I replaced the stock dynamic microphone element in my MH-31 microphone with an electret element, as described originally by M0UKD. As far as I can tell, it works at least as well as the original microphone.
The stock mike never impressed me, so I had the W4RT voice booster installed. I did not notice any improvement, but the mike picked up RF on occasion. Signal reports deteriorated over time and finally the microphone seemed to stop working altogether.
It appears that the original mike element failed, so I chucked it and the modification in the trash and followed instructions per M0UKD, just taking note to reverse the polarity of the electrolytic cap so that the positive side faces the +5V phantom power.
I tweaked the layout a bit:
- I used existing holes for the two switched capacitors.
- A short run of 30 Ga wirewrapping wire worked nicely to bring power up the side of the microphone.
- The electret element I had on hand looks like it was meant to mount on a circuit board and had 0.1″ spaced leads, so I used Dupont connectors. It can be detached again if necessary and it avoided applying heat to the element it self.
- I just hot-glued the microphone in place. I don’t know if adding a lot of wadding around it really helps anything.
- I forgot to add the last 1nF capacitor until after I had screwed it together, luckily, there’s a convenient place to tack it down just below the switch.
- As others have recommended, I removed the metal weight from the microphone. Less to carry up SOTA hills.
I tested the modified microphone into a dummy load, with pickup by a nearby receiver in both SSB and FM modes. Voice quality seemed fine in either setting, so I tweaked the respective microphone gain settings on the FT817. Hopefully, I’ll have a chance to try out the new microphone “in the wild” in the next week or two.