Icom IC-M601 / IC-M602
It's not too hard to equip the rather recent Icom IC-M601 and the almost identical IC-M602 marine VHF radiotelephone with a discriminator output.
Before you start, please check whether channels 87H and/or 88H (AIS1 / AIS2, 161.975 and 162.025 MHz) are available. If needed, an Icom dealer can program these for you as additional channels.
IC-M601 / IC-M602
The device can be opened by unscrewing 2 x 6 screws at the back. The shielded high-frequency unit can be found underneath a metal lid. The lid can be opened carefully using a wide tip screwdriver.
The transceiver contains two TA31136 receiver-IC's: one for DSC on channel 70 and one for the chosen VHF channel. The latter IC is used to make a discriminator output. It is located near the aluminium housing, close to the flatcable recess. As the IC pins are separated 0,65 mm (0.025"), good soldering skills are required, as well as a soldering iron with a fine tip.
Inside of the radiotelephone
Pin 9 of the 31136 contains the unfiltered audio
The 10 k resistor, soldered to pin 9
A 10 k miniature resistor is soldered to pin 9 of the receiver IC. The inner lead of a piece of shielded audio cable is soldered to the resistor's other lead. The braid can be connected to the metal shielding close to the IC.
The cable is lead outside through the flatcable opening. Take care to replace the lid correctly.
A 3.5 mm chassis socket can be placed in the front panel side, close to the microphone connector. Because the case is rather thick, the chassis terminal can be fixed to the panel with some hot glue. To align the chassis terminal's tip with the front, some plastic can be milled from the inside.
The negative lead of a 10 uF electrolytical capacitor is connected to the 'hot' solder lug of the chassis socket. The braid of the shielded cable is connected to the ground lug of the socket.