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Testing the keying circuit.
Turn both the volume control and the sidetone control full anticlockwise. Connect a Morse key to CON3 (unless you
have fitted the switch to the board) and a 12v supply to CON1. Press the Morse key or switch. The relay should give a
click as it pulls in (energises). Release the key and note that there is a short delay before the relay clicks again as it
drops out (de-energises). Now connect the headphones to CON2. Touch the upper terminal of the volume control
with a metal object as before and turn the volume control clockwise until the buzzing noise is heard. Press the Morse
key and observe that the buzzing noise is muted. Release the key and observe that the bussing noise returns when
the relay drops out.
