Add the Astatic D-104 Microphone
to the Icom IC-706MKII

By W8CWE


Introduction

It was about a year ago that I purchased an Icom IC706MKII HF through 2M radio for home base use. Although I quickly came to love this rig, I have always had some difficulty with the low audio output from the radio. Out of the box audio reports were “low audio”, “sounds thin”, and “your HT has twice the audio.” I quickly set out to do some investigating.

The first thing I did was to go through the multiple posts found on the Icom 706 discussion group at Vartel.com. I found this discussion board to be a wealth of knowledge and feedback from other 706 users. I found many posts dedicated to the same audio problem I was having. The first fix attempt I did revolved around the stock hand microphone. This fix consisted of removing the mic element from the shell, drilling a small hole in the front of the shell where the mic element mounts, and then reinstalling the element. This really helped improve the volume, but didn’t help the poor frequency response. This fix also introduced a new problem with “breathiness” on fricatives (Ps, Ts, etc.). I was not satisfied.

The second thing I did was to purchase the Heil HM-10 microphone with the Icom in-line preamp. This really improved the frequency response, but even with the mic gain set at 10 and the preamp in-line, I still was unable to obtain decent output power for SSB work. I had been running with the Heil microphone until now.

The Challenge

I had read a number of posts on the Vartel.com 706 discussion group concerning the interface of the Astatic D104 microphone to this radio. It had been tried by a couple of folks with marginal results. Most reported overmodulation and an extreme “tinny” quality to the audio on FM. I actually traded some Email with a few folks and was told by one that the D104 wasn’t compatible with the Icon 706 series radios. That’s all I needed to hear. Tell me I can’t do something and I will immediately go off and figure out how to prove you wrong.


A Few Words About the D104 Used

I need to point out right away that the D104 I used may not be the “standard” typical microphone. Most D104s I’ve seen and owned were wired with a 5-wire coiled microphone cable. The one I used was purchased used from a fellow ham and was wired with a straight, three-wire cable. I have had this microphone wired to a number of radios in the past. I chose to use this one only because it was not currently being used on anything else in the shack.


Materials Required

RJ45 Connector (Computer Twisted Pair Network Connector)
8-conductor Twisted Pair Cable
RJ45 Crimp Tool
Alternatively, you can purchase a short twisted pair network cable from your local computer store and cut what you need off of this.
Soldering Iron / Solder
Shrink Tube
1/2W 33K Ohm Resistor
1 Small Tie Wrap (nylon wire tie)


Preparing the RJ45 Connector

1. Install the RJ45 connector onto a short (1-foot) length of the twisted pair cable.

2. Cut the twisted pair cable so that only about 2-inches or so are left protruding from the RJ45 connector.

3. Using a small scissors, cut the cable jacket back from the twisted pair cable to expose approximately 1-inch of the individual twisted pair wires.

4. Prepare the end of the microphone cable to be soldered to the twisted pair wires.

5. Slip a section of shrink tube over the end of the microphone cable. Use the appropriate size and length to completely cover the junction you will make between the twisted pair cable and the microphone cable.

Identify the Correct Twisted Pair Wires to be Soldered

1. Identify the correct twisted pair wires corresponding to the RJ45 connector wire positions. They are as follows:

Pin 4 = PTT
Pin 6 = Audio
Pin 7 = Ground

RJ45 FEMALE CONNECTOR(Micropnone Connector Port on Radio)

RJ45 FEMALE CONNECTOR(RJ45 on Microphone Cable)

Solder the RJ45 Connector to the Microphone Cable

1. Prepare the three twisted pair wires for soldering by stripping a small amount of the individual wire insulation from the wires. Slip a section of appropriately sized shrink tube over the ends of each wire. Make sure that the size and length of the shrink tube will allow complete coverage of the solder junction you will make in the next steps.

2. Solder the Audio wire from the microphone cable to the appropriate twisted pair wire coming from position ??? of the RJ45 connector.

3. Solder the PTT line from the microphone cable to the appropriate twisted pair wire coming from position ??? of the RJ45 connector.

4. Solder the Shield from the microphone cable to the appropriate twisted pair wire coming from position ??? of the RJ45 connector.

5. Dress each of the individual solder connections by sliding the shrink tube you placed earlier over the exposed wires. Heat the shrink tubes until each is securely affixed over the corresponding solder junctions.

6. Verify that none of the solder joints shows any exposed wire.

7. Cut the remaining twisted pair wires back so that they are flush with the outer twisted pair cable outer jacket. Separate them so that the ends are not in contact with each other.

8. Dress the entire assembly by sliding the large shrink tube you placed on the mic cable earlier over the entire solder junction. The shrink tube should overlap both the outer twisted pair cable jacket and the outer microphone cable jacket. Heat the tube until it is securely affixed over the junction.

Modify the D104

You will now be required to open the base of the D104 microphone and add a resistor in-line between the Audio wire and the solder pad where it currently resides.

1. Remove the base plate (if installed) from the bottom of the D104. Save the three screws.

2. Remove the 9V battery from the microphone. You will no longer need to have a battery in the microphone. The microphone circuitry will now be working from the 8V DC which is on the 706 microphone line.

3. Insulate the battery connector snaps with shrink tube or electrical tape. Secure the battery connector and excess wire to the 9V battery holder with a small tie wrap.

4. Desolder the audio wire coming from the microphone cable from its associated solder pad.

5. Solder one end of a 1/2W 33K Ohm resistor to the now free end of the audio wire.

6. Dress the connection with a piece of shrink tube.

7. Solder the remaining end of the resistor to the audio wire solder pad.

Testing and Adjusting

Initial testing can be done using an open frequency and a scanner. The best results will be obtained by actually talking with someone on the air and adjusting the mic gain on the radio appropriately. Start with a setting of 2 and work from there. You will probably end up having to back it down a little from this initial setting. Keep the level pot in the base of the D104 fully open. Choking the audio level using the pot seems to remove much of the base response from the audio.

If you find that the microphone is still too hot, experiment with different resistor values. I ended up using the 33K Ohm value. You may find that a different value works better for you (or more than likely, works better for those you are talking with). Play with different Mic Gain settings as well.

If the final output seems too tinny (not enough base in the audio), you may be able to experiment with different capacitor values added in-line with the audio wire. I actually discovered a different method for lowering the frequency response of the microphone. I had a couple of spare D104 element heads. I took one of these spares, opened it up and proceeded to punch a series of pin holes in the element foil. This seems to relieve some of the tension on the foil and results in a much warmer response. I’m sure there is a way to do this electrically (so as not to destroy a perfectly good mic element for future use), but I was unsuccessful in my attempts to do this. Since I had a number of spare elements to play with, I wasn’t inhibited in punching holes in the one I used.

Final Results

FM – With these modifications, the audio is simply superb with a mic setting of just 1. This means that you adjust the Mic Gain to 0 and then turn it up until 1 is just displayed. Everyone I talked with (7 different local operators) picked the D104 hands-down over the Heil HM-10. The biggest difference is the audio level (louder) and the crispness of the audio (“crystal clear” as one guy put it).

SSB - With the same Mic Gain setting, audio is even more impressive on SSB. With the D104 and Compression enebaled, full power output is now realized (average output of 60W with peaks nearing 100W). Previously I was lucky to see peaks of 30-40W with the stock mic or Heil HM-10. Recent 6M openings (week of January 3rd) were a joy to work wit this new configuration. I have never had so many "first call" contacts working 6M DX. All signal reports were of the "Great Audio" variety.

Caveats - Keep it down! Overmodulation is very easy with this setup. I heven't tried this in a mobile environment. The small amount of unshielded CAT5 cable may allow stray RF to enter the radio. A ferrite core around the CAT5 cable may help.