How to test cable & phone lines

Written by raul avenir
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How to test cable & phone lines
(Jupiterimages/Polka Dot/Getty Images)

Cable and phone lines don't serve only one function anymore. Cable customers often receive Internet, phone and cable TV through their cable lines, while telephone lines often carry phone and Internet services. These lines seldom cause trouble, but when they do, they can halt any information coming into or going out of your home. Diagnosing the problem can become a guessing game. Know how to test and troubleshoot phone and cable lines before calling a repairman and parting with your cash.

Skill level:

Things you need

  • Multimeter tester

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    Test the Telephone Cable

  1. 1

    Open the network interface device (NID) and unplug the modular connector inside from its jack. The NID is a plastic box typically mounted on a wall nearest to where the phone line runs into your house. Get a working telephone, plug it into the jack, and listen for a dial tone. No dial tone means the problem lies with the telephone company. Call the telephone company from another phone, and inform them of the problem. If you can hear a dial tone, then the problem lies within your household wiring.

  2. 2

    Disconnect any other electronic gadgets or devices that you have plugged to the phone lines such as fax machines, modems, security gadgets and caller ID. Listen for a dial tone. A dial tone means one of the gadgets or devices has malfunctioned and caused the problem. To detect the faulty device, plug in each item one at a time and listen for a dial tone. If the dial tone stops after a device is plugged in, that device is defective and is the cause of your problem.

  3. 3

    Plug each phone set into the test jack inside the NID, and test each one for a dial tone. Defective phones can affect the whole circuit, which is why you have to know that all the phones are working. Replace any phone set that is defective, and replug all the phones to their jacks.

  4. 4

    Open the telephone junction box if you have one. A junction box is directly wired to the NID and distributes phone wires throughout your house. Insert the plug of a working phone into its test jack and listen for a dial tone to know if there is power running from the NID to the junction box. No dial tone means that there is a connection problem from the NID to the junction box. Trace the cable from the junction box to the NID and look for cuts or damages.

  5. 5

    Lift the handset of each phone and listen for a dial tone. The absence of a dial tone indicates a problem in the telephone cable from the hub or NID up to the jack. Trace the line from each jack to the junction box to see where the problem lies.

    Test the Coaxial Cable

  1. 1

    Get a multimeter tester and set it to an ohms scale of around 100 ohms. Coaxial cable generally operate at 50 or 75 ohms rating, so the scale should be set at or higher than these ratings. Disconnect both ends of the cable from their connections.

  2. 2

    Place a multimeter probe stick on the centre wire emerging from the coaxial connector on the wall at each end of the cable.

  3. 3

    View the multimeter reading and interpret the resulting data. A reading of about 50 ohms or 75 ohms indicates that the cable is working properly from end to end. A reading displaying infinite resistance indicates a break somewhere along the inside of the cable. Zero indicates a potential short between the inner cable and its shield.

  4. 4

    Pull off the cable connector to expose the cable shielding. Place one multimeter probe on the cable shielding and the other on the inner cable. View the display and interpret the information. If the reading shows infinite resistance, it means that there is no short between the shield and the inner cable. Anything below infinite indicates the shield and inner wire may be touching somewhere along the cable. Replace all defective cables, but test the replacement cables before installing them.

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