What to check

The power supply, once started, only provides 20v for typically 1 minute before timing out.

You will need to find the 900-VS4 video distributor that feeds this group of affected phones.

At the 900-VS4, start the PSU timer by shorting ""SIG"" to ""-"" wires, (you will need to check at phone or PSU for the wires colours), check the 20v + -  input terminals voltage and that it is continuing to the monitors.

Corrective Action

Beware of tucking excess unsheathed coax behind the 900-VS4 pcb as this can cause shorts.

Use spare conductor as required, or Rewire.