I have a linear power supply for an old video door phone system with 2 voltage outputs +15VDC , +18VDC and they share the same GND.
The +15Vdc is not working because the power supply is faulty. Can i install a 15Vdc SMPS to work alongside this old power supply? The DC outputs feed the central unit of the video door phone outdoor station and it has one GND terminal. When i wire the new SMPS should the GND output from the old P.S. and the negative terminal from the new SMPS connect to the same terminal on the outdoor station ? Will it work?

 

yes, it will work

 

Based on the description, YES, providing a substitute 15 volt supply should work quite well.