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I'm installing my own avionics - question

jgbmo

Active Member
I've got a PS Engineering PM2000 intercom, a King KMA24 audio panel, a Garmin GNC355 gps/comm, and a Garmin SL60 gps/comm that I'm installing in the RV7 I'm building. All of the radios are used except for the GNC355. The intercom works fine between the pilot and copilot, PTT works, too. I can hear audio coming back through the audio panel and intercom from the gps/comm radios and it seems fine. The problem is that when I use the PTT, instead of my voice being transmitted I get a tone (200 hz, I'm guessing) and no intelligible voice. I suspect I have a wiring screw-up on the mic side, but am unsure where to look.

Any help is appreciated!
 
Funny tone on one or both radios when PTT? (selected)
Assuming you have the correct radio selected for xmit on the KMA24, I'm going to guess that the audio out of the KMA24 to the radios is either on the wrong pin (at one or both ends), not connected, broken wire, or you have COMM 1 and 2 audio swapped.
Sounds like you are keying an open wire on the radio audio input.
 
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Both of the radios transmit the same way (with the tone and no voice). I wired them the same way into the audio panel, so I'm pretty sure I connected pins the wrong way at the audio panel--or from the mic side of the intercom to the audio panel. Things get a bit confusing since King, Garmin, and PS Engineering use slightly different terminology and logic in their wiring diagrams.
 
I'm afraid I don't know what "transmit interlock lines" are? Nothing related to a flux capacitor, I trust ;-)
 
Based on install manuals I found on the web, I think this is the wiring between the audio for the intercom, audio panel, and radios. (does not include power, etc. Just the audio and PTT stuff)
See if this matches your wiring diagram or wire list.

oops, Add KMA24 pin "1" to the the intercom ground row. (KMA24 has a power ground, "speaker" ground, and and "everything else" ground.)
pm2000.jpg
 
I'm afraid I don't know what "transmit interlock lines" are? Nothing related to a flux capacitor, I trust ;-)

A transmit interlock is a wire from PTT for #1 to an interlock or mute input on #2, and vice versa. So if the antennas are close together you won?t get annoying feed thru on the radio not in use. To test if you need this, just turn off #2 and transmit on #1, see the the problem goes away. Not all radios have this input.
 
Based on install manuals I found on the web, I think this is the wiring between the audio for the intercom, audio panel, and radios. (does not include power, etc. Just the audio and PTT stuff)
See if this matches your wiring diagram or wire list.

oops, Add KMA24 pin "1" to the the intercom ground row. (KMA24 has a power ground, "speaker" ground, and and "everything else" ground.)
pm2000.jpg

Sorry, but I know nothing about the KMA 24 and am surprised it has comm audio out and audio in.

From what I gahter from your chart, KMA audio out should go to Mic hi on the radios and audio in should go to the Phones hi pin on the radios. Need to be carefull here, as many radios have both headphone pins and speaker pins. You want the 600 ohm headphone pins. The radios may also have an audio in pin that is designed for high impedence sources such as stall or traffic warnings. Mic output (i.e. Audio Out) needs to go to a pin called mic or mic hi on the radio, not something called audio. At least for modern Garmin radios. This is possibly your problem.

You will need to review the KMA manual to understand what direction In and Out refer to, as I may have it backwards. I am guessing here on what they mean. I assumed they meant in or out of the KMA.

You;ll want to tie the two phone lo's together with the KMA other ground and tie the two mic lo's to the KMA other ground as well. Not really sure if this is the best config for the KMA. Maybe Bob Turner can give guidance on that. Again, I have no experience with the KMA so can't advice well on ground ties. Most modern audio panels tie all of the lo's on a common ground bus in the a/p.
 
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Thank you to all! I'm going to go through my wiring with the chart and info you all have provided to check after breakfast this morning. Regarding the interlock, I think the audio panel provides that functionality. Regardless, I get the tone even if only one radio is installed in its rack, so I doubt that's a problem--but I'll check it out.

Thank you all!
 
I think I may have found my issue, but wanted to verify before I start desoldering and un-wiring some things. I did not separate audio and airframe/electrical grounds. There is some co-mingling of these grounds. Could that be the issue? If so, I will completely separate them. Thank you!

P.S. the PTT and audio in all seems correct. My comms transmit when PTT is pressed for both radios (whichever has been selected on the KMA24). Also, audio in works for both radios--I can clearly hear audio from comm, depending on which buttons were pressed on the KMA24.
 
Sorry, but I know nothing about the KMA 24 and am surprised it has comm audio out and audio in.

From what I gahter from your chart, KMA audio out should go to Mic hi on the radios and audio in should go to the Phones hi pin on the radios. Need to be carefull here, as many radios have both headphone pins and speaker pins. You want the 600 ohm headphone pins. The radios may also have an audio in pin that is designed for high impedence sources such as stall or traffic warnings. Mic output (i.e. Audio Out) needs to go to a pin called mic or mic hi on the radio, not something called audio. At least for modern Garmin radios. This is possibly your problem.

You will need to review the KMA manual to understand what direction In and Out refer to, as I may have it backwards. I am guessing here on what they mean. I assumed they meant in or out of the KMA.

You;ll want to tie the two phone lo's together with the KMA other ground and tie the two mic lo's to the KMA other ground as well. Not really sure if this is the best config for the KMA. Maybe Bob Turner can give guidance on that. Again, I have no experience with the KMA so can't advice well on ground ties. Most modern audio panels tie all of the lo's on a common ground bus in the a/p.

The signal names in the center column are "radio-centric". For instance, KMA24 pin P is "COM 1 MIC AUDIO", so that goes from the KMA to the radio mic input, whatever Garmin calls that. The KMA seems to have a differential headphones output, with a dedicated ground (also for the speaker outputs), but the PM2000 uses a common ground. So, you'll have to tie them together.
But yeah, putting all the signal names for all the different units in the chart was getting a bit overwhelming so I simplified it. "goes inza's and goes outza's".
Grab the KMA25 install manual, Fig 2-5 and draw arrows on it for the pins in my chart. That's what I did.
 
I think I may have found my issue, but wanted to verify before I start desoldering and un-wiring some things. I did not separate audio and airframe/electrical grounds. There is some co-mingling of these grounds. Could that be the issue? If so, I will completely separate them. Thank you!

P.S. the PTT and audio in all seems correct. My comms transmit when PTT is pressed for both radios (whichever has been selected on the KMA24). Also, audio in works for both radios--I can clearly hear audio from comm, depending on which buttons were pressed on the KMA24.
The radio equipment grounds can be co-mingled (see below), but the headphone jacks MUST be isolated from the airframe. The jacks come with special washers for this purpose. If you've done this, then continue your efforts.
Having all of the avionics share the same ground is good practice to prevent feedback loops. I have all my avionics grounds going to the same physical forest of tabs, and that is also grounded to the airframe - no issues.
 
The radio equipment grounds can be co-mingled (see below), but the headphone jacks MUST be isolated from the airframe. .

Well, this is a bit strong. Isolating the jacks is best practice, and helps eliminate ground loops (alternator whine, etc) but shouldn?t cause your problems.
Just to be clear: Are you reporting what you hear, when you push the PTT? Can you get a pirep from some other plane (or handheld 100 yards away) and see if they receive your voice, or 200Hz tone?
 
The headphone jacks have been properly isolated using fiber washers, and the 200 hz (and that's a guess as far as the frequency) sound only occurs when you PTT transmit one either of my two comm radios, picked up by a handheld 40 to 50 feet away. The panel is currently on a bench in my workshop/basement until I get the transmit issue resolved.
 
If it?s on the bench:
How are you connecting the transmitters to an antenna? Typical com antennas (quarter wave whips) must have a decent ground plane (with coax shield attached to it) to operate properly.
How are you getting dc power to the radios? Another thread noted a marginal power supply caused similar problems.
 
I've got a coax cable (RG400) that I made connecting the radio to an antenna with BNC connectors. I used a friends SWR meter to make sure that the cable and antenna were tuned (SWR of 1.6-1.7). Power is from a 12 volt lead-acid battery that I recharge periodically. Both radios produce the same results (the tone) when I transmit. They both receive signals properly from my King KX99 hand held.

Thanks!
 
The headphone jacks have been properly isolated using fiber washers, and the 200 hz (and that's a guess as far as the frequency) sound only occurs when you PTT transmit one either of my two comm radios, picked up by a handheld 40 to 50 feet away. The panel is currently on a bench in my workshop/basement until I get the transmit issue resolved.

In case you don't know this, you need one fiber washer and one plastic shoulder washer. Two fiber washers will not isolate the jack from the airframe. Not saying this is the answer to your issue (I still question your wiring arrangement) but will likely cause whinning issues later.


Larry
 
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