The µBITx comes with an electret microphone capsule, and the circuit is designed to support this (with “mic bias” of around 9v applied to the element).
Constructors can use this element and a press-to-talk switch to create their own microphone. A number of examples of homebrew microphones can be found on the BITX20 list.
One such example from a list member uses a roller pen as the mic enclosure with the element poking out the end (and presumably glued or jammed in place).
Getting more gain from your electret microphone
Dave WI6R says, “To simply get more gain with an Electret Mic you might try dropping another 47 Ohm chip resistor on top of the existing R63 by soldering another chip resistor [or a standard through hole resistor] one end at a time.”
Using a cheap commercial alternative
Many constructors have purchased cheap Baofeng style electret microphones from eBay.com or Aliexpress.com. You can use the existing connector (Kenwood style dual 3.5″ and 2.5″ connectors), but it is a bit of a fiddle to align two jacks to the correct spacing. Simply cut off the original connector and replace with another fitting.
You could use a Kenwood/Yaesu/iCOM style 8 pin plug, or a 4 pin plug, or you could separately wire up two stereo 3.5mm plugs, so that you can use the jacks supplied with the kit.
The wiring diagram for a Baofeng mic can be found here.
Be warned that some of these microphones sourced from China are not of a high quality. They may be factory rejects and have wiring issues, internal circuit boards may have quality issues, or the cable may have discontinuities. The wiring on the cable varies considerably (colours are not standardised) so disassemble your mic first to identify wiring colours before wiring up suitable plug(s) for your µBITx.
There appears to be no need to modify the Speaker/Mic at all. A red LED light on the Speaker/Mic will light up when the PTT is pressed.
Using a Dynamic Microphone
Dave WI6R, observes that in referring to the uBITX Schematic:
“The Gain of the mic pre-amp is controlled by R63 in the emitter of Q6 to C62/R64. It basically decouples the audio bypass of R64 by C62 to limit gain for the high output electret mic. Replacing R63 with a Zero Ohm resistor and putting a 10K at the Mic input should allow a Dynamic Mic to work with a Mic Gain Control.
“R60 that supplies Mic bias to the electret-condenser microphone needs to be removed. If there is not enough gain, you may have to reduce the value of R64 to around 470 Ohms. Look at your transmitted signal on a scope , or listen to your audio on another receiver, to verify you are not causing distortion.