Repairing the Yaesu MH-27 microphone to correct low audioParts Needed: (per mic)
1 ea. 0.1 uF Tantalum cap @ at least 10-12 volts (C5). Suggestion: Radio Shack
#272-1069 0.1 uF Polyester Film @ 50 volts
1 ea. 1.0 uF Tantalum cap @ at least 10-12 volts (C6). Suggestion: Radio Shack
#272-1434 1.0 uF Dipped Tantalum @ 35 volts
KC2RDX, who has successfully completed the repair, wrote to say that he used
Radio Shack parts with total success and provided the above RS part numbers. I
checked the Radio Shack website and they appear to be available at most stores.
Now, on to the good part.....
The engineers at Yaesu, for whatever reason, used 50 volt electrolytics in
designing the MH-27 mic. As you know, an electrolytic needs voltage for the
capacitance to stay "formed". Well the 5 volts in the mic just isn't enough and
they gradually lose their ability to pass AC signals. This prevents audio from
the mic getting to the radio.
You will be working with Surface Mount Technology here, but the caps that need
replacement are fairly large in size. Open the mic by removing the three
external screws; one in the hangup button and two in the black plastic case.
Gently unplug the mic cord and set it aside. Remove the three smaller screws in
the PC board "sandwich". The "on air" LED, the electret element and the UP/DOWN
switches are not physically attached to the case so they may be pulled from
their locations. You can now remove the PC boards leaving the rubber keyboard
membrane in the case.
The boards are held together by headers at each end - a 2 pin header at the top
and a much larger one at the bottom near the mic cord plug. Use a solder sucker
or solder wick to free these. You may now separate the two boards. On the
inside surface of the back board, you will see 4 black cubes approximately 3/16
inch on all sides. The two that are right next to each other are the audio
coupling caps. With the mic cord connector facing downward, the one for the mic
audio (C5) is on the right and is nearest the edge of the board. The one on the
left and closer to the center of the board is C6, the Touch-Tone (TT) audio
cap. Mine were marked "1R" and "1".
Simultaneously heat both terminals on the end of C5, the outboard cap, and
remove it. Replace it with the new 0.1 uF cap, observing polarity if
applicable. Tack solder. Neatness counts! Also replace the 1.0 uF TT cap (C6),
the inboard one of the pair, while you're in there. Even if your tones are
working now, their days may be numbered! Remember that C6 is a 1.0 uF cap, not
a 0.1 uF.
Make sure nothing is shorted and solder the two boards back together. Screw the
boards back into the mic. Re-install the electret element, the "on air" LED and
the UP/DOWN switches. The pot you can see after re- assembly is for Touch-Tone
audio only. There is no pot in the mic for voice audio. Re-install the mic cord
and test it. See note below. If all is well, close it up.
NOTE: Although not likely just because of the cap replacement(s), audio levels
may need adjustment. There is a pot internal to the radio that controls ALL
audio coming from the mic. Adjust the internal radio pot for voice level first,
then adjust the pot in the mic for TT level. Since the MH-27 was used on more
than one model of radio, you're on your own in locating the internal radio pot.
If you can't find it, just adjust the mic pot for TT levels. Again, level
adjustments due just to cap replacement will not likely be needed. But, if like
me, the first thing you did to try and get your voice audio back was to adjust
the pot in the mic, you may have changed the TT levels.
--- On Sun, 3/7/10, LHOTP@ATT.NET <LHOTP@ATT.NET> wrote:
From: LHOTP@ATT.NET <LHOTP@ATT.NET>
Subject: [Yaesu] MH-27 Mic DTMF inop
To: Yaesu@contesting.com
Date: Sunday, March 7, 2010, 9:56 PM
The DTMF on my MH-27 microphone is intermittent/ weak at best; voice
reports are A-OK, so I assume the problem is isolated to the DTMF
portions of the circuit board. I looked online and found that one
likely bet is aged capacitors. I don't have the skills to R&R the
capacitors myself. The mic is attached to a FT-2500M, if that matters.
Does anyone know of a place to have it repaired for a reasonable price?
I'd rather not spend more than about $30 to fix it when a new 2m is
barely four times that price.
Jon KE5WGZ
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