I received very poor audio reports when using this microphone. After searching the web I found that the problem was universal and there were several fixes, depending on the actual version of speaker mic. I checked them out along with my own ideas. They all made a difference but none were as effective as I hoped.
In my unit the electret element was hanging, by its connecting wires, in a cylindrical 'dock' that was far too big for it. Unlike many examples though, there was a small hole from the dock to the outside world.
If you do not have a hole you must make one, or move the microphone element so that it listens through a small hole.
Microphone elements seem to be 6.5mm or 9mm diameter. My dock was 7.5mm. and as I could not find a new mic to fit I added packing round it so that it was a tight push fit in the dock. The front of the mic must be isolated from the internal space that contains the loudspeaker.
The results of this restoration of the original designer's intention were disappointing. The tonal balance was all wrong and the sound was very coloured. 'Head in a bucket' is an apt description. Something else was going on.
Incorrectly fitted curly cord. This is a known condition, but not recognised as spoiling the audio. It does!
Audio level too high. This causes distorted TX audio and can be reduced by speaking more quietly, but it is well worth reducing it electronically.
Here is a picture of my unit, after modification. There were also repairs needed because I damaged the board while hooking up test connections and configurations.
The six solder pads for the wire connections
are marked on the board, from left to right...
MIC PTT GND SP SP SP
(Right click the image for a much larger view.)
|MIC||TX Audio||Ring on big plug,||RED|
|PTT||Push to Talk||Sleeve on big plug,||BLACK|
|GND||Ground||Sleeve on small plug,||WHITE|
|SP||RX Audio||Tip on small plug,||GREEN|
Check that the wires in the curly cord connect to the expected plug contacts (above). If they do not, make your own connection table.
Make sure that the wires connect to the correct pads on the circuit board.
(I had to interchange the GREEN and WHITE wires. These are the usual culprits.)
When the GREEN and WHITE wires are the wrong way round the mic insert is grounded through the loudspeaker. This makes the speaker to function as a second microphone, in series with the electret. The speaker favours low frequencies and is very resonant. No wonder the TX audio quality is strange!
The TX audio wire (RED) carries the DC supply to the speaker/mic TX LED as well as the audio. The current is limited by R1 ('222', 2K2) left of the MIC pad. I accidentally destroyed mine. It is no great loss. With it gone I replaced it (with 2K7, the nearest I had, no problem). This makes it easy to connect a resistor (mine is 4K7, selected on test) in series with the electret element. The junction of the two resistors and the RED wire is dressed away from the circuit board. There is plenty of room for 1/4W rsistors.
When testing note that if the speaker mic is plugged in, it is the speaker mic that functions when the TCR PTT is pressed (and the regular mic in the TCR is inactive). Also, because of the RF generated in TX mode, any nearby or connected analogue multimeter is likely to indicate a voltage, even a reverse voltage. (This is because of the protective diodes fitted in it.)
Take great care when moving the cicuit board. The LED track connections will break away with very little force and I broke mine. I have re-fitted the LED with supporting sleeving between the board and the LED and a wire link to the PTT pad.
The speaker/mic double plug follows the 'Kenwood' standard. (The alternative 'Icom' or 'Yaesu' system is different and works by connecting the ground side of the microphone to cicuit ground.) I usually use a parallel resistor to reduce the mic output. With my Baofeng this killed the TX LED. My first tests were made with the resistor in series with black wire to the mic element. There is plenty of circuit track available for a ground connection. There is no reason why this unorthodox configuration should not be employed and the original R1 left in place.
As a final step I have added layers of acrylic felt (from a craft shop) above and below the circuit board. This reduces the colouration produced by the small mylar loudspeaker and improves the sound of received signals.
John Everingham. G4TRN
Recoded to HTML5 with typos corrected and minor additions, June 2022.
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© John Everingham