VOX

The audio signal coming form P4 ( squelshed transceiver ) is amplified by U1 that works as a high gain amplifier , the output of the U1 is a highly amplified audio signal, this signal is then rectified with a 5.1 volts zener diode that also protects the input of schmit triger U3A aginst overvoltage ( 12V ) and compared at pin 2 of U3A. U3A will swicht state when the voltage at pin 2 is above 2/3 of Vdd ( 5V ) and bellow 1/3 of Vdd.the signal is then stabilized at C7 and finalliy delivered to the PTT switching circuitry flip-flop. R20 adjusts the voltage were U1 switches state ( high means unsquelsh ). To adjust R20 turn on the radio ( squelshed ) turn the volume control to middle and adjust R20 ( multi-turn ) until the voltage at pin 3 of U3A switches form 0 V to aproximatly 5V, this should be just a little less that 1.7 V, at this poit any signal above the squelshed background noise will triger the output of U3A. For the other Radio this is exactly the same.