Explanation of how the delay circuit works.
Om Blake Bartosh has shared with us the explanation as to how the delay circuit works as you may be eager to try it out on your Bitx. The Audio Mute circuit operates in the following way. The 555 timer is connected to the PTT switch and generates a 100 msec pulse when PTT is engaged and another 100 msec pulse when PTT is released. The 555 timer is configured as a debounce circuit, so contact bounce of the PTT switch does not initiate any new pulses. The output of the 555 timer and the PTT switch signal are combined in logic to form two signals, a mute command and a transmit command. The mute command turns on Q16 of the Exciter board which shorts the audio signal to ground, and no sound is heard from the speaker. The mute command occurs immediately upon engaging PTT, but the transmit command is delayed by 100 msec. This delay ensures audio transients due to switching from receive to transmit mode do not cause a "pop" in the speaker, but is not so long as to be noticeable to the operator. The transmit command energizes the relay coil of K2 of the PA board via a 2N3904 transistor and the rig switches to transmit mode. Upon releasing PTT the transmit command is immediately turned off, K2's relay coil is de-energized and the rig reverts to receive mode. 100 msec later the mute command is turned off and the speaker is activated again. The Audio Mute circuit is powered by 12V+, and a voltage regulator reduces the 12V+ voltage to +5VDC required by the 555 timer and the 74LS00 quad NAND gate IC.