What to check

The Monitor when rung from the door or the ON button pressed sends a pulse down the SIG wire to start the central power supply timer, which then supplies 20v to monitors for about 1 minute before timing out.

At the door, ringing any button should start the power supply timer which then powers the camera and the lamp above it as well as sending 20v to the monitors.

If the SIG wire to a group of monitors is open circuit, then pressing the buttons will NOT start the timer and the lamp above the camera will NOT come ON.

If ringing a different button, starts the timer, then the monitors should work until the power supply 'times-out', this implies that the SIG wire to this group of phones is 'open circuit'.

To check the "SIG" wire at the 900-VS4 video distributor, you should have voltage between "SIG" and "-",(you will need to check at phone or PSU for the wires colours).

Although phones send a pulse to the PSU, you can manual simulate this by shorting "SIG" to "-" and then you should have 20v DC at the 900-VS4 supply terminals.

On some 200 series, the lack of 20v will prevent speech from connecting.

Corrective Action

Reconnect "SIG" wire or use a spare conductor.