I'm not sure what it would do? Just a transformer? I think I would test it on just one phone first. One phone that you don't mind destroying.
12V is within the reasonable range for the phones. I assume you mean a powed transformer with a 12V DC ouput. (A
Wall Wart) You could even start as low as 9V for testing.
I opened one up last night and noticed one thing right off. Both leads, (red and green) coming in have a resistor on there. I'll try to read the color bars and so we know about what it draws.
The next things after the resistors is a full bridge rectifier. I still think the aidio signal rides on DC. The rectifier might have something to do with how the ringer works.
I decided to try and trace the PCB to get a schematic. I need to get better at that. In this case there are only about 11 components and no surface mount, so I think I can do it.
(Also this is a schematic I want to have. Ya know, for posterity.)
Here is what I have so far.
This IS WRONG! (but close) All the components are presents, but I am missing a connection or 2, and I haven't really started indentifying component values.
I was wondering though. How does the existing intercon work? Have you tried to see what kind of singal goes from consol to consol?
Intercom make and model might help, too.
--tim
Edit: RE-read you first post. Your intercoms worked fine being plugged into the phone system, so they work like phones without a ringer.
If you destroy an intercom, can you replace it cheaply?
I think I would try it on just one old phone first and then see if that phone still works as a phone or is damaged.
Then if all is good, try 2 phones in parallel. (NOT like in my first picture/link) You need it to work in parallel, since that is the way the whole systems is wired.
I don't think I would just hook it up to the whole system right away. Too risky,
:endEdit