I have been doing some research on APRS recently, and Brian, VK2AH, kindly donated a MFJ-1270B.
Over the years I have heard so many reports that the MFJ-1270B does not accommodate twist (an amplitude / frequency slope usually the result of lack of pre-emphasis, usually the result of audio overdrive of the transmitter, and usual for hams). More detailed discussion is at The extent to which APRS works is often an accident.
There is no shortage of bad signals to test the tolerance of the TNC, with two local overdriven digis, and travellers through the area. Continue reading MFJ-1270B TNC review