Jump to content

A Stumper


Recommended Posts

I got my plane back a month ago after a pretty heavy surgical panel redo.  In doing so there are some squawks to work out. Panel light connectors disappeared, overhead light disconnected but overall everything is working well.  The stumper is that I noticed two days ago that when I key the mic to transmit my manifold pressure on the JPI 900 rapidly increase from say 24” up to 30” progressively.  When the mic button is released the MP goes back to normal.  Anyone have an idea on this stumper of the day? Thanks

Link to comment
Share on other sites

I have a similar problem with my UBG-16... whenever I key the mic on comm #1, GNS-480, all of the values on my engine monitor go down..CHT and EHT bar graphs go down, along with the digital value read outs.  Doesn’t do it when I transmit on comm #2, SL-30.   It’s a RFI issue that I have learned to live with.

Link to comment
Share on other sites

Does the airplane speed up, too?    ;)

There's an interference path between something transmit related and the MP measuring system.   Does it happen with both coms?   Does it happen if the co-pilot PTT is keyed instead of the pilot?  

Short-term fix would be just never say anything to anybody.  ;)

  • Haha 1
Link to comment
Share on other sites

There is a finite magnet that some installs call for. It  blocks RF into the instrument around its leads. I have one airplane  EI oil pressure guage that goes to zero while you transmit and flashes red. Gets your attention. 

-Matt

Link to comment
Share on other sites

54 minutes ago, MB65E said:

There is a finite magnet that some installs call for. It  blocks RF into the instrument around its leads. I have one airplane  EI oil pressure guage that goes to zero while you transmit and flashes red. Gets your attention. 

-Matt

I suspect your spell check changed ferrite -> finite.    A ferrite bead or collar is a pretty common cure for noise on a low-frequency signal wire.   Where to put it is sometimes tricky.  ;)

 

  • Like 1
Link to comment
Share on other sites

What is your JPI grounded to?  There is a very specific callout in the install where it should and should not be grounded to (engine I believe).  

So here’s my smart a$$ remark - when you speak into the mic there is less a restriction on the amount of hot air extruded and therefore the manifold pressure rises.   

  • Haha 1
Link to comment
Share on other sites

Dang, this is the Saturday night tough crowd. :angry: The airplane generates significantly more power when the mic is keyed.  My straight and level cruise speed goes into the red arc with rapid acceleration, its amazing.  My CHTs go down and fuel flow drops to 7gph while power seems to be at 100% or more.

Kidding aside, I'm not smart enough to try comm 2 in one outing so I'll try that next time.  I haven't determined grounding or even brought it to the mechanics attention until next week.  I will try PTT on both sides as well.  Hopefully something simple.  Thanks gang 

  • Haha 1
Link to comment
Share on other sites

It could be simple,it could be hard. 

I tried to resolve similar issue with my EI engine monitor. It was pretty bad when transmitting on certain frequency range from both COMs literally wiped the screen off. It happened after my GTN 750 was installed. I went to the extend of taking everything apart, running new shielded wires, ground everything everywhere, install new engine ground strip, repositted transducers and pressure sensors.

It helped but it didn’t go away completely. I still can see sometimes quick flicker on the monitor screen when transimmitng on 13x.xx frequencies from my GTN 750. I just live with it now. 

Link to comment
Share on other sites

2 hours ago, MooneyBob said:

It could be simple,it could be hard. 

I tried to resolve similar issue with my EI engine monitor. It was pretty bad when transmitting on certain frequency range from both COMs literally wiped the screen off. It happened after my GTN 750 was installed. I went to the extend of taking everything apart, running new shielded wires, ground everything everywhere, install new engine ground strip, repositted transducers and pressure sensors.

It helped but it didn’t go away completely. I still can see sometimes quick flicker on the monitor screen when transimmitng on 13x.xx frequencies from my GTN 750. I just live with it now. 

I did have an issue with Tower at KCAE not picking me up two separate times in their airspace on 119.50 but not sure if it was on their end or not. Subsequently communications went fine on the same frequency. That makes no sense. Nonetheless, this is new territory for me diagnosing this stuff. BTW my #1 is a 750, #2 a 650. 

Link to comment
Share on other sites

I think the first thing is to determine if it is RF related or electrical?  Does it happen with your Boom mic PTT button on the yoke as well as the hand Mic?

Does it happen on both comms?

Chances are great it will be RF related.  Did they replace your Coax cables to the Comms when they upgraded the panel?  Chances are good that they left the old RG58/U (original equipment, black) coax in place. RG58/U is grandfathered on your aircraft, but is no longer legal on new, certified aircraft.  My guess is that they broke/damaged the shield at the rack connector.  Those always take a lot of abuse when they are working behind the panel. 

  • Like 1
Link to comment
Share on other sites

I vote Ground.  Separate the grounds for the PTT/panel and the JPI.  Both should be to a good airframe ground. usually over on the copilot side under the com access panel.   Also check the airframe ground to Engine.  Should be ground strap on copilot side of the firewall.  Clean all connections till shiny bright. Could also clean the battery ground to the airframe.

 

  • Like 3
Link to comment
Share on other sites

I'm almost positive we have all new Coax but I will take a look.  I've gotta change the oil so I'll take a look at all grounding, and wiring etc while it drains.  I will report back with some photos.  Hopefully a simple fix. Thanks for the feedback on this.

Link to comment
Share on other sites

Ok, more interesting info on this but all seem interrelated.  I changed the oil yesterday and looked over everything and ran it.  By this point it was at night.  Looked at the airframe to engine grounding and it could be cleaned up some but looks ok. I didn’t have time to look under the panel.  When running the engine I used panel lights from the rheostat which worked fine. 

Heres what we know and found out last evening.

1.  Tower couldn’t here me two separate occasions on 119.5

2.  PTT makes MP go up in flight but not last night on the ground.

2. Flown for 25 hours since avionics shop with only above issues noted. Below are issues noted last night.

3. Plugged music into the music input and observed weird volume fluctuations going real high then low last night.

4.  The message box on the 750 said stuck mic, and TX wasn’t in the annunciator.

5.  Sometimes PTT would work for TX and sometimes not. Voice command worked at startup but didn’t later in the run up.

6. AMPs not displaying on the 900 but hasn’t since the shop(squawk). 

7.  Basically, the players in this are between the 750, 650, and the 900. Everything else is working well that I have observed so far. 

I’m going back up today to take a closer look at the grounding from the battery up to the firewall and take some photos. This one has me baffled as it doesn’t make any logical sense but certainly seems like a possible RF and grounding combination. Other folks have used this shop with good results FWIW. Any advice on what else to look for is much appreciated. Hopefully we can figure this one out here. I know the shop can and will but this is a real interesting puzzle.

Link to comment
Share on other sites

16 minutes ago, Junkman said:

PTT switch/wiring?

PTTs have worked well until last night along with the previous MP PTT issue, and comm issue on 119.5.  Didn't get to it today so hopefully more info here soon and we can dig through connections and wiring.

Link to comment
Share on other sites

Another weird experience between a GTN and JPI900...

@Marauder had an issue where the JPI would cause a squelch break on the GTN...

The solution involved installing an iron donut on a wire?

Some things are very sensitive to odd electrical behaviors... continue to check all sources of power and ground...

PP thoughts only, not a mechanic...

Best regards,

-a-

  • Like 1
Link to comment
Share on other sites

Here is what I've done and haven't.  Cleaned all the wires and established more secure battery connection. TX  mic still  locked up so went home.  Came back to the airport this am to fly down to the avionics shop.  Everything fired up and worked fine for the 30 min flight except for two inadverdent TX issues for a brief period.  The TX raising MP phenomenon continued as well.  Get to the destination and cannot replicate the mic TX issue but isolated the issue to the comm1 (GTN 750) for raising MP.  Frankly, comm 2 (GTN 650) may not be involved at all.   Moved the coax(brand new shielded coax all the way around) connecting the antennae to the 750 away from the JPI connections and called it a day.  Flight home and on a separate 30 minute flight trying to replicate the TX issue yielded no issues

Haven't confirmed grounding of anything and where everything is grounded on my end.

I'm not convinced this issue is resolved but really is strange.  

Link to comment
Share on other sites

I had a maybe not so similar won’t transmit but stuck mic Tx issue- it was an old washer on my pilot Mic jack such that the PTT was getting grounded with the vibration of the plane - but only sometimes.  I’m not implying that this is your issue but perhaps worth checking on because it’s easy.  Also make sure that the barrels solder tabs are not touching any of the mix jack housing if it has a housing.  Finally check the wire routing for the Ptt from the yoke - that exit point is a potential strain / shear point.  

Those are the limits of my thoughts about mic wiring. 

  • Like 1
Link to comment
Share on other sites

Sometimes when troubleshooting wierd electrical problems, I start by shutting everything off that I can, and I mean everything.  Lights, radios, heats, everything.  Then start testing by trying to duplicate the problem while switching each thing on one at a time, followed by more testing before switching the next thing on.  I can usually narrow the problem down pretty quick to the offending device.

So in this case, I would start by shutting everything down and keying the mike, check for a change in manifold pressure.  Then turn on the beacon and key the mike.  Then try the transponder etc.

  • Like 1
Link to comment
Share on other sites

6 hours ago, bradp said:

I had a maybe not so similar won’t transmit but stuck mic Tx issue- it was an old washer on my pilot Mic jack such that the PTT was getting grounded with the vibration of the plane - but only sometimes.  I’m not implying that this is your issue but perhaps worth checking on because it’s easy.  Also make sure that the barrels solder tabs are not touching any of the mix jack housing if it has a housing.  Finally check the wire routing for the Ptt from the yoke - that exit point is a potential strain / shear point.  

Those are the limits of my thoughts about mic wiring. 

We did reconnect the plugs for the headsets and this may be why stuck mic hasn’t shown up again. Also, we isolated all MP issues to GTN750.  We couldn’t replicate the stuck mic to isolate exactly where it was occurring but did resecure the connections on the pilot side. I’ll take a look at the other side too. Thanks

 

2 hours ago, glafaille said:

Sometimes when troubleshooting wierd electrical problems, I start by shutting everything off that I can, and I mean everything.  Lights, radios, heats, everything.  Then start testing by trying to duplicate the problem while switching each thing on one at a time, followed by more testing before switching the next thing on.  I can usually narrow the problem down pretty quick to the offending device.

So in this case, I would start by shutting everything down and keying the mike, check for a change in manifold pressure.  Then turn on the beacon and key the mike.  Then try the transponder etc.

I’m going to try this approach. As a point of info the MP raises with the engine off as well so if it’s 29.9 it’ll raise to a specific point which was 30.4 yesterday.  If we can isolate to the components involved then I’ll probably start with where everything is grounded and go from there. Thanks for the help:)

Link to comment
Share on other sites

There are two ways to wire the mic phone jacks.   One is with a PTT one of the phone terminal would go to ground with a PTT in a hand held mic or headset with PTT on the cord.   The other would to be to leave that wire off.  Probably need to know how yours are wired.   A rub or a stray strand of wire would cause the TX to be stuck.   Next is your PTT buttons on the yoke.    They also connect to ground when the PTT button is pushed closed.   Sounds rubbing wire ish.

Also sounds like a bad audio panel ground.

What's the MP circuit consist of?

 

 

  • Like 1
Link to comment
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.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.