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Write up: How to install an external mic using a CH721 head

Techholder88

Member
Joined
Apr 18, 2008
Messages
35
Do you have a CH-721 head and have a need to run the wires behind the radio?
IMG_4675.jpg
I needed/wanted to utilize the external mic feature on my CH-721 head instead of running a mic cord across my Truck radio touchscreen and needed a bit of time to figure out how to do it, now that I understand the system, I can illustrate it and hopefully clear it up for anyone who wants to know.

First you will need to figure out what wire does what from the mic. I opened up a spare one I had and there are 5 wires that are present.
Red is PTT
Black is GROUND
White is MIC HIGH
Brown is MIC Low
Blue is for the CG disable input or mic hangup function (which I will not be using for this)

How you will connect the mic to the option cable is going to be beyond the scope of this discussion, if you have physical connectors to connect the 17 pin portions of the mic to something else and the part numbers let us know what they are. I happened to stumble upon a mic which had an RJ45 connector from the factory but you don't need to go looking for one to make this work. I assume an older Harris M7100 style mic would work as the wiring scheme is pretty much the same. If you have a mic with cut wires, it will be a perfect donation for the project.

Depending on how you want to connect the wires to the rear of the head is going to be dependent on which option cable you use or if you make your own.

I elected to make my own because I didn't want to make a bigger hole in my dash.

I connected the wires as follows:
MIC Ground (BLACK)--- PIN 7 of back of CH-721 head (Blue Wire of a standard DB25 Cable) SHOWS AS GROUND on Datasheet
Mic PTT (RED wire)---- PIN 22 of back of CH-721 Head (Yellow/Black wire of a standard DB25) SHOWS AS INPUT 1 on Datasheet
MIC HIGH (WHITE) ----- PIN 24 of back of CH-721 Head (Grey/Black wire of a standard DB25) SHOWS AS EXT MIC on Datasheet
MIC LOW (BROWN) ----- PIN 23 of back of CH-721 Head (Green/black wire of a standard DB25) SHOWS AS CUTST/FPROG on Datasheet
CG disable I did not use because it is unnecessary for my use

If you use the CH-721 option cable CA-011854-001 you will connect 2 of your wires to Input 1 (or Input 2) and a ground, as well as pin 24 for External Mic High and pin p23 for External mic low.

MIC Ground ---- Ground PIN 2 or PIN 4 of the 6 pin plug
MIC PTT ----- INPUT 1 Pin 1 of the 6 pin plug
MIC HIGH ----- EXT MIC Pin 24 of the DB25 plug on the cable
MIC LOW ----- CUTST/FPROG Pin 23 of the DB25 plug on the cable.

Input 2 would use PIN 5 of the 6 pin plug and ground would use PIN 2 or PIN 4


If you are using the 44 pin option cable CA-012349-001 at the rear of the radio you could connect as follows:
MIC Ground ---- Ground Pin 1 of the DB25 connector on the cable PIN 8 of the DB44
MIC PTT -------- Input 1 Pin 21 of the DB25 connector on the cable PIN 18 of the DB44
MIC HIGH ----- EXT MIC Pin 24 of the DB25 connector on the cable PIN 6 of the DB44
MIC LOW ------ EXTALO Pin 17 of the DB25 connector on the cable PIN 27 of the DB44

If you wanted to use input 2 use the 6 pin plug with pin 5 of the 6 pin plug which connects to PIN 25 of the DB44
If you use the input 2 for PTT, use the 6 pin plug with pin 2 or 4 for ground which connect to PIN 7 of the DB44

In RPM or RPM2 you will need to go to External IO option and assign PTT to Auxillary input 1 (or Auxillary input 2)

Capture.JPG

L3 Harris recommends using a FM-017900-001 dust cover on the front of the head if you use this external mic, but it is mostly an expensive dust cover. As long as you chose to only use either the mic port on the front or the method mentioned here no damage will occur.
 

merlin

Active Member
Joined
Jul 3, 2003
Messages
3,094
Location
DN32su
Um, the acessory jack on the rear of the head has pinout for external mic, as does the DB44 on the rear of the radio.
 

merlin

Active Member
Joined
Jul 3, 2003
Messages
3,094
Location
DN32su
L3 Harris recommends using a FM-017900-001 dust cover on the front of the head if you use this external mic, but it is mostly an expensive dust cover. As long as you chose to only use either the mic port on the front or the method mentioned here no damage will occur.
That little thing ? only thing makes them expensive is usually comming in lots of 50.
 

redbeard

OH, PA, WV Regional Admin
Database Admin
Joined
Feb 5, 2003
Messages
1,368
Location
BEE00.348-3.1
That little thing ? only thing makes them expensive is usually comming in lots of 50.
L3 Harris recommends using a FM-017900-001 dust cover on the front of the head if you use this external mic, but it is mostly an expensive dust cover. As long as you chose to only use either the mic port on the front or the method mentioned here no damage will occur.

Keep an eye on eBay, I'll have some front mic cover kits available soon. And much cheaper than the $80 each Harris wants.
 

Teotwaki

Member
Joined
Aug 3, 2008
Messages
549
Location
SoCal
Various issues have kept me from posting them online yet, but I will have them at Hamvention next weekend. I'll be in the flea market area at 8441/8442.
I won't be going so I'll have to ask PRCguy to cruise by your sales spot for a look-see.
 

Techholder88

Member
Joined
Apr 18, 2008
Messages
35
Do you have a CH-721 head and have a need to run the wires behind the radio?
View attachment 159775
I needed/wanted to utilize the external mic feature on my CH-721 head instead of running a mic cord across my Truck radio touchscreen and needed a bit of time to figure out how to do it, now that I understand the system, I can illustrate it and hopefully clear it up for anyone who wants to know.

First you will need to figure out what wire does what from the mic. I opened up a spare one I had and there are 5 wires that are present.
Red is PTT
Black is GROUND
White is MIC HIGH
Brown is MIC Low
Blue is for the CG disable input or mic hangup function (which I will not be using for this)

How you will connect the mic to the option cable is going to be beyond the scope of this discussion, if you have physical connectors to connect the 17 pin portions of the mic to something else and the part numbers let us know what they are. I happened to stumble upon a mic which had an RJ45 connector from the factory but you don't need to go looking for one to make this work. I assume an older Harris M7100 style mic would work as the wiring scheme is pretty much the same. If you have a mic with cut wires, it will be a perfect donation for the project.

Depending on how you want to connect the wires to the rear of the head is going to be dependent on which option cable you use or if you make your own.

I elected to make my own because I didn't want to make a bigger hole in my dash.

I connected the wires as follows:
MIC Ground (BLACK)--- PIN 7 of back of CH-721 head (Blue Wire of a standard DB25 Cable) SHOWS AS GROUND on Datasheet
Mic PTT (RED wire)---- PIN 22 of back of CH-721 Head (Yellow/Black wire of a standard DB25) SHOWS AS INPUT 1 on Datasheet
MIC HIGH (WHITE) ----- PIN 24 of back of CH-721 Head (Grey/Black wire of a standard DB25) SHOWS AS EXT MIC on Datasheet
MIC LOW (BROWN) ----- PIN 23 of back of CH-721 Head (Green/black wire of a standard DB25) SHOWS AS CUTST/FPROG on Datasheet
EDIT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

CG disable I did not use because it is unnecessary for my use


If you use the CH-721 option cable CA-011854-001 you will connect 2 of your wires to Input 1 (or Input 2) and a ground, as well as pin 24 for External Mic High and pin p23 for External mic low.

MIC Ground ---- Ground PIN 2 or PIN 4 of the 6 pin plug
MIC PTT ----- INPUT 1 Pin 1 of the 6 pin plug
MIC HIGH ----- EXT MIC Pin 24 of the DB25 plug on the cable
MIC LOW ----- CUTST/FPROG Pin 23 of the DB25 plug on the cable.

Input 2 would use PIN 5 of the 6 pin plug and ground would use PIN 2 or PIN 4


If you are using the 44 pin option cable CA-012349-001 at the rear of the radio you could connect as follows:
MIC Ground ---- Ground Pin 1 of the DB25 connector on the cable PIN 8 of the DB44
MIC PTT -------- Input 1 Pin 21 of the DB25 connector on the cable PIN 18 of the DB44
MIC HIGH ----- EXT MIC Pin 24 of the DB25 connector on the cable PIN 6 of the DB44
MIC LOW ------ EXTALO Pin 17 of the DB25 connector on the cable PIN 27 of the DB44

If you wanted to use input 2 use the 6 pin plug with pin 5 of the 6 pin plug which connects to PIN 25 of the DB44
If you use the input 2 for PTT, use the 6 pin plug with pin 2 or 4 for ground which connect to PIN 7 of the DB44

In RPM or RPM2 you will need to go to External IO option and assign PTT to Auxillary input 1 (or Auxillary input 2)

View attachment 159772

L3 Harris recommends using a FM-017900-001 dust cover on the front of the head if you use this external mic, but it is mostly an expensive dust cover. As long as you chose to only use either the mic port on the front or the method mentioned here no damage will occur.
Important Update: upon further investigation, the MIC LOW connection to CUTST/FPROG is NOT required, necessary, or desired. This needs to be struck from the record here for using the back of the CH-721 head. CUTST/FPROG is used for factory flashing so not something to mess with by the end user. I apologize for not seeing this earlier.


This thread is the only place i have found that explains the diagram for option cable CA-011854-001

In summary, you only need 3 connections for the DB-25 Connection on back of the CH-721 head: MIC HIGH, PTT, and GROUND.
The 4th connection: MIC LOW may be used for the connections on the back of the DB-44.
 
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