4X1MK Ron Gang, Kibbutz Urim, Israel

Modifying the Yaesu MH-31 Microphone by adding a Heil mic element,
replacing the Yaesu dynamic cartridge with a higher gain, more sensitive Electret element:

Opening the MH-31 stock microphone case, I saw there was room to add the HC-4
element in the place of the metal weight that sits below the Yaesu microphone element.

Removing the weight was tricky, as beside it being secured with screws and washers,
Yaesu had generously added glue to hold it in place.

I sprayed some Radio Shack contact cleaner on the glue, and I believe it helped loosen
 the glue, such that I was able to VERY carefully pry off the weight bar
 without damaging the plastic around it.

In order to accomodate the HC-4 capsule, it was then necessary to destroy
the inside halves of the plastic protrusions which had held the screws of the weight bar.

Next, I driled a 5 mm. hole in the front of the microphone case over the spot where
 the voice hole of the Heil capsule is situated.

(Later, I used the sponge in the Heil package to hold the cartridge in place,
pressed upon on the other side by the PC board.)

Electrical connections....


I had found that the most agreeable position of the switch for my QSO partners
 before modification was when the little 0.33 uF (if I remember right) electrolytic
 cap was switched in series with the mic, so I rewired the whole thing so that in
position 1 the Heil element switched in and in position 2 the Yaesu element in
 series with the electrolytic cap is switched in.

Before buttoning it all up, a added a little piece of black earphone sponge
 between the element and the hole drilled in the front of the mic case,
 in order to keep dirt from entering the element.

While everything works as it should, one problem has always been the low output of the microphone, and the necessity for me, being a "soft speaker" to really get close up to the mike.
It turns out that there is a 680 ohm resistor installed in parallet to the output in the microphone....
if this is eliminated, the mike becomes much more sensitive.
This may affect the frequency response of the Yaesu element and it may be necessary to experiment with the value of its series capacitor...

or one may wish to substitute an inexpensive (i.e. cheap) electret element for the Yaesu dynamic element, with even more mic gain (sensitivity) and possible even better audio fidelity... more hi-fi for pleasant ragchewing.

This diagram tells it all.  The only new components are the mic elements and the 7.5 K ohm resistor. The slide switch on the back of the microphone case now selects the DX or ragchewing sound.    The 680 ohm resistor was placed in the output of ONLY the electret mike, as the output of the electret mike is so overpowering, this brought it down to a more manageable level.   (Further attenuation is probably in order, as when switching to the electret mike, I find I must reduce the mic gain.  You'll see what works best for you.)

The electret elelment with an optional rubber shield around it, as lifted from an old cassette recorder.
The Heil HC-4 element is hidden under the blue sponge.

The microphone being put back together.  
Be sure to situate the electret element right over the grille slits so the sound will get in OK.
Note the 7.5 K resistor added to bring the DC voltage supply from pin 2 to the electret element.

The yellow padding material helps hold the electret element in place and  provides acoustic damping.
All that is left to do is put on the back cover, secure the screws, and insert the plug (left corner of frame).

left:  An unmodified MH-31 (used on the VHF-UHF radio) ,
right:  A modified MH-31 connected to the FT-920.
(Note the 5 mm. hole to let the sound into the Heil capsule obliterating some of the print
on the mic face....  That's the only external difference between the two.)
The two mikes rest on the venerable D-104 microphone, their mounting brackets
screwed onto the D-104's upper face screws.  The allows me to use both hand mikes as
table mikes.  It isn't elegant, yet it works!

copyright 2004 by Ron Gang, 4X1MK

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