[ICOM] Dualing mic jacks issue

[Gary P. Fiber]

John,

I did not read you original message very close and I went on about how 
the mic is wired hot. It will pick up audio anytime you key the radio 
from the back mic jack. Are you using AFSK or FSK keying, I assume it 
much be AFSK since your are using the mic jack.

Seems FSK would be a better choice and use the 13 pin accessory 
connector or wire the 13 pin audio input connector up for AFSK as its 
well after the mic circuit and you should not see the load, however you 
will Still have the mic hot and it will pick up ambient noise when ever 
you key the rig in SSB from the rear.   My IC-706MK II is the same as 
your original so far as accessory connectors are concerned. The Later 
IC-706MKIIG, had the 9 pin data connector which can take the mic out of 
the loop when you choose that in the set mode, unfortunately is wired 
for packet not AFSK RTTY.
Both the front and rear mic jack end up at the radio mic amplifier 
stage, so essentially you have both in parallel. As another mentioned 
likely a nice load being placed onto the mic line bring the impedance 
lower that the 600 ohms it normally is..
I assume the mic alone the radio drives up good output power on SSB?

Now as you mentioned you have low SSB output with any the mic plugged in 
in either the front or rear mic jack with the RTTY interface also 
installed. I am wondering if some how the phantom 8 volts is low or 
missing when the sound card is plugged in. If your not capacitive 
coupled between the sound card and radio that could place a load on the 
phantom voltage. Though we assume that sound card inputs are capacitive 
couples it could be its straight into a transistor or op amp circuit 
coupled through a resistor.

Most all Icom mics have some sort of preamplifier built in. The real old 
rigs, IC-720, 730 etc had the microphone amplifier on a separate card 
with in the mic. the newer rigs the mic element has a small FET 
amplifier built into it and draws its bias voltage from the actual mic 
audio line, where the very early Icoms ie: 720, 730 etc not only had 8 
volts on the mic line but the mic's preamp also used Pin 2 of the 8 pin 
mic connector for preamp operating voltage.

Fixing it could be a small buffer circuit between the sound card and 
radio or possibly even some passive matching transformers. The old MFJ 
mic switch and utilize only one mic connection would also relive this 
since only one would be connected at a time. But if you could FSK keying 
would really fix it since the mic audio line is not used but that does 
require equipment capable of FSK keying and wiring up the 13 pin 
accessory connector.

Good luck

Gary K8IZ

On 1/12/2012 8:17 PM, John Geiger wrote:
> I have an Icom 706 original and I recently made up a cable to go between the mic jack and my computer sound card for RTTY. It is working great on RTTY-already bagged 2 new RTTY countries (PJ2 and CP).  I also get the receiver audio off of the mic jack, so I dont have to tie up the speaker on the rig.  I figured it would be easy to plug this cable into the rear mic jack, and leave the microphone plugged into the front jack.
>
> That is where the unusual issue occurs.  With both the RTTY cable and mic jack plugged in, RTTY still works fine, but the microphone will only put out a few watts on SSB-it wiill send a FM and AM carrier ok, though.  This occurs regardless of which jack the soundcard cable is plugged into and which jack the microphone is plugged into.
>
> Is this typical for the 706?  Any idea why it might be doing this?  Any ways around it other than switching cables as needed?
>
> 73s John AA5JG
> ----
> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/
> To support QSL/QTH.net: http://www.qsl.net/donate.html
>


-- 
Gary Fiber K8IZ
GROL PG-19-6691 with shipboard radar endorsement
Washington State resident