XTR 4000.1D

[Perry Babin]

R580 feeds that transistor. You need to follow back from the terminal of R580 that is not connected to that transistor.

Is there a Q71 anywhere in the amp?

The attached is much smaller but has similarities up to the output driver IC. The supply for the output driver should be the same.

 

[Hexhclass]

I did not see a Q71 but there is a Q171 but it is not directly connected to the IC.

Pin 11 is directly connected to Q153 (DF) center leg and Q157 (BR) center leg

 

[Perry Babin]

You'll have to follow the circuit back through the traces.

Q171 is the level-shifting transistor for the audio circuit.