Alt Field CB Tripping When PTT is Pressed

[JohnM]

I am having some strange issues with my Alt Field 1 circuit breaker that I am hoping someone can shed some light on.  I have a 1989 J where 6 out of 10 times I use the PTT on Comm 1 (GTN 750) the Alt Field CB trips.  This does not occur if I am using Comm 2 (Kx-165).  As far as I can tell by the wiring diagram there is no connection between Comm 1 and the field circuit.  The voltage regulator was tested and came back okay.  The only thing I can think of is the RF cables from the Comm radios run across/near the voltage regulator wiring harness and that maybe there is some bleed over during transmission that is causing the field circuit to trip.  Has anyone run into this issue before?  I appreciate any help that can be provided.

 

[Aerodon]

I used to think that the RF was nicely contained in a coax cable, but not so much anymore.  I recently heard of an ELT being triggered every time a a transmission was made.  The unshielded ELT wires are bundled with the coax.  A friends Lancair has a problem with the engine monitor - every time one radio is used, the indications drop to zero.  No problem with the other radio.  And when we swapped the antennas around, the problem follows the 'antenna'.  Our next step is to try overbraid on the coax.

Those look like Nav coax cables near your ACU.  (and there is an unused splitter that you can take out).  And ACU's are notoriously finicky,  I've had at least 2 with intermittent faults that suddenly allow high voltage and have damaged radius and alternators.  Caused enough pain that whenever I suspect a problem , I go for a new one.  Plane Power or Zeftronics.

My 1986 M20K has 2 x zeftronics.  Your 89 looks like a Mooney branded unit with serial number 14.  I don't trust anything without serial number of 1000 or more.

Personally I would replace that - its the quickest and easiest way of seeing if the problem goes away.  If not, you have a spare one to sell.

Have you got good RG400 cables for your transmitters?  with or without intermediate connections?  Anything new, all it takes is a bad crimp....

 

Aerodon

 

 

 

[JohnM]

The ACU was the first thing we started looking at, it was sent down to Don Maxwell for inspection and was told it was good to go.  I got the plane in December and this issue showed up about 2 months ago.  Maybe would happen once every 5 or so flights, I initially thought it was just random CB trips.  It wasn't until recently that I realized it was happening on PTT presses.  I have not done any panel work since I have had it.  Before we sent the unit down to Don we were looking at Zeftronics but were having a difficult time finding a direct replacement for it.  

 

The splitter in the back is for the VOR and is hooked up.

Edited May 12, 2023 by JohnM

[Ragsf15e]

I would check (and probably just redo) the radio ground.  The vr is another good thing to look at though…

Heres another thought… swap the antennas in the back of the radios and see if it switches to comm 2.  That way you’ll know if it’s a shielding issue with the antenna or a power issue with the radio.  I know it’s sometimes difficult to get to the back of the radios…

 

[JohnM]

Good idea, I will give that a try.

[larryb]

So that splitter in the bottom of the pic, the one with one port open, what radio does it go to? What is the label for that open port?

It is generally a bad idea to leave a splitter port un-terminated. Strange reflections can result. So the first thing I would do for this strange problem is terminate that port. The part in the link below would do nicely.

If that didn't do it, I'd un-screw the regulator and zip tie it somewhere else, away from those RF cables.

Back in 1992 I had a new Ford Explorer. I installed a vhf/uhf ham radio and any time I transmitted the cruise control would drop out. On a whim, I found the cruise control module, removed it, and opened it up. I found the circuit board had a nice ground plane design, but manufacturing hadn't bothered to solder all of the components on both sides of the board. In other words, the ground plane wasn't grounded. I soldered all of the component ground wires on the top of the board and the module never dropped out on transmission again. The designers had made a good design, but manufacturing took a shortcut and caused a hard to find problem.

Larry

https://www.mouser.com/ProductDetail/Mini-Circuits/BTRM-50%2b?qs=xZ%2FP%2Ba9zWqacVsew%2FI1UMw%3D%3D

[EricJ]

Grounds are a likely culprit.    Make sure the shield on the alternator field wire is grounded on all segments, and by that I mean on each side of the firewall.   It looks like there may not be any shielding in the bundle coming out of the regulator in your pic.   That may not be catastrophic, but it's something to look in to.    Also make sure the comm radio coax cable shields are grounded, although the test suggest above about swapping the cables should show whether it is cable related or not.

A few things happen when the PTT is pressed, a significant one being that the selected comm radio suddenly draws a lot more current than it was previously.   If there is a ground issue somewhere that additional current may be aggravating it enough to cause the problem, since the regulator will try to react to the increased load.   

Good luck with it.   These things are sometimes pretty puzzling.

[ArtVandelay]

I assume you have an ammeter, does that show any jumps in the current when PTT is pressed?

[Ragsf15e]

6 hours ago, ArtVandelay said:

I assume you have an ammeter, does that show any jumps in the current when PTT is pressed?

Possibly, but most of the Mooney ammeters are set up to just show current to/from the battery, not the total power being drawn by the electrical system.  

[larryb]

The event likely happens so fast that you will never see anything on the ammeter or engine monitor. You would need a digital oscilloscope to see a glitch like this. 

When the field CB trips it is either the field wire contacting ground, or it is an overvoltage scenario where the voltage regulator intentionally grounds the field in order to trip the CB and end the overvoltage condition. If this problem is indeed related to the transmitter, then the voltage regulator is seeing a phantom voltage spike and tripping the CB intentionally.

[JohnM]

I have a JPI 900 and have looked at the amps and volts when the events happen.  There is no spike when the PTT is pressed, I agree it probably happens too fast for the engine monitor to pick up.

[carusoam]

Check to see what the JPI is set for seconds per data point…

Newer ones can get one per second…

Standard setting is a goofy six seconds…

neither will be fast enough to catch this transient with any accuracy…

PTTs are a simple switch to ground…

See what else is accidentally connected to it….

Reviewing the health of the field wire is a great idea… alternators are known to stop working when the field wire stops being connected….

PP thoughts only, not a mechanic…

Best regards,

-a-

[JohnM]

I just wanted to follow up with the resolution to the issue.  The shop swapped the RF cables on the radios (Comm 1 to Comm 2 antenna and vice versa) and the issue went away for a few months.  Plane went in for an extensive avionics upgrade and the problem came back, this time not associated with PTT on the radios.  Finally on a local flight CB tripped, and no power came back when CB was reset.  Shop found that the field wire was loose/broken within the connector to the alternator, replaced the connector but now the alternator was not putting out any voltage.  One new alternator later and all seems well, no CB trips or issues since the alternator was replaced about 40 hours ago.  

Edited September 2, 2024 by JohnM

[Fly Boomer]

1 hour ago, JohnM said:

I just wanted to follow up with the resolution to the issue.  The shop swapped the RF cables on the radios (Comm 1 to Comm 2 antenna and vice versa) and the issue went away for a few months.  Plane went in for an extensive avionics upgrade and the problem came back, this time not associated with PTT on the radios.  Finally on a local flight CB tripped, and no power came back when CB was reset.  Shop found that the field wire was loose/broken within the connector to the alternator, replaced the connector but now the alternator was not putting out any voltage.  One new alternator later and all seems well, no CB trips or issues since the alternator was replaced about 40 hours ago.  

Thanks for the follow-up!