All Mode Talkback (AMT) not working

[UltimateX]

Hi,

I am trying to trace a fault in a multimode 10/11 meter transceiver (schematic and block diagram attached) that has fault preventing the talkback feature from working.  The talkback feature is basically the ability to listen to yourself during transmit no matter which mode (AM, FM, USB, LSB) you choose to use.  The volume of this talkback, monitored through an external speaker connected to the radio, is controlled via an AMT volume control on the front of the radio (VR112).  The fault on this radio is that in AM, USB and LSB you can hear the mic audio coming through the speaker no matter where the AMT control is set, but the AMT, as feature, does nothing on FM.  I have already checked the AMT pot with my DMM and it responds normally as the knob is rotated back and forth through its range. Any suggestions on the test points to check that might reveal where in the circuit it's failing to use the AMT pot and likewise, presumably, why FM is failing to engage the feature at all?

Thanks