What to check

When a video telephone is called from the door panel or the phone "ON" button is pressed, a gnd pulse is sent on the "SIG" wire to start the central power supply timer, which then supplies 20v to video distributors and the monitors for about 1 minute. During that timed period, the lamp above the door unit camera will be illuminated. 

To test at the monitor, press ON and then measure for DC voltage between "20v" and "-".

If there is a fault with the monitor not sending the pulse down the "SIG" wire to start the timer, you can manually simulate this by shorting "SIG" to "-" and pressing "ON" to turn the screen on.

Check the SIG wire at the telephone you should have about 12v DC voltage between "SIG" and "-".

Corrective Action

Use spare conductor as required or rewire.

Additional notes

Beware of tucking excess unsheathed coax behind the 900-VS4 PCB as this causes shorts on the pcb tracks.