Avionics Work on my NXCub

[ve6yeq]

There are two different kinds of “interlocks” being discussed here:

1. Audio mute to prevent you from hearing other transmissions while talking which can confuse some people if they are not practiced at multiple simultaneous conversations. This is controlled with DIP switches on both the GMA-245R and for Bluetooth audio on the headset controller.
2. RF Pre-amp interlocks which prevent the large amount of RF energy from one transmitter overloading or even permanently damaging the very sensitive RF pre-amps by powering it down.

In my Carbon Cub EX-3 with GNC-355A and GTR-20 using the default antennas, I can definitely hear the RF pre-amp on the edge of overload so after this discussion, I will look at adding the necessary wires to disable the pre-amps when one is transmitting. I already have my Lightspeed ANR mounted in the helmet set to mute and I think I might try it with the GMA-245R as well.

 

[lrobichaux]

@ve6yeq please let us know if you get better results after installing the interlock. Mine seems to work great now.

 

[Cactus Charlie]

I was very surprised that this mod worked when wired as documented. I had expressed my doubt about this mod several times.

A day before I heard the report I had emailed Pete at CC saying basically that I did not understand the mod as drawn and asked if there was a pin numbering error. I asked if pin 12 was the intended GMA pin.

Pete replied late Monday - "That is a preliminary sketch and indeed there was a pin number mixup with the GNC 355. The upcoming service document will have the correct pin number there, pin 12."

Pin 12 is "COM 1 MIC KEY* OUT". That connection would pull COM 2 "TX Interlock In" to ground and de-sense the receiver when COM 1 was keyed.

 

[Neal]

Definitely going to wait for them to move this out of BETA. Thanks for the info.

 

[lrobichaux]

@Neal ... Hey - I've been messaging with @Cactus Charlie on the COM issue, and I'm concerned that Pete / CC may now think that the COM1 / COM2 monitoring issue is resolved (based on my feedback to CC / Pete ... as I am the "beta" test he referred to) when @Cactus Charlie is convinced it's not. I wanna send an email to Pete and copy all of us to make sure that everyone is working off the same information. Can you DM me your email address so I can loop you in? I have @Cactus Charlie 's email address.

 

[Neal]

Sent you my email. I suggest conversations be put in this thread and not private when able so we can all learn from them and contribute.

I know Pete is watching, you're welcome to participate too

 

[Cactus Charlie]

Pete knows the beta mod instruction was incorrect. What I don't know is whether he knew it was incorrect before I raised the issue.

Sit tight and wait for the released mod instruction then undo the incorrect mod (if made) when implementing the correct mod.

 

[Neal]

I'm not paying my avionics guy until I know for sure. I don't understand why the GMA dip switch is not sufficient as it blanks out the other radio while transmitting unless someone wants to be hearing chatter while talking. Is the dip switch not a sufficient fix?

 

[Cactus Charlie]

Ve6yeq has explained why it is desirable to prevent high levels of RF getting to the receiver front end. I don't know what power he runs at his amateur radio station but I run 500 W which can destroy a proximate receiver.

The much lower power output of these aircraft radios may not be sufficient to cause damage but it may be sufficient to overload the receiver. If the receiver is hard muted by the DIP switch selection you won't be able to hear the overload.

I don't have this radio configuration so I can't offer any test data. All I can do is look at the installation manuals, the mod instruction, and any available schematics.

 

[Neal]

Yeah, I remember that, thanks for reminding me. I thought our output is 10W? I could be wrong.

 

[lrobichaux]

Ok - spoke to Pete a few minutes ago and confirmed with him the following. The COM1 / COM2 interlock modification that was shared with @Neal by Pete a few days ago is based on my emails with Pete from a couple of months ago when I first recognized this problem. Further, CC is moving forward in creating a service bulletin / "final fix" based on this solution. They have not changed or updated anything. Not sure how @Cactus Charlie got the impression that CC recognized that their first fix was an "incorrect fix," because Pete confirmed to me on the phone earlier today that CC's final solution is indeed the solution I had installed. They are just "cleaning up" and formalizing fix into a proper service bulletin.

So, @Cactus Charlie over to you. Do have specific reasons why you believe the CC interlock solution is flawed?

 

[Cactus Charlie]

As I posted earlier today -

Pete replied late Monday - "That is a preliminary sketch and indeed there was a pin number mixup with the GNC 355. The upcoming service document will have the correct pin number there, pin 12."

Why he says GNC 355 not GMA 245R is unknown to me. No GNC 355 pins are shown in the mod instruction I saw.

De-sensing the GTR 20 requires its pin 5 to be pulled to ground. The mod, as published, ties pin 5 to GMA 245R P2401 pin 11. Pin 11 is an analog audio signal not a ground/open signal. Pin 12 is a the ground/open PTT out signal and this signal is suitable for pulling GTR 20 pin 5 low.

It is still my belief that the mod instruction should have shown GTR 20 pin 5 tied to GMA 245R pin 12 not to pin 11.

Again - Please read Pete's reply. I would not have posted that the mod instruction was wrong unless I had been told it was wrong.

 

[lrobichaux]

Why he says GNC 355 not GMA 245R is unknown to me. No GNC 355 pins are shown in the mod instruction I saw.

Agree, the mod instructions given to me reflects GMA 245R pin 11, not the GNC 355.

Pin 11 is an analog audio signal not a ground/open signal. Pin 12 is a the ground/open PTT out signal and this signal is suitable for pulling GTR 20 pin 5 low

What would be the result if GTR 20 pin 5 was connected to GMA 245R pin 11 instead of pin 12? I'm trying to decide if I need to go talk to my avionics shop to determine if they exactly followed the incorrect instructions. Would it do no harm, but just not have the intended positive effect?

Again - Please read Pete's reply. I would not have posted that the mod instruction was wrong unless I had been told it was wrong.

My apologies, I was moving a little too fast. Now that I've slowed down just a bit, re-read the entire thread, and compared everything to the original instructions sent by Pete, I can see where CC incorrectly referred to the wrong pin. I'm now left wondering if my avionics shop (which enjoys a very good reputation) figured that out during the install, or whether they blindly just installed the interlock as incorrectly sketched out by CC.

 

[Cactus Charlie]

I doubt any damage would be done by connecting GMA pin 11 to GTR 20 pin 5. The fact that your on-aircraft check showed modulation present for all 4 tests shows the audio output was not damaged (at least at the time the test was run).

If you ask an avionics shop to implement a manufacturer's mod instruction I doubt they would research it like I did. I expect they would simply implement it as instructed.

I still don't understand why you were told the mod instruction was correct and I was told, by the same person, that it had the error I suspected.

 

[lrobichaux]

I expect they would simply implement it as instructed.

No sense in taking a chance. I'll go visit them this week, explain the situation and ask them to check their work against the updated instructions.

I still don't understand why you were told the mod instruction was correct and I was told, by the same person, that it had the error I suspected.

This is an easy answer. It's because Pete emailed me the "fix" when it was first proposed by CC on June 20th, and it appears that they then researched it further. It took me nearly seven weeks to do the install and evidently during that time CC discovered their error. Wish Pete would have let me know, but I assume he thought I would not wait as long as I did to complete the modification.