Here is the reverse-engineered circuit diagram of the amplifier at the base of the loop, as far as I have been able to determine it.
The transistor is most likely an n-channel FET, it could also be a MOSFET.
Can you measure the range of V-tune and V+ against ground? The amplifier does not have to be connected during this measurement.
Update: I have updated the circuit diagram with a few more details and I have removed it here, see further down below for the new version!