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Installation of FT817 RF Speech Clipper into:
Yaesu's MH-36 DTMF Microphone (see pictures at end of description)

  by Tony Wallace VE3TNW   bravhart@sympatico.ca  

What is described here is my own experience at installing the FT817 Clipper by DF4ZS into the MH-36 mic, which I now use exclusively for ssb.  At the time of installing, my aims included the ability to test the clipper in on-air A-B comparisons which had me include a switch so that I could easily flip between raw audio from the electret element and the processed output of the approx. 1” x 1 ½” circuit board. This is NOT necessary for anyone else to do and so will be left out of these instructions, but will still appear in some of the pix as I wasn't about to undo the switch installation at the time of scanning.   Be aware that this modification will most surely void the warranty on your MH-36 so decide if you're willing to forego this convenience, and make sure it's working BEFORE you start the mod. Read ALL the instructions especially "6" before deciding whether or not you're up to it.

Dress all leads and install such that there's no possibility of shorting under stress after soldering into place. Be sure to refer to the various and sundry pix to be found hereabouts while reading these instructions, lest ye become thoroughly and extremely hostile towards me and be likewise confused by my attempts at explaining this process.   One of the pictures is a freehand sketch of the main features on the MH-36 PCB and is annotated in text and numerals. These numerals refer to a variety of points on the schematic diagram that came with your clipper. This, along with the accompanying text and pictures gives you 3 ways to check your understanding of the process. If you like success, use them all…and frequently.You'll need… a keen eye, a steady hand, a soldering iron of 40 watts or less with a very tiny long and pointed tip, and the skill to go with it. Oh, yes… if you can't pick fly scat out of pepper, I don't recommend you do this. Get somebody who can.

1)  Remove the three Philips head screws from the rear of the mic, and gently lift (wiggle?) the rear cover from the facedown front half. Mine wants ever so much to cling to the cable strain relief that I need to focus on separating these and the cover just lifts away after this.

2)  Remove the cable's connector from the PCB. Also, remove the 2 screws that hold the rectangular chunk of steel plate from the cover's inside. Yaesu presumably put this piece of WWII surplus battleship hull here to give the mic a better "feel" to it for the price we paid! Nip off (slur unintended) about ½ the height of the threaded post as noted in the picture. Mine was cut off to about 1/3 less with side-cutters. This way you'll have enough headroom for the clipper and can still restore the mic to stock complete with steel if you wish at a later date.

3)  At the top, you'll see 2 wires, one red, the other black running between the electret element there and the 2 solder pads just southwest of the PTT micro switch. Desolder the ends from these solder pads and lift out of the way for now.

4)  If your clipper came with a mic element attached, remove it and the leads from the PCB.  It won't be needed. Turn the clipper component side down over the MH-36 PCB in the area shown in the pix. The area between these needs covering with vinyl electrical tape to ensure insulation between them when the cover is replaced. After doing so, attach the red wire from the MH-36 mic element to point #2 on the clipper board. Crosscheck (text, #s, and images) and make sure you are on the right bit of real estate before soldering! The black mic element lead (#3) goes to the left hand terminal of the PTT switch, where it is soldered in place.

5)  A ground lead from #5 on the clipper is soldered to the left hand pad as in the sketch. The audio out signal from #4 on the clipper is soldered to the right hand pad.

6)  This step is the "finickiest" part of the surgery, in my estimation, and requires the greatest acuity. Look carefully at the sketch and locate the very small plated through hole on your MH-36's PCB that is to provide +5 volts to the clipper. Strip the white lead coming from the clipper (#1) just enough to provide enough bare wire to poke into this hole without it protruding through the other side more than 1/16". If it's screened over, carefully scratch the surface of this hole just enough to reveal metal. Very slightly, pre-tin the pad leaving the hole open! Poke the white lead home and hold it in place while you quickly solder the joint in place making absolutely sure you don't short to any other land or component around there. Check with an ohmmeter as well as visually.

7)  Check and re-check all of the steps involved for accuracy, good solder joints, and lead dressing that precludes any possibility of shorting during reassembly and use. Better safe, than sorry!

8)  Replace the mic cable into its socket, the PTT lever into its position, and carefully put the cover over the whole works. You will probably find that the clipper needs to be moved into place slightly and leads may need tucking in to be free of pinching. DON'T FORCE anything into submission! There ought to be plenty of room to maneuver things into place without so much room that things are going to "rattle" around. Be patient, and, as I eventually did, use a bit of foam tape if there's too much room when you've found the ideal placement arrangement. When all's right in the world, you can replace the mic cover without force and put in the 3 screws and "hanger button".

9)  If you're like me, wait ‘till the usual oral emissions subside, take it apart again to replace the cable, the PTT lever, or both, then, reassemble! Assuming all went well in the long run, it's time to taste the pudding. Stoke up the FT817 and plug in the mic as usual. Nothing should happen! If the transmission light comes on without "tapper" down (that's a QRP Hammer), turn off the rig, and GO TO Step 1 above! Check and recheck everything until you find the cause. Fix it, do 7 through 9 again, then return to this paragraph.

Once functional, you may have to tweak your mic gain in the FT817's menu to get ALMOST full power on the power meter whilst talking comfortably and with no over modulation. You ought to be able to see that you're now getting near full power all through your words, as you talk on-air, AND get through more often and in more adverse conditions than before. Such, is the joy of RF Speech Processing!!

72 de VE3TNW, Tony  bravhart@sympatico.ca  


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