Ron Gang, Kibbutz
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
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.)
I WANTED THE SWITCH
ON THE BACK OF THE MIC CASE TO SWITCH BETWEEN
THE YAESU ELEMENT ("hi-fi") AND THE HEIL CAPSULE ("dx").
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
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
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
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
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 mikes rest on the venerable D-104 microphone, their mounting
screwed onto the D-104's upper face screws. The allows me to use
hand mikes as
table mikes. It isn't elegant, yet it works!
2004 by Ron Gang, 4X1MK
to the 4X1MK ham picture gallery
now for something completely different.....
may want to enjoy my paintings.