Icom IC-A21
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I used one of these in my DIY "ASOS" system, to detect mic "clicks".
Squelch circuit
Noise components of the signal are output from IC1004 on pin 9. This is marked IC4 on the board and is an MC3372P. This then goes through the squelch control pot and is rectified to a DC voltage. When high enough, this voltage turns on Q1008 pulling its output low. For an active high signal monitor the output of inverter Q1009. This signal goes across the ribbon cable to the logic board and is called 'sql'.
