Recommended Posts

2 hours ago, NotarPilot said:

, I pulled the number 1 IFD 440 CB to see what happened and that only caused the intermittent TX problem to go steady TX

The symptom you relate — pulling the CB for #1 IFD-440 causes intermittent #2 IFF-440 to transmit properly  — is a puzzle worthy of Click and Clack.  Unfortunately they are off the air.  

Or did you mean that pulling the #1 CB causes the #2 to transmit all the time?  

 

 Do you get the same intermittent transmitter operation if you use the old standby microphone and its PTT?   Or is the problem only with the yoke-mounted switch?  
 

And what event seems to clear the intermittent mic problem?  Does it clear up after 30 seconds or so and then resume at a subsequent thumbing of the PTT?  

Share this post


Link to post
Share on other sites

*Members that donate $10 or more do not see advertisements*

On 11/7/2019 at 2:25 AM, Jerry 5TJ said:

The symptom you relate — pulling the CB for #1 IFD-440 causes intermittent #2 IFF-440 to transmit properly  — is a puzzle worthy of Click and Clack.  Unfortunately they are off the air.  
I don’t recall them ever discussing avionics on that show so I’m not sure how helpful they would have been.

Or did you mean that pulling the #1 CB causes the #2 to transmit all the time?  

Trying to diagnose the problem I pulled the #1 IFD CB and they #2 IFD went from intermittent to full on nonstop TX.  

I also pulled the audio panel CB in the past which had no effect on stopping the intermittent TX problem. 


Tonight I did some troubleshooting on the ground the while the #2 IFD was doing the intermittent TX thing with the engine off I tried physically removing the audio panel from the tray to see what would happen and it didn’t seem to affect anything as the intermittent TX problem persisted.

I decided to get under the panel and wiggle some wire bundles around in hopes of possibly isolating the problem as this seemed to be the fix before.  Wiggling one bundle seemed to be the culprit and holding it in a particular position seemed to make it stop while another position held made it transmit.  So it seems to be a wire that’s causing some sort of interference but now I just have to figure out which one and possibly replacing it.

 Do you get the same intermittent transmitter operation if you use the old standby microphone and its PTT?   Or is the problem only with the yoke-mounted switch?  
I haven’t tried anything with the old standby mic and this problem doesn’t seems to be associated with the PTT switch on the yoke.  I know you have some experience in this field but correct me if I’m wrong but I don’t think the PTT switch will work with the audio panel removed, will it?  Also, on my PMA450A I have both yokes set to transmit on the #1 IFD while #2 is intermittently transmitting. I recently flew from both seats and both PTT switches correctly transmit on the #1 IFD as configured.  

And what event seems to clear the intermittent mic problem?  Does it clear up after 30 seconds or so and then resume at a subsequent thumbing of the PTT?  
The IFDs have a feature where it detects a stuck mic after continuous transmitting of (I believe) one minute after which it will stop that radio from transmitting temporarily.  After some unknown time it’ll reset and the problem will persist.

So what do you think Jerry? My personal thought is perhaps repinning the TX wire going into the back of the tray for the #2 IFD.  That’s the same tray that was installed for the previous GNS 430s that were installed circa 2006.  I’m not sure how difficult or easy that would be to do.

Edited by NotarPilot

Share this post


Link to post
Share on other sites

The part I don't get is how pulling the power on #1 radio affects the #2 radio PTT operation.  Perhaps an effect from the "fail-safe" features of the PMA450A.  Ignore that symptom for now...

Your test of wiggling the wire bundle sounds significant.   The PTT switches in the yoke are routed to the audio panel.   The audio panel then routes the PTT signal (grounding the PTT pin enables transmission) to the appropriate radio. 

Here's a bit of the PMA450A Schematic:

502828415_PMA450ASch.thumb.jpg.849e7ae480514cc0cc915a34b0a6437c.jpg

 

You can see that the PTT runs from the audio panel to each radio. That's how the correct transmitter is keyed based on the XMT selector switch of the PMA450A.

If you remove the PMA450A from its rack and can reach the pins of J1 down inside the rack, check the resistance of pin 30.  It should show a high resistance relative to aircraft ground. 

If you wiggle the wire bundle and that resistance drops to a lower value (a few dozen ohms or less) then you may have found the culprit -- an intermittent short to ground on that wire.  

 

 

  • Like 2

Share this post


Link to post
Share on other sites

Here's my theory: You have a 'weak/intermittent' short to ground on the the #2 comm radio's PTT input.  Something internal to the the PMA450 is providing a 'pull-up' path to the #1 comm's PTT input which prevents #2 from going to TX most of the time (i.e., when the 'short' is weak...when it is a 'strong' short to ground #2 transmits) AS LONG AS #1 has power. When you pull #1`s CB, that 'pull-up' is now gone and #2 goes to TX continuously.

One way to check this is to put BOTH transmit (pilot and co-pilot) select switches to comm #2...my guess is that comm #2 will go to continuous TX as the pull-up path to #1 will likely be disconnected.

You're wire wiggling pretty much confirms an intermittent short, and Jerry's suggestion to check pin 30 to ground (if you can get to it) is a good one.

Good luck with tracking down exactly where the wire is shorting...NOT fun!  You might look for where wire bundles come in contact/wrap-around metal structures, especially sharp edges.  Could be inside a connector back shell...shield wire/foil contacting the back of a pin.

Edited by MikeOH
  • Like 3

Share this post


Link to post
Share on other sites

Thank you both Jerry and Mike for your input and suggestions. I kind of understand what you guys are saying but I’m no avionics expert so I gotta get someone a lot smarter than me to check out the things you suggested.  I think I’m on the correct path of getting this sorted out.

Share this post


Link to post
Share on other sites

I would check where the wires exit the back of the yoke tube.  The PTT is just switched ground.  If built into the headset circuit, you can have I believe it is tip on the mic phono plug go to ground that will also activate the PTT.   So you need to check your headset phono plugs.  this would allow for a PTT in the headsets.

You may be shorting say a PTT circuit to B+ which could fry the A/P 

if the insight has an audio alarm B+ in the audio circuit fry it too.

Not an EE or any other letters behind my name.

Share this post


Link to post
Share on other sites
3 minutes ago, Yetti said:

I would check where the wires exit the back of the yoke tube.  The PTT is just switched ground.  If built into the headset circuit, you can have I believe it is tip on the mic phono plug go to ground that will also activate the PTT.   So you need to check your headset phono plugs.  this would allow for a PTT in the headsets.

You may be shorting say a PTT circuit to B+ which could fry the A/P 

if the insight has an audio alarm B+ in the audio circuit fry it too.

Not an EE or any other letters behind my name.

I don’t think that’s the issue because when the PMA450A is set to transmit on IFD #1 and you press the PTT switch the #1 IFD will transmit meanwhile the #2 IFD will transmit independently.

Share this post


Link to post
Share on other sites
On 11/2/2019 at 1:52 PM, Baker Avionics said:

Well, all I will say is there is a reason why I am not a fan of Garmin, LOL.  Aside from that, it seems like you may have had a ground issue or maybe a bus issue.  Now, IF there was a "surge" would it be evident in everything that has power to it?  Mags, Lights, switches, CBs, etc....is everything working now?  Was it mainly things on the Avionics Bus and nothing on the battery bus?  Those are the anomalies that need answered and explained to go any further.  Did anything else "burn up?  Were all of thiose units hooked to the same grounding point and there is an issue there?  

 

Inquiring minds want to know!!!!!

Did you say mags?They are not connected to ships system bus at all...they are self generating 

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.