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Help me get my Audio Panel Install working

Flying Canuck

Well Known Member
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This is reaching out to a very special someone(s) out there that likes reading schematics and can see what I've done wrong. Only the brave and willing need read on. First some background.

I have just installed a second COM (MGL V16) and a PMA8000B Audio Panel, along with a NAV radio (MGL N16) and control head (MGL Razor). I moved the existing GTR200 as well. I designed and built all of the wiring harnesses - the GTR harness is adapted from the factory supplied harness. There were other changes to unrelated systems that I'll detail in a moment. Most everything works as expected. I have 2 snags.

1. Noise. I've got a fair amount of new noise in my headset, from master switch on onward. Gets worse when I power on the avionics (including both COMs and the Audio Panel), but not appreciably when I power on individual items.

2. This is the show stopper. I show a Pilot PTT Stuck full time on the GTR200 (COM1). This comes from the Audio Panel, so not an actual stuck PTT. Doesn't matter if I transmit on COM1 or COM2.

I've triple checked most everything, my actual wiring is decent, all connections that I intended to make are correct. You can see the as built schematic for the Audio Panel harness where I've also added the COM1 and COM2 harnesses. I use shielded 2 and 3 conductor cable, shields are terminated to ground at the audio panel only. Shield ground is passed through the Molex connectors.

For troubleshooting the stuck PTT, I've tried quite a few things.
- verified that I don't actually have a stuck PTT.
- used multimeter to check the PTT connectors in my harness to see if anything is grounded. I found that the PTT pin on the COM1 connector is grounded when the system is powered up, but not otherwise.
- I swapped COM1 and COM2 connections, the problem went away on the GTR and didn't appear on the V16

As you can see from my drawing, I've wired TX Interlock. In updating the drawing I saw one thing that may be an issue, but I don't see why. The V16 connector joins audio in ground to pilot PTT. I believe I have this wrong, but I don't see how this would cause issues with PTT on the other radio. I also used a possibly incorrect ground pin on the V16 for the pilot mic, but again, don't see how that would affect the other radio.

If you are still reading, you are awesome and I thank you.

If you are observant, you will see that I don't wire a mic lo. I rely on the PTT itself to provide the lo. I can't find where I got this from, but it wasn't my idea. Maybe it's an issue.

I think that covers my issue. If you can decipher my drawing and see something I did wrong, I would be so appreciative.

As promised above, here is a list of all of the changes that I did at the same time as this upgrade.
- created a secondary avionics buss running off of a 15A circuit on my VP-X. This is switched on with my avionics master and feeds 4 traditional circuit breaker protected circuits. COM2 (V16) and Audio Panel are each one of these circuits.
- installed N16 and Razor
- installed Delta Pop Aviation VHF antenna under left wing for COM2
- installed Bob Archer NAV antenna in left wingtip
- some physical relocation of panel items
- new radio rack with space ready for my GPS175 which should arrive in a couple of weeks.
- replaced my keyed ignition switch with a pair of magneto toggles and a start push button
- access panel installation - this is the only thing I'm really sure has nothing to do with my issue.

There, that's it. Thanks for getting this far. I'd love to get this wrapped up before long. I'm missing on some really nice flying weather and the constant head scratching is leaving a divot.
 

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Okay. Took me a bit to realize the vertical column of components in the center of the drawing are in reality simply harness connections (Molex, Dsubs, Deutsch etc....). Is there a reason to use those? Must be for ease of maintenance and/or installation. I try to steer away from those when it comes to audio circuits but if I do I like to use use D-Subs with metallic backshells with the shields attached to the backshells to carry the shielding across the joint. Despite all this I doubt those connectors are your real problem.

So, did you make sure the headphone and mic jacks are isolated from any sheet metal mounting holes with insulating washers? Your diagram leaves off the jack circuits.

I am not sure the MGL radio should have pins 14 and 11 tied together. In fact usually the microphone circuit between the radio source and the audio panel is a 3-conductor shielded wire, not a two-conductor shielded wire. It would make more sense to have the audio panel PTT circuit tie into pin 14 and then have pin 11 connect back at the audio panel backshell, getting there via the third wire in a three-conductor shielded wire (you show a two-conductor). So right now with your schematic I think pin 11 keys Com2 PTT all the time (pin 14). Maybe the TX interlock between the two comms has the MGL over-riding the GTR with it's stuck mic due to pins 11 and 14 being interconnected.

Unfortunately if you are dealing with a pre-made GTR harness where the supplier made some short cut by using two-conductor twisted pair instead of three-conducted twisted pair for the mic circuit then I am at a loss. And I could see where their creativity may have caused confusion on the MGL wiring if using their GTR wiring as an example.

So a quick test would be to cut loose the wire going to pin 11 at the MGL transceiver and then connect it to the twisted pair's shield. That will leave only the COM2 PTT ("key") wire connected to pin 14. All that pin 14 wire does is tell the transceiver to "key" when it's grounded. In fact I would change the terminology on your diagram at the GTR unit, MGL unit and both COM1 and COM2 intermediate Molex connectors to "KEY" instead of PTT since the physically PTT only talks to the audio panel and the KEY circuit comes out of the audio panel to key whichever selected COM radio to transmit.

There are some folks on VAF who really know their stuff so stand by to get some more knowledgeable advice than I can provide.
 
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Without seeing the schematic, not sure what happens if you use a common ground rather than a Mike Lo input for the mike lo - but it may be your noise source?
For sure, connecting pins 11 and 14 (PTT and ground) together on the V16 cannot be correct.
 
Okay. Took me a bit to realize the vertical column of components in the center of the drawing are in reality simply harness connections (Molex, Dsubs, Deutsch etc....). Is there a reason to use those? Must be for ease of maintenance and/or installation. I try to steer away from those when it comes to audio circuits but if I do I like to use use D-Subs with metallic backshells with the shields attached to the backshells to carry the shielding across the joint. Despite all this I doubt those connectors are your real problem.

Two reasons I'm using Molex, ease and cost. Didn't say two good reasons. I shied away from D-Sub because I had issues getting consistent results and the pins get expensive. I have become much more comfortable with D-Sub though and the cost is just money. I had not thought of the sheilding benefits of the metallic backshell. That makes a lot of sense. I agree this isn't the cause of the PTT issue, but it could well be the main cause of noise.

So, did you make sure the headphone and mic jacks are isolated from any sheet metal mounting holes with insulating washers? Your diagram leaves off the jack circuits.

I didn't change the jack circuits, they are left alone from my initial GTR200 harness installation. I did double check the insulators.

I am not sure the MGL radio should have pins 14 and 11 tied together. In fact usually the microphone circuit between the radio source and the audio panel is a 3-conductor shielded wire, not a two-conductor shielded wire. It would make more sense to have the audio panel PTT circuit tie into pin 14 and then have pin 11 connect back at the audio panel backshell, getting there via the third wire in a three-conductor shielded wire (you show a two-conductor). So right now with your schematic I think pin 11 keys Com2 PTT all the time (pin 14). Maybe the TX interlock between the two comms has the MGL over-riding the GTR with it's stuck mic due to pins 11 and 14 being interconnected.
You are correct, I'm using 2 conductor for the microphone circuit from the MGL, only because of what is likely a flawed assumption on the PTT/lo joint.

Unfortunately if you are dealing with a pre-made GTR harness where the supplier made some short cut by using two-conductor twisted pair instead of three-conducted twisted pair for the mic circuit then I am at a loss. And I could see where their creativity may have caused confusion on the MGL wiring if using their GTR wiring as an example.
That's on me, not the supplier. It is a 3 conductor wire, I had joined the lo to the PTT initially on the audio panel. I cut that lead at the audio panel while troubleshooting. I can reactivate the third conductor if I redo the connector.

So a quick test would be to cut loose the wire going to pin 11 at the MGL transceiver and then connect it to the twisted pair's shield. That will leave only the COM2 PTT ("key") wire connected to pin 14. All that pin 14 wire does is tell the transceiver to "key" when it's grounded. In fact I would change the terminology on your diagram at the GTR unit, MGL unit and both COM1 and COM2 intermediate Molex connectors to "KEY" instead of PTT since the physically PTT only talks to the audio panel and the KEY circuit comes out of the audio panel to key whichever selected COM radio to transmit.

Good ideas all.

There are some folks on VAF who really know their stuff so stand by to get some more knowledgeable advice than I can provide.

If it gets better than this then I'm in for a real treat because this was VERY helpful. Thanks!

I am seriously considering redoing all of the audio circuitry connectors as D-Sub with metallic backshells. Apart from removing pins, I'm really comfortable with the D-Sub now. The PMA connectors are high density 44 pin and weren't bad to work with at all.
 
The audio panel has a drawing for connections to two radios. Follow it faithfully. Make sure your headset jacks are not grounded. Best way is to use the correct fiber washer and/or nylon washer. PTT should connect from the audio panel to each radio separately and not be tied to ground.

Molex connectors can work. They also add a layer of problems. Direct cable from audio panel to radio is more customary.
 
Molex connectors can work. They also add a layer of problems. Direct cable from audio panel to radio is more customary.

Direct cable connection is viable, I can build a complete harness connecting the audio panel to the 3 radios, 2 headsets and a music port with new wire and new DSub housings for about $200. Replacing my 7 Molex audio connections with DSub connectors with metal backshells would be more work and cost about $50 less. Nice thing about that idea is that the second attempt at something usually turns out much better and it would also be sure to be the cleanest both in presentation and in noise. I also wouldn't have to build the harness in the plane.

Guess I'll sleep on it and decide.
 
interlock is wrong

see note 11 on GTR manual. Interlock goes from in to out and out to in between the 2 radios. It should not be hooked to the PTT or the audio panel.

Jay
 
I assume the intercom uses the mini-Dsub connectors. Those can be a challenge for folks new to them and you have to look very closely for strays before crimping, when using 22 ga. I would find at least 10% of the time, I would leave one stray strand outside the connector and have to repeat the insert before crimping. I would be looking for a stray strand that didn't make it into the connector before crimping, especially on the Lo wires and shields.

Larry
 
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Don't start over yet. I stand corrected. I looked at the PMA8000B install manual. It does show using a single twisted pair for both the transceiver audio and mic circuits. The Mic Lo is not used at all so your original GTR harness is good. Mic Lo pin at the radio is not used when an audio panel or ICS is used upstream. So keep the twisted pair going to the MGL just eliminate the connection to Pin 11 in it's entirety. Dont' worry about draining Pin 11 to the shield as I previously suggested. That's a waste of time.

Also, on the PSE audio panel J2 you show sending avionics ground to three areas, Pins 10, 11 and backshell. Usually the wire going to the backshell isn't part of the "power ground" feeding the unit. If I were you I would remove that backshell wire and then optionally pin it in pin position 13 to get Karaoke mode for Music 2 (why not and most people find this setting less mentally confusing than constant interruptions in the background).

And great catch by Jay above. The transmit interlock communicates strictly between the transmitters and doesn't get the audio panel involved.

You are close enough I wouldn't haul off and re-harness the whole thing. Just address the referenced pinning and leave the Molex's as is. I think you will be happy.
 
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A big part of the confusion is that Garmin, MGL and PMA all have their own interpretations of how this is all connected.

For the mic lo -

Garmin shows Pilot Mic Lo and Receiver Audio Out Lo being joined at the audio panel.
PMA shows Pilot Mic Lo not being wired at all
MGL doesn't show an audio panel configuration example
I think I'm okay with either way.

For TX interlock
- PMA not involved
- Garmin says COM2 key to COM1 TX interlock in, COM1 TX interlock out to COM2 TX interlock
- MGL has a single TX Interlock for in and out and just looks for it to be grounded at the other end, so COM1 key should work, right?
I don't think my TX interlock wiring is wrong. If it is, it it is only that I'm not using the GTR's TX interlock out. Should I be connecting both COM1 interlock out and interlock in to COM2 TX interlock? Seems counter intuitive.

Interesting side note on TX interlock - Garmin says it's to protect the receiver when another COM is running. MGL says it's to prevent 2 COMs transmitting at the same time and doesn't say anything about protecting the receiver.

The PMA J2 diagram clearly shows the shield ground being tied to the 10 & 11 ground pins and the airframe ground. The backshell has 2 lugs that are supposed to take the shields as well. So I think that is correct as well.

That leaves the bad pin 11 connection on the MGL. I'll go cut that tonight and see if that resolved the stuck ptt. If it doesn't, I'll move the tx interlock on the gtr off of the ptt and onto the interlock out.

I'm not convinced that this will address the noise, but we'll see. I'll hold off on re-harnessing for now. If the noise is tolerable, I can skip the re-harness.

Thanks again to everyone for their time in looking at this.
 
I'd love to report success but no joy. I cut pin 11 and it made no difference to the stuck mic. Maybe marginally less noise but hard to say for sure. I also removed the TX interlock in from COM1 and that made no difference.

I've removed all of the audio wiring harnesses and brought it home with my supplies and tools. Tomorrow, I will be ordering the necessities to rebuild the whole harness with all direct connections for audio.

Quick questions, does the music port need to be insulated from ground as well? Mine didn't have the fiber washer behind the front retaining nut, can't say if there ever was one - it's possible, but if there was it wasn't lost recently.

One thing that was better today after cutting pin 11, my COM2 antenna isn't showing a flashing ANTENNA! warning when booting the MGL control head anymore. It never persisted and I was hearing audio as much as I would expect inside the hangar.

Thanks everyone who's watching this post.
 
Quick questions, does the music port need to be insulated from ground as well? Mine didn't have the fiber washer behind the front retaining nut, can't say if there ever was one - it's possible, but if there was it wasn't lost recently.

Yes, it must be isolated from the panel ground. Otherwise you will very likely get a bunch of unwanted noise on the music input, especially if you are using a USB power port.
 
That's what I figured. I never use the music input, so never really noticed an issue. I included a new jack in my purchasing so it will be correct when it is re-installed.

I'm going through my single harness design. It is going to include all new d-sub connectors to the PMA8000B, Garmin GTR200, MGL V16 and MGL N16. It will also include new pilot and pax headphone and mic with PTT jacks, the music port and one auxiliary audio input - from my Skyview. The MGL radio connectors will go on to connect to the Razor control head with a Can buss connection and both COM radio will have RS232 links to a selector switch in front of the Skyview. Of course there is also power to everything which will be done with Molex 2 pin connectors.

I will run the mic audio lo from the audio panel to the 2 com radios. In both cases this will be a split of the single lo for each com at the audio panel as is shown on the GTR drawing with the GMA panel.

The only thing I'm not 100% on is the TX interlock. Here is the description of TX interlock on the MGL.
The TX interlock is both input as well as output. It is realized as input with a pull up of 2200 ohm to 3.3Volts similar to the PTT inputs but also has an output transistor that can switch this output to ground.

The TX interlock is grounded by the internal transistor whenever the transmitter is active. Should this line be grounded by an external device while the transmitter is not active, the PTT inputs as well as PTT commands from the communications interfaces are disabled.

If the TX interlock is grounded by an external device while the transmitter is active it will not have any effect on the current transmission.

It is common to connect this line to the corresponding TX Interlock of a second V16 to prevent simultaneous transmissions.
What I can't figure out is if you TX on the GTR, it grounds the out pin on COM1, wouldn't that just ground the in pin too if they are tied together going to the MGL? I think it should be setup with the MGL PTT going to the GTR in pin and the GTR interlock out going to the MGL interlock pin. Any thoughts? I'm going to raise this question on the MGL forums. Someone has to have figured this out before.
 
My new "no connections" harness is built and I'm just back from installing it.

Noise isn't gone, but significantly better and definitely acceptable. I don't have a lot of time listening to my radio without the engine running so it might well be comparable to before I made any changes.

Stuck PTT issue is gone (good riddance).

I'm receiving fine on COM1 (GTR200) and COM2 (V16), but I can't hear anything from COM1. I see the RX light up when I tune in ATIS, so I know the radio is receiving. I didn't try any transmissions, I'll probably put my canopy back on and pull it outside and do some more tests tomorrow. I think I'll also make some pins on sticks so I can double check continuity on the COM1 audio circuitry without having the pull the harness out.

It does appear that my NAV radio (N16) is working, I see data at the Skyview. Not expecting to pick anything up, nearest VOR is 56 NM away and I'm in a hangar on the ground.

It's definitely a step forward, hopefully I can figure out this issue and verify everything else and get this thing back up in the air where it belongs. I miss flying.

My GPS 175 (and Dynon ARINC module) is sitting in Corona, CA waiting to be shipped to me, so stay tuned for that installation in February.
 
For test probes for avonics connector pins down inside the rack backplates I use thin welding rod and sharpen the end like a pencil. Then I run heat shrink down the length of it to insulate it leaving a bit of both ends exposed. The blunt end for the test lead and the pointy end to stick down in the tray to ring out connector pins.
 
I like to use spare pins inserted into the connector mating pin for testing. I move the spares from place to place and the meter taps the spare that is plugged in. Keeps from having to stab something into the connector.
 
Claude,

From your schematic it appears you connected the shields at both ends. The shields are only supposed to be connected at the audio panel.
 
Claude,

From your schematic it appears you connected the shields at both ends. The shields are only supposed to be connected at the audio panel.

That's just because that diagram only goes out to the intermediate connectors when I was passing the shield through a Molex connector. The only end devices shown are the two com radios. Shields were only grounded at the audio panel.

That diagram is my first harness and that no longer exists. My version 2 harness did away with all intermediate connections and went direct device to device. I haven't updated the diagram to reflect the new harness, I built off of a spreadsheet which I have attached a screenshot of.
 

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Claude,

From your schematic it appears you connected the shields at both ends. The shields are only supposed to be connected at the audio panel.

Ray, I thought the same thing originally until I realized those green shield wires in the center of the drawing were passing through Molex connectors en route to the ultimate destinations. Basically quick disconnects.
 
Claude,
On your spreadsheet comments column (far right) why are you tying Com Audo Lo's to Mic Lo's at the PMA? Same mis-match between Com Mic Lo's and PMA Audio Lo's. Something seems amiss in the comments section.
 
Do you by chance have the volume turned down on the GTR200?

Bill

I did check the volume knob. I did not check the Receiver Out Gain setting in the config. Since I use the the Receiver Audio instead of headsets for output, this would need to be set properly. The default is 100% so it's likely fine but I'll check that today.
 
Claude,
On your spreadsheet comments column (far right) why are you tying Com Audo Lo's to Mic Lo's at the PMA? Same mis-match between Com Mic Lo's and PMA Audio Lo's. Something seems amiss in the comments section.

That's how I understand it to work. There is only one lo per com on the PMA. I did that tie in based on the GTR200 diagram for connecting to a GMA 240. It connects the COM 2 AUDIO LO on the audio panel to both the PILOT MIC LO and RECEIVER AUDIO OUT LO on the radio.
 
Well another day and another update. My COM1 audio out issue is fixed, I figure that was a slightly loose seating of the radio in the rack. I did determine that I have no mic on either headset, but I'm pretty sure I know what's wrong. Multimeter test indicate that I've swapped the hi and lo coming off of the jacks at the PMA. I've traced that back to my spreadsheet, I swapped the labelling on my sleeve and ring. Ran out of time and stamina today, so that will get fixed tomorrow. The HD pins are actually quite a bit easier to remove than the standard pins so this won't be too difficult. I did fix another pin swap today - pilot mic PTT was wrong. This one should have, and eventually did raise the alarm when it being swapped with audio lo didn't cause a stuck PTT, that was because it was actually swapped with audio hi.

Got some help and put my canopy back on today, so once I fix that last audio issue I should be ready for some outside tests and if everything passes on the ground I may even make it to a flight test.

Once again, thanks to all of you who have studies my diagrams and spreadsheets to spot my mistakes. It's so helpful having extra eyes on this stuff.

EDIT: I've attached my updated schematic for the all in one harness. It only shows PMA connections and the Transmit Interlock between the COMMs. I've intentionally created this one in a layout similar to the diagrams in the PMA manual.
 

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Finally!

Went out and swapped 3 pairs of pins (Skyview Audio was swapped too) and then tried it out. Everything audio panel related works as expected now. Pulled the plane outside and did a radio check on both COMs and it was good both RX and TX on both radios. Still have a little noise but I think that's the Skyview audio, that's still using a Molex connector and the portion Skyview side of that connector isn't shielded at all. I get most of the noise before I turn on the avionics buss when the Skyview gets passed through as a failsafe and along with the mics are the only audio sources up at that time.

My serial port sharing switch only seems to work in one direction, I can't tune the radios from the Skyview. I also can't set or swap standby. Not a big deal, just means I'll repurpose a serial port from my ELT once I can feed that from my GPS175.

While it hasn't been a topic on this thread, my keyed ignition replacement seems to work well - although I couldn't generate enough battery to start today, the start button definitely triggers the starter.

I updated the diagram in my last post to reflect the final working configuration.

Thanks for all the help with this. Hopefully this can serve as a reference for anyone trying to do an audio panel on their own plane.
 
see note 11 on GTR manual. Interlock goes from in to out and out to in between the 2 radios. It should not be hooked to the PTT or the audio panel.

Jay

And great catch by Jay above. The transmit interlock communicates strictly between the transmitters and doesn't get the audio panel involved.

I'm resurrecting this old thread after reading the GTR 200 installation manual, which doesn't seem to align with the 2 quotes above. But...maybe they are only referring to Transmit Interlock OUT.

Note 11 in the current manual reads "If two COM radios are installed this input and output connects to the other COM radio. *Transmit interlock IN to * Transmit interlock OUT and *Transmit interlock OUT to *Transmit interlock IN. *Denotes an active low signal."

However, Section 3.6.6.1 on page 39 reads: "Pin 5, TX INTERLOCK IN - Active Low Input that ‘desenses’ (protects) the GTR 200/200B receiver when another communications radio is transmitting. This input comes from another communication radio's interlock output or MIC KEY line.

My COM 2 will be an existing KX-155 w/ ILS that I plan to keep in my new panel. Best I can tell it does not have an interlock feature. I do agree with a post I read at some point (from Larry I believe) that says most people just turn down the volume on the other COM, but if can wire the GTR 200 interlock to the Audio panel MIC KEY out, why not?

Edit: And, figure D-4 on page 82 shows a connection from a GMA 240 to the GTR 200 Transmit Interlock In
 
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