RACORDER - The Marine Black Box
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1. Use a shielded audio cable with the shield soldered to a solder lug so that it can be mounted to a screw in the radio to serve as a strain relief. The Racorder module provides this function.
2. A low pass filter to reduce the possibility of RF getting into and out of the radio via the audio line is recommended. This consists of a ferrite bead and one or two 100 pf capacitors.
3. The main circuit consists of an audio mixer made from two resistors which connect to the receive audio at the volume control and the transmit audio at the deviation control.
All the components can be mounted on a small perf board about .6" X 1.0". 30 gauge wire-wrap wire or 26 ga stranded hookup wire are suitable for the connections.
Connect a 100 kohm 1/4 watt resistor in series with a .1 mf capacitor from the audio cable to the surface mount resistor indicated in the diagram. Use 3" of wire (white lead on the Racorder module).
Connect a 10k ohm 1/4 watt resistor from the audio cable to the volume control board at the pin closest to the front of the radio. Use 2.5" of wire (blue lead on the Racorder module).