---WIRING AN EXTERNAL SPEAKER-MIKE FOR AN ICOM 24AT H/T--- ---Written by Noah Lehmann-Haupt KB2NDR--- ---May 13, 1992--- DISCLAIMER I am writing this document for the use of the general Amateur Radio community. It is being done in order to try to assist amateurs in making their radio system work. I hereby do NOT GUARANTEE IN ANY WAY that the following procedure will DEFINITELY work for your radio. If for some reason it does not, I hold NO RESPONSIBILITY for any im- proper actions taken on your part or on mine. * * * This was written for use with an Icom 24-AT, but should work with all "like" radios (NOT the W2A). However, it has not been tested on any- thing but the 24AT. * * * INTRODUCTION Recently, I have noticed a great deal of people who are start- ing to get into their own "homebrewing" of equipment. I, for one, am always looking for the cheap way out (if you can call owning a TS-440S and an Icom 24-AT cheap.) A couple of months ago, I had some spending cash available, and since the the big thing I wanted was a Yagi and a tower, and that was TOO big, I decided to get something that I could really use in the Shack, which was an external mic for my HF rig, and possibly for my H/T. Flipping through the AES catalog, I noticed the Heil BM-10 Boomset. That seemed pretty nice; it had stereo headphones, a very nice Heil mic, with the HC-4 element installed, and was basically just what I was looking for...except the $85 price tag. After a long fight with my willpower, I being the victor, I decided it would be a nice addition to the hamshack. The only thing was that it came wired only Icom, Yaesu, and Kenwood HF rigs, and NOT for any type of H/T. Thinking that I could figure some way to wire it to my IC-24AT, I said "why not?" and bought the thing, with the high-freq. response HC-4 DX'ing mic element installed. I also got the separate, lower freq. "Rag Chewing" HC-5 Mic element. When I got the whole setup in the mail, The first thing I did was to put together a make-shift PTT switch, by hooking up a regular push button switch to a 1/4" Phono Plug with a long wire (this plugged into the "Foot- switch" jack on the mic.) This would mean I could use the mic without VOX for those time I had a cough, or something noisy like that. The mic , by the way, was pre-wired for my Kenwood HF rig, with an 8-pin female plug on the end. Let me get to the point, so after some feeble attempts to wire the thing to my H/T, I finally got it. PARTS The following parts are needed for the wiring to work correctly: 1. A 2.5 mm Sub-mini 2-conductor audio plug (Radio Shack (RS) 274-279) 2. A 33K Ohm resistor (RS-271-040) 3. A .47 uF Tantalum Capicitor (RS-272-1433) 4. A MALE 8-pin Radio Plug (optional--for mics wired for Kenwood HF rigs Any MALE plug, depending on you rig, may be used, but is not essential) 5. 3-conductor (or 4) shielded wire (optional--RS-278-777) 6. A Soldering Iron 7. X-Acto or similar knife PROCEDURE In this procedure, we are assuming you are connecting a mic wired for a Kenwood HF rig--modify the instructions accordingly. First, note the color, or just note, which wire is which: Mic, Shield, and PTT. Solder the mic wire to pin 1 on your male plug, or connect the mic wire to the mic output of your microphone. Next, connect the PTT center to pin 2 of the MALE 8-pin plug, etc. Connect the PTT ground (if any) to pin 8 of the 8-pin plug. The mic shield goes to pin 7. After this has been completed, connect all grounds to the Shield of the phono plug. Then connect BOTH the PTT center and the mic center to the tip of the phono plug. The next step can be modified in any way, but generally, you now want to cut the PTT and mic center wires below the phono connector and install a 33k resistor in series with the PTT center line (make sure you know which wire that is) and install the .47 uF cap. in series with the mic center line, making sure the + end is toward the plug. Wrap the entire connection with electrical tape or some sort of insulating connection, making sure not to short either of the lines to ground accidently. Again, this procedure should work with any mic, not necessisarily a Heil mic, and not necessisarily a Kenwood rig. The only things specific to the Kenwood rig are the pin connections on the 8-pin plug. The resistor and capacitor are universal. ** NOTE ** In a recent QST, Icom's Tech Talk column/ad gave the above procedure, but they said to connect the mic and PTT to the center ring on a STEREO plug. That may work, but the above setup has been tested and SHOULD WORK on a 24AT radio. If you have any problems feel free to contact me anytime: Noah Lehmann-Haupt KB2NDR 627 west 247th street Bronx, NY 10471 USA I have included a .GIF file in this ZIP file containing a ROUGH (hihi) sketch of what the setup should look like. Remember, it was drawn by hand.