Hi fozzies, I have just been to check the continuity ( which in fairness I had not done to the new module) across the call button and they make and break every time, the voltage into the module is 12.3v and is constant whether the button is pressed or not, I did notice that the psu is not that dissimiler to a laptop psu, being fed by the mains via a figure of 8 cable, which in turn feeds the intercom module and 2 keypad modules in parrallel, is this a standard psu for this kind of setup?
as I'm now suspecting that there is a possibility that when it was installed just over 3yrs ago, it just about coped but is beggining to fail due to overload!
It's a weird one because it's almost giving the symptoms of a corroded cable, sometimes working, sometimes not.
I think I will replace the psu with something a little more substantial and replace the cables from it to the intercom module also.
The pabx system is realitivley new Samsung unit and after talking to the installers of this, they assure me that if it works in one direction it will work in the other direction as it is just a 2 wire system, and thinking about it, when the module seizes the line, the connection is always made.
It's just getting the module to do it's job and seize the line on the first button press every time.
Thank you fossies for your helpful advise.
Chris