I presented this project at the Williamson County Amateur Radio Club's annual Project "Show and Tell" meeting in December, 2019. It received such a warm welcome that I thought it would be useful to place a description and photos of the project here so that those who wanted to build something similar, or who could not attend the meeting, could see what was done.
Throughout this document, you may click on any thumbnail photo for a larger version of the image.
I recently purchased a Yaesu FT-991A that came with a Yaesu MH-31 handheld dynamic microphone. Contacts I made through our local VHF repeater reported that my audio sounded muffled compared to my audio quality they were used to hearing with an external computer microphone I was using on a Baofeng UV-82HP handheld. Reports were that I was "full quieting" to the repeater, but the audio was muffled.
I took apart the MH-31 microphone, and the audio pathway consisted of a dynamic mic element, a 680 ohm resistor in parallel with the mic element connections, and a switchable series 0.33 uF electrolytic capacitor. While some inexpensive mics have been known to be behind a piece of solid plastic that could cause the muffled sound, this mic element was very nicely designed with the front slats open and a thin piece of fabric screening between the mic element dome and the slats. So the audio was able to reach the mic element without a problem. A cursory Internet search noted that a few others have had similar experiences with this mic. The most common fix was to replace the mic element, but I wanted to leave the mic stock for now. I also was starting to miss having a desk microphone.
I thought it would be a good idea to adapt the computer microphone that was used with the Baofeng HT for use with the Yaesu base station.
The computer desk microphone I used came with a Creative Labs SoundBlaster sound card from several years ago. I hot-glued a small button for push-to-talk (PTT) that came in a Radio Shack "Surprise Package" box in the 1970s (it looks like it came from a keyboard of some kind). The button has a good feel and makes consistent contact. These were connected under the base with a new 2-wire-plus-ground shielded cable to a 1/8" stereo phone plug. This was the configuration I used for both the Baofeng UV-82HP and the uBITX HF kit. The piece of fabric and twist-tie over the mic is to provide a pop filter.
There were two things that needed to be addressed in this project:
The Yaesu mic jack has a +5 volt DC power output (unused for the stock mic) available to power condenser microphones, so power was not a problem. Converting the jack styles would be handled later in the project.
I used a small solderless breadboard to wire together an interface circuit that provided power to the mic element, a DC block capacitor for the mic input, and a small equalization filter on the mic input to lessen the harshness of the sound slightly. Power for the condenser mic element is provided current-limited with a 10K ohm resistor.
Component choices went through a few iterations of testing before I finally settled on the circuit in the schematic below. Note that this schematic also has the circuit diagram of the original MH-31 stock microphone.
Through experimentation, I discovered that the original MIC GAIN parameter in the Yaesu FT-991A needed to be reduced from 50 for the stock microphone to 15 for the new microphone.
I was using an old "biscuit jack" that was used for connecting T1 lines or RS-232 serial connections over RJ-45 to break-out the Yaesu's mic jack to individual wires. It turns out that this was a good size for a compact project box for the interface and to convert to the 1/8" phone plug. All the parts in the schematic were soldered point-to-point and protected with either heat-shrink tubing or electrical tape. It isn't too pretty inside, but it is functional and compact, and is easy to add or remove as needed. To get the phone jack mounted, I increased the size of the notch already in the biscuit jack's lid, and cut a large notch in the bottom half to accomodate the mounted jack.
When completed, the interface box sits nicely on the desk using the stock cable from the original mic, and the desk mic plugs right into the interface.
While this works well and the audio is generally crisp and clear, the sound still emphasizes the highs too much for some operators. It's also a relatively inexpensive computer microphone, and it has its limitations for general use. I think that a better quality dynamic mic would have overall improved sound and a wider dynamic range. However, this project is a great way to have an inexpensive universal microphone for many different applications, at least in the short term. I will probably purchase a commericially-produced desk mic at a later date.
During testing with a dummy load connected to my transceiver, I also noticed a small amount of hum entering the audio path. Nobody has realized or complained about it, but I believe this is RF leaking into the unsheielded cable (or possibly through the DC power path). This is another reason to consider a new desk mic in the future.
Finally, I will likely investigate other dynamic mic elements to replace the one inside the stock MH-31 mic. That mic is still a great form factor for field operation of the very capable Yaesu FT-991A radio.