Home Intercom Retrofit

[sgreenstein@xxxxxxxxx]

I recently moved into a home with an old M&S Model 350 intercom.  The
Speakers all seem to work ok,  but the master unit is in bad shape. It
doesn't turn off the program/set time buttons do not work.

I found the DCM3-4 unit online as a retrofit replacement. This looks
like it'll work, but everything I see regarding the installation
indicates that I have to replace ALL the Speaker stations as well.
After inspecing one of the speaker I'm puzzled as to why.  The unit is
extremely simple,  the speaker with a switch to toggle between a
speaker and mic setting.

Can anyone tell me why you would make the old speaker incompatible with
the old system.
Thanks