Arrghh -- sorry, phone call and I hit the wrong button.Continuing ...
When the radio goes into TX, that cap has to charge up via R127 -- which takes a LONG time. ?During that time, Q12 is also powering up and C122 has been sitting at zero volts, so it starts charging as well. ?This is the response of the Microphone Amplifier to the rising edge of the TX signal -- and the net result is that the Balanced Mixer (T4/D15/D16) is knocked out of balance until the amplifier stabilizes. ?The result of that is a spurious RF transmission that looks like the one I have attached as a file. ?The RF rises almost immediately to about 5-8 watts, within 5 ms drops down to about 3 watts, builds back up to 5-8 watts in another 5 ms, then slowly decays over a period of 60 ms decays down to close to zero. ?(All of this observed with the PTT line held).
That's pretty much the equivalent of a DOT at 20 WPM. ?You can hear that TX burst clearly on a monitor receiver.
When sending CW that initial burst may be masked by the first element as long as the speed is slow enough (< 20 WPM).
Since the unintended RF burst is caused by the transient response of the stage being powered up, a more optimal solution would be to keep that stage powered at all times, but shunt the audio to the base of Q12. ?Reducing the size of C124 would help shorten the burst.
73, Bob, WB4SON
Bob