CAIRO is a standard for wiring between radios and accessories. It is sometimes used in RAYNET to allow remote control of radios, and offers an advantage that any CAIRO accessory can be connected to any CAIRO radio. That would be very useful for me, for example

• Headset with VX-7, 8900, 897 or any future radio.
• Signalink with VX-7, 8900, 897 or any future radio.
• Use switching boxes in the shack rather than reaching behind to swap plugs.
• Experiment leaving the radio closer to the antenna to reduce feeder loss in weak signal situations in RAYNET.

I tried making a passive CAIRO adapter for the Yaesu VX-7 following the
Passive Series Circuit. I also made a breakout cable to allow it to work with a PC headset. I used a 2K2 resistor rather than a 4K7 in the adapter based on the information in the Yaesu manual.

Sadly results were poor:

• Hum pickup was very bad. I think this is because in the passive circuit the MIC LOW line is routed back up the PTT line, through the PTT switch and back down the shield. This allowed huge scope for pickup even on my short cable.
• I could PTT by touching the Mic and Ear sockets together. PTT is made by connecting MIC LOW to system ground. This could be achieved by touching the metal cases of the MIC and EAR sockets together. I could use plastic cases, but this still leaves open the possibility of shorts elsewhere. It did save me having to build a PTT switch.

An Active Adapter should perform better. If I have time I’ll try to build one for the Yaesu 8900 and any other radio that uses its 6 pin mini-DIN connector. This circuit will clearly separate PTT and mic, and cause the mic signal to be sent through its quasi-balanced shielded line. The phantom powering arrangement makes it significantly easier to tie the MIC LOW line to system ground. It’s not true balanced line, and not like the common mode phantom power technique used in theatrical systems.  I hope it works well.