My homebrew ubitx project is based on the v2 design (so it's very similar around the audio stages except for a couple of added components to enable a "one wire" electret mic hookup).
I tried using a speaker mic (built-in electret element and speaker; the PTT switch completes the electret element path to enable PTT). When the electret element appears in the mic input path, a PNP transistor switches the ubitx to transmit state.
Here's the issue - when I hit PTT on the speaker mic, I get a brief (maybe 100 mSec) feedback - high pitched audio oscillation. If I use a separate mic and speaker, the feedback/oscillation doesn't happen at all. It only happens at the beginning of transmit; when I release the PTT on the speaker mic, no oscillation.
I suspect this is because both (rx & tx) audio paths to the balanced modulator are powered up for a moment. C52 (47 uF at the low level rx audio amp Q70) and C64 (47 uF at the mic amp Q6) would seem both to allow a short period of "opposite state" power to their respective circuits based on RC time constants involved.
Has anyone tried a speaker mic with ubitx, or had this (feedback) issue?
Thanks,
Dave
