Transmit sidetone from Spectra conventional

[kc7gr]

Fellow BatWingers,

I'm in the process of adapting an aircraft-type audio switching panel to work with a pair of Spectra radios in a mobile configuration. The panel has provisions for transmit sidetone input when used with a headset, and I would like to use this feature with the Spectras, just to remind me that I have a live mic.

In looking at the command board schematic in the detailed service manual, it appears that there are two uninstalled components -- R425 and C425 -- that, if installed, provide a path between the MIC HI line and the input of the radio's sidetone amp, U401B. However, the output of that same amp goes straight into the audio filter IC, with no sign that it puts its output anywhere to the outside world.

My (tentative) plan at this time: Install C425 and R425, and run an additional output tap in the form of a .01uF capacitor from the output of U401B to an unused pin on the options connector (they're both remote-mount A9 series radios, with the Systems-9000 heads). From the options connector, run that line into the audio panel's sidetone input.

My concern: Will adding those two components also make mic audio appear at the radio's speaker during transmit?

Has anyone tried this before? Any other comments?

Thanks much.

73 de KC7GR.
(Bruce Lane)