HT 750 Accessory Connector

kmoose · Post by **kmoose** » Wed Apr 21, 2004 2:52 pm

First off, let me explain what I am trying to do. In each and every aircraft that flies on a US Forest Service contract, there is supposed to be what we call a "Forest Service Plug". This is a connector that allows an operator to adapt a portable radio into the aircraft audio system. When connected, this portable takes up a space on the aircraft audio panel, and can be selected on the audio panel so that when pilot pushes his PTT key on his yoke, or his cyclic, or wherever it is, it keys the portable and routs the pilot's microphone to the portable. It also routs receive audio from the portable over to the pilot's headset. The idea behind this is that a Forest Service guy can jump in any aircraft at a fire, with a portable and the appropriate wiring harness, and use the aircraft's audio system to talk to people over the portable. All the Forest Service Plugs have the same pinout. It is a standard cannon plug, with a standard pinout. We use a portable in this position as an AUX FM in the aircraft. Currently, we are using Bendix/King EPH portables. However, my boss has asked me to try to build a harness that would allow us to use our HT 750s in this position. So, being the smart guy that I am, I bought two remote speaker/mics for the HT 750. My thinking was: A remote speaker/mic utilizes the same functions that we need for the Forest Service plug. Audio HI and LO, Mic HI and LO, and PTT. the only other connections are a +12 VDC and GND, for power to the portable. I have that covered. I bought a battery eliminator for a cigarette lighter plug. I simply cut the cigarette lighter end off, and terminate the DC HI and LO into the appropriate pins in the cannon plug. However, I am having trouble figuring out the pin out on the remote speaker/mic. I was hoping I would get a wiring diagram with them, but no such luck. So I figure, "Big Deal, I can just look at the pin out for the Accessory Connector, and find the 5 pins I am looking for(Audio HI and LO, Mic HI and LO, and PTT)" But I am having trouble figuring it out. First off, there is no pin labeled PTT. Maybe they were afraid of infringing on Nextel trademark?

Also, I am not sure where to start counting on the accessory connector. I did find something of a pinout on the main site, under the programming cable building section, but I still am not sure how to make the PTT work. Can anyone help?

Sundown · Post by **Sundown** » Wed Apr 21, 2004 8:50 pm

Use the pinout from http://www.batlabs.com/images/1550svc.gif... Speaker is obvious, as is Mic. I think PTT is Opt Sel 2 (pin 5), and there's probably a pin looped back somewhere in the plug to allow the radio to sense there's a speaker/mic attached.

Sundown · Post by **Sundown** » Wed Apr 21, 2004 8:50 pm

http://www.batlabs.com/images/1550svc.gif even

kmoose · Post by **kmoose** » Thu Apr 29, 2004 2:56 pm

OK, I almost have this thing working. The problem is: everytime the radio receives, the audio output starts to "motorboat'. For the life of me, I am not sure why. I even tried isolated the audio amp output from any DC by installing a capacitor inline, but that didn't help either. Anyone have any ideas? I seem to recall that I have had this problem before, when I hooked up a SINAD meter to the RTX4005B. *shrugs*

wireless.friendly · Post by **wireless.friendly** » Sat Feb 04, 2006 2:22 pm

Anybody know how Professional Series portables, like is HT750/1250/1550, detects that speaker microphone is connected? I want use radio with another speaker microphone, but simple usage of pins 1, 2, 4 & 7 does not bring to success. Silence in RX mode & no PTT in TX mode...

And also:

I check microphone settings in CPS software: Per Radio > Misc > Microphone > Accessory Connector Mic. Enable & found that this option is NOT checked & NOT available for change... Any ideas? Or may be just speaker microphone must be connected? & this option will appear?

wireless.friendly · Post by **wireless.friendly** » Sat Feb 04, 2006 2:31 pm

Or may be anybody has a schematics of ANY speaker microphone for Professional Series portables?

If YES, please give me it to e-mail wireless.friendly # gmail.com (replace " # " with a "@") or just give a link to it...

pema307 · Post by **pema307** » Sat Feb 04, 2006 5:59 pm

An ohmeter check of my AAHMN9053B mic shows pin 6 tied to pin 7 (ext mic enable) and pin 5 is PTT.

wireless.friendly · Post by **wireless.friendly** » Sat Feb 04, 2006 9:57 pm

pema307

You mean that pins 6 & 7 just shorted inside of mic?

pema307 · Post by **pema307** » Sat Feb 04, 2006 11:40 pm

Yes, pin 6 (option select 1) is tied to pin 7 (ground).
PTT is activated by shorting pin 5 (option select 2) to pin 7.

wireless.friendly · Post by **wireless.friendly** » Sun Feb 05, 2006 2:52 am

pema307

Thanks!

wireless.friendly · Post by **wireless.friendly** » Wed Feb 08, 2006 8:57 am

pema307

I check the original/genuine Motorola HLN9716 (aka HLN9716, aka AAHLN9716) audio adapter for Professional Series, but pins 6 & 7 does NOT shorted.

I check it when external speaker mic has been connected to adapter and disconnected - no differences - pins 6 & 7 always un-shorted.

ffemt1764 · Post by **ffemt1764** » Wed Feb 08, 2006 9:50 am

Interesting as my speaker mic part number is HMN9052C ... unfortunately my ohm meter has joined the dodo bird and i cant check to see if its shorted at the above mentioned pins