Troubleshooting Intercoms

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The intercom systems provided by PTI Security systems are built into the keypads and are designed to work with Aiphone brand master stations (LEF models, or NEM models with adaptors). The intercom substations use either a two or three conductor shielded wire to communicate to these master stations. In addition to the substations in the keypads, there may be one or more independent LED model substations placed in strategic locations of the facility.

Wire Specification:

The intercom system uses 18 Gauge shielded cable for the connections, which is provided by PTI Security Systems. This cable is either two or three conductor, depending on the intercom equipment.

  • For systems that consist of a single master station and one or more substations, 2-conductor shielded cable is used.
  • For systems that have more than one master station, or that include background music adapters, overhead speakers or All Call adapters, 3-conductor shielded cable is used.
  • Systems that use more than one master station, but do not have communication between masters, may use a two-wire system. An example of this may be a system that has two master stations placed beside each other to allow for the connection of a large # of substations.
  • The maximum distance the wire can be pulled from the master station to any substation or keypad is 1600 feet.
  • Each substation has an individual cable that is home run to the master station location.
  • Only one substation or keypad can be connected to any one station input on the master intercom. Placing two or more substations or keypads onto the same input on the master is not permitted.
  • Substations do not require power.
  • Substations inside keypad have their circuitry isolated from the keypads other functions.


Background Noise or Hum:

The most common complaint of intercom systems is a hum or other loud background noise when a substation is selected, and the main causes of this problem is:

  • Incorrect (unshielded) wire was used.
  • Shielded wire was used, but the shield is either not connected to ground, or the ground it is connected to does not meet the specifications for a good earth ground.
  • Splices in the line that broke the shield, and it was not spliced correctly.
  • Data transmission wires (RS485A type of communications used for electronic devices. Generally, limited to four thousand total feet of wire length. Used for connecting the Falcon XT to AI Devices.) that run in close proximity to the intercom wires are unshielded, or their shields are not connected to a good earth ground.
  • Routing of the intercom cable in too close of a proximity and in parallel with a high voltage cable (over more than a few feet).

Constant call signal from substation:

Substations signal a call back to the master station by the press of a button that shorts the two wires. This creates a "beep" sound from the master as long as the button is held down. If the wires are shorted for any other reason, this can simulate the button being pressed.

A defective master station can also cause a constant call signal on a substation input. Usually this is due to a surge damaging the master stations electronics. Note that when a master station is damaged like this, there will often be more than one input that will register as a constant incoming call. Proper attention to correct installation and grounding of the intercom system will go a long way in helping prevent damage by surges.

To troubleshoot, disconnect the incoming substation wires from the master station. If the call signal stops, the problem is either in the wiring or the substation itself. If the call signal remains, the problem is in the master station.

Note that substations themselves have almost no electronic components in them. A speaker, pushbutton, a couple of resistors and a capacitor. It has no semiconductors so is not very susceptible to damage from a surge. (This doesn't mean that surge damage to a substation is impossible. Given enough energy, the substations can still be damaged.)