[1000mp] Re: Mark V Audio Distortion with MD 200 Micrphone, the cure

[kc6mmi]

Excellent post Mike.
thank you!
Tnx
Rich
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Sent: Saturday, August 11, 2007 2:06 AM
Subject: 1000mp Digest, Vol 40, Issue 10


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> Today's Topics:
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>   1. Mark V Audio Distortion with MD 200 Micrphone, the cause and
>      cure (Mike Schatzberg)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 10 Aug 2007 10:37:53 -0400
> From: "Mike Schatzberg" <cherokeehillfarm at earthlink.net>
> Subject: [1000mp] Mark V Audio Distortion with MD 200 Micrphone, the
> cause and cure
> To: <1000mp at mailman.qth.net>
> Message-ID: <000f01c7db5c$09d8dd30$6501a8c0 at mike>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Hello MP Fans:
>
> Over the years my Mark V has suffered on and off with RF distorting my
> audio.  Certain positions of my yagi could occasionally result in audio
> distortion during transmissions.   I normally use my MD 200 microphone to
> drive some outboard compressor/limiter with a downward expander to control
> noise.   As you know, this microphone was developed to work with the Mark 
> V
> specifically.   I have previously needed to use ferrite snap ons to 
> control
> the problem.  The problem is especially pronounced when running higher 
> power
> with an amplifier.
>
> Recently, I realized that the RF was entering the audio chain via the MD 
> 200
> microphone.  I started to examine the microphone carefully, and here is 
> what
> I discovered:
>
> 1. The element housing is floating, and not connected to chassis ground. 
> It
> seems to act as an antenna, and picks up RF.
> 2. The powder coating on the housing does not permit continuity between 
> the
> upper and lower portions of the housing.
> 3. The vibration damping mounting bracket is not grounded to the base
> either.
> 4. No chassis ground is carried in the cables to the microphone board, 
> which
> is grounded to the floating housing.
> 5. The base of the microphone and its switching board are indeed chassis
> grounded.
>
> Here's how you fix the problem:
>
> 1. At the point of attachment of the mounting bracket to the housing, 
> remove
> a small amount of the housing powder coating around the holes which are 
> used
> by the gold plated retaining pivot screws.  This will permit the mated 
> upper
> and lower housing halves to be electrically connected.  ONLY REMOVE the
> coating from the small area around the screws, from the surfaces where the
> upper and lower housings meet.  When the housings are reassembled, these
> contact points will not be visible
> 2. Remove the coating from the mounting bracket, at the very bottom, ONLY
> FROM the surface that mounts onto the post which extends from the 
> microphone
> base.  The post is at chassis ground already, and this change will ground
> the mounting bracket also.
> 3. Next, the MD 200 can accept a second cartridge, there is a switch in 
> the
> base to select either the stock variable pressure element, or the optional
> cartridge which you can install.  This means there are two extra wires in
> the microphone housing cable, which are not being used if you DID NOT
> INSTALL THE ADDITIONAL CARTRIDGE.  Remove the top housing and examine the
> microphone board.  You will see the two small pads, next to the microphone
> element connector, which is white.  One pad is marked, GRD 2, this is the
> unused microphone ground for the optional cartridge.  Solder a small gage
> wire to this pad, it will connect to the internal wiring, which is 
> connected
> to the base.  The other end of the small wire can be connected to the 
> board
> ground, which can be obtained at the near by pad with the mounting screw
> which attaches the board to the housing.  Providing this link will ground
> both the board to the housing, and the housing to the base, once the other
> end of the cable is grounded.  Reassemble the housing.
> 4. Remove the base cover, you will see the small slide switch which 
> selects
> the microphone cartridge.  In front of this is a row of pads which appears
> to be the termination of the cable conductors.  The 4th pad from the right
> is the 2nd cartridge microphone ground, you can confirm this fact now, by
> using your VOM to see that the cartridge housing is common now to this pad
> (make sure that the upper housing is plugged into the base of course). 
> Tack
> solder another short wire to this pad, and solder the other end to the 
> board
> ground, also at the point that the board is retained by the screw at the
> lower left hand corner.  This board is already at chassis ground, so
> completing the wiring to the housing, makes everything common.  Reassemble
> the base cover, and install the cartridge mounting bracket, and the
> cartridge housing into the bracket.
>
> I made some measurements and found that the housing screens are common to
> the housing itself, so grounding the housing grounds the large screens.
>
> In my case, the RF distortion was eliminated completely, and the audio
> quality was greatly improved.  It is surprising that Yaesu produced a
> microphone without grounded shielding for the element, with as large a
> screen area as this microphone has.  Obviously, take care not to damage 
> your
> microphone either electrically or mechanically during this modification,
> which does require moderate experience to complete.  This modification is
> done at your own risk, and no liability is assumed by the author for 
> errors
> or damage.  Check your work carefully, it should work well for you.
>
> I will eventually take some pictures of the mods on the two boards and 
> place
> them on my website.  If you have any questions, please let me know.
>
> 73 and Happy DXing,
>
> Mike
> W2AJI
> Administrator: 1000MP Reflector
>
>
>
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> End of 1000mp Digest, Vol 40, Issue 10
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