I have been tasked to replace a handset for in suite that was beaten to peices.
I've contacted my local rep and installed an IS-92BZ as a replacement. It is not the unit that came out as it was dated 1991, has no model number on it, and is a different style although still 4+1 wires.
This new one has a one-transitor circuit board, likely an amplifier. The oldies do not have this circuit board.
Problem is this unit howls with feedback when lifted from it's wall mount. I think it is still carbon style mic like the old unit and does not need the amp.

What I need is the schematic for thr MA-771 intercom amplifier.
Please contact me if you have one, and we can discuss options from there.