continuity/short. Modern multimeters can do all sorts of weird and wonderful stuff when you connect a long floating wire (= antenna) to their high impedance input.However, 1MOhm resistance between the wires shouldn't have much effect on the phone function, even if it wasn't a spurious reading. Assuming it is POTS (not ISDN or something similar), it would only attenuate the signal slightly. Even with the 100V or so of ringing voltage it would cause only about 100uA of current, that's negligible .

author: elecdonia
author_rank: Frequent Contributor
timestamp: August 23, 2018, 09:21:43 pm
content: When different metals (for example iron and copper)touch each other and also get corroded from exposure to moisture, the chemical reaction can cause behavior that resembles "a diode."So I think your measurement of open circuit in one direction and a diode in the other direction is significant.Another reply mentioned that telephone cables are often stapled down with iron staples. The wire inside the cable is copper. So if a staple cuts through the insulation that is an iron-to-copper connection that might act like a diode.Analog telephone wiring often suffers from corrosion because there is 24 to 48 V DC voltage applied between the two wires all the time that the phone is on-hook. The voltage drops to about 6-8 volts while a telephone set is off-hook and being used. But if you think about it, the telephone set is on-hook most of the time. That continuous 48 V DC, combined with even a small amount of moisture, will result in corrosion.If the phone occasionally works but you hear noisy crackling sounds in the receiver this is another sign of corrosion somewhere in the wiring.So, long story short, replace that old cable. I suggest using CAT5 cable. It is very easy to find at any large home/hardware store. CAT5 wire has 4 pairs. you only need to use one pair for a single line phone or two pairs for a 2-line phone. The standard wiring scheme for using CAT5 cable with analog phone lines is blue/white pair for line 1, green/white pair for line 2. The other two pairs aren't used.Also look closely at your existing telephone jacks in the walls. If there are any signs of rust or corrosion replace them. These parts are easy to find in big home stores.Finally, the modular line cords between the wall jack and the telephone set can also fail. They are cheap. So get new ones if you see any signs of damage or corrosion. The pins on the ends of the modular plugs should be very shiny gold color. Also the pins inside the modular jacks should be shiny and gold color. If you see any bits of green fuzzy stuff that is corrosion. Best solution is to replace all items that are corroded.

author: amyk
author_rank: Super Contributor
timestamp: August 24, 2018, 12:14:20 am
content: I agree with others that the presence of a "phantom diode" points to corrosion.It's the same reason whyhttps://en.wikipedia.org/wiki/Foxhole_radioworks.

author: miguel.rocha
author_rank: Newbie
timestamp: August 24, 2018, 12:37:55 am
content: Thanks everybody, now i understand i little more what could be going on.I'll be replacing the cable if possible, if not i'll try the wireless solution some have recommended.Thanks again.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: August 24, 2018, 02:29:24 am
content: In my experience any solid wire that was stapled in place will break. The staple impact creates a microfracture that will break. This was 90% of the phone line problems I saw. (I was not a telephone repair man.)The weirdness you have seen reminds me of a case where I had a house with a phone problem that made no sense. I was taking to my co-worker outside about it. That's when I noticed an "extra" phone line laying the ground. It was in a puddle of water.