XCOM 760 radio problem

Can anyone explain EXACTLY how to set up an XCOM 760 transceiver push to talk (PTT) button? I have the PTT lead (7) coming out the 15 pin connector on the back of the radio connected to the PTT metal tab on the microphone jack. Along that wire (connecting #7 to the jack) I inserted one lead going to the PTT switch, and the other lead on the PTT switch goes to ground. The radio receives transmissions well. However, when I press the PTT button, I get a loud squeal and it does not transmit. I have actually set this up completely independent of my airplane’s power system ( I also have a problem getting my MGL radio (V6) to properly work when the airplane engine is on, in that the EFIS and radio work fine with the engine not running, but goes haywire when the engine is on) with a spare battery. Anyway, I would think it should be simple to get the radio to function properly (including the PTT). Everything that is supposed to be grounded is grounded to the negative terminal of the battery, so grounding isn’t the problem,  Any ideas?

Steve

N750HS

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Permalink Reply by stephen hetz on February 27, 2019 at 5:04pm

Well, I'm definitely not an EE.  I, in fact, do have all the grounds (from the jacks, the radio case) tied at one point to the 16 gauge black wire that goes to the negative terminal of the battery (remember, this setup is currently entirely independent from the airplane and just lays in my copilot seat.  The jacks are laying free as well).  I'm using Lightspeed 3 headphones and haven't tried another brand (I don't own another brand).  As best I can tell, the jacks seem to be functioning properly as far as the mechanical aspect goes when the plugs are pushed in.

I also have been in contact with Michael from XCOM (via email from Australia.  He has answered most of my emails within 15-20 minutes of when I send them during regular business times in Australia.).  To quote him, a lot of properly setting up a radio is "witchcraft" and he strongly advised getting an avionics tech to look things over.  Of course, I've done that (the avionics tech has been trying to help me fix this for several months).  Michael at XCOM thinks the problem is my lack of proper shielded wiring.  I suppose he must be right.  It's puzzling, though.  It seems to receive fine.  But once I push the PTT button, it briefly transmits (I can hear "sidetone" in my headphones) but within a second or two, stops transmitting (based on others listening and the lack of sidetone in my earphones).   I'll look at making my own connector and harness with shielded wiring, but will likely take the easy way out and order the harness from XCOM.  And frankly, thanks for taking the time to try and help me puzzle through this.  It makes sense to ask about even the most mundane process or procedure to make sure it hasn't been forgotten or missed.  Any other suggestions are welcome.  Thanks.

Permalink Reply by Gary Welch on February 27, 2019 at 11:59pm

My radio is the Flightline 760, so not exactly the same. I did a lot of research, especially over on the Vans Air Force website because those guys analyze everything to a gnats ass. All the advice I got there was to "run shielded wires or you'll have problems". When I asked Flightline directly they said "no need". 

I was skeptical but I ran mine un-shielded and the radio has worked perfectly. HOWEVER, I don't have a any kind of sophisticated avionics in my plane either. 

FYI - getting my radio to work right did take quite a while. Damn PTT would not work at all. That is until I discovered a second ground wire missing....

Good luck figuring this out. Wiring can be a PITA!

Permalink Reply by Gary Welch on February 27, 2019 at 11:51pm

The official diagram shows power going to both pin 9 & 10. Your diagram is only showing power to pin 9.

Permalink Reply by stephen hetz on February 28, 2019 at 10:54am

I noticed that as well as I was drawing the wiring diagram.  Not sure that'll make any difference but I'll include #10 with the power.

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