ZL1NCs Information Pages
UV series radios achieve COS from P65/AURX supplying a signal to switching transistors that supply power to the final audio driver chip when the squelch is to be opened. When a valid signal (or PL tone / DCS) is detected, P65 goes high. Q18 is forward biased, pulling down the base of Q19 (which was held high by R90). Q19 is now forward biased and conducts BAT1 to pin 2 of the TDA2282 amplifier.
Aha! Pin 2 of the TDA2282 is the perfect place to pick off a +Ve going COS (PL) trigger.