Improving the Passive CAIRO adapter for VX-7

As a last minute experiment before bed I connected the PC headset microphone directly across the VX-7’s mic input. The VX-7 is designed to provide VOX with compatible headsets, so must be able to work with the mic permanently connected.

The VX-7 did work, along with its VOX function. This gave an idea for improving the Passive Adapter. Disconnect Mic Low from PTT and instead move it to system ground. The Microphone signal no longer has to pass through the PTT circuit.

There is still connection between the PTT circuit and the microphone circuit. I could use a transistor to try to reduce this, but now the unshielded length is system ground and there’s a 2K2 resistor between the PTT circuit and the microphone circuit. A small capacitor was added across the PTT terminals to act as a path to ground for induced currents.

The circuit is now:

Testing the circuit I find

  • Much quieter monitoring the mic on the “Wave” display, and in the VX-7’s monitor function. The capacitor greatly reduces the pickup of mains hum when I touch a screwdriver (and myself) to the PTT line.
  • Hum when transmitting, perhaps due to RF getting into the circuit. I could also investigate the drop in DC bias when PTT is closed, but the hum appeared in VOX mode too, and also when I tried with the microphone connected directly across the radio’s mic input with short crocodile clip leads.

I expect this circuit will have problems with low impedance dynamic mics or low impedance transformer isolated active accessories. It violates the CAIRO requirements in this way. It could possibly be fixed using a P Channel MOSFET to switch on the microphone and resistor circuit when PTT is pressed. The gate would normally sit at the DC bias level of 3V pulled up by a resistor. The PTT switch will pull the gate down so activating the microphone circuit. This would remove the VOX feature. It could be the next circuit to try.

The complete assembly with PC headset adapter is:

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