I have written this final chapter for some locals who own the same type aircraft as I have that were helping me so I will cut and paste it here to save some time and effort.
"Since you two have suffered through all my rants about this you should get the first report of the solution.
My gut tells me I should take the time to summarize the whole affair and submit it to the EAA for publication
as it might save others a bit of frustration and it is much more likely to occur in the future with the new
products that split the display unit from the actual “work” unit. This issue will probably not occur for the traditional set-up
where the “knobs and dials” are part of the transponder unit and/or com radio. In addition, it might, but is
unlikely to occur in a metal aircraft. When I asked at the end of my final conversation with the Dynon product manager whether there
was anything in the installation manual or user’s guide that would have prevented the whole thing, he
confirmed that there was not and that they probably need to make some additions to their instructions. As
close as I can remember he said: “we have not paid enough attention to composite aircraft and failed to
remember everyone does not build an RV.”
The line that would have prevented 6 months of headache is this: “Ground all the avionics cases together.”
As you both know instructions emphasis over and over that all the ground wires from the harnesses need
to go to a separate avionics ground buss. And, I assume you know I check that over and over as well
as checking every other possible item suggested – cable type, internal cable grounding, .....
Here is a great illustration (see attached) of why this particular issue was so difficult to track down.
I constructed this cable. It is clearly overkill as far as wire gauge but is what I had laying around. #10 leads
soldered to #4 backbone. From left to right they were connected to: com remote unit; Transponder Remote;
Display unit; com panel unit, intercom panel unit, ADSB-out, and a spare. The backbone runs behind my
buss tray and connect directly to the #4 battery cable at a common terminal on the right.
It is not the fact that the grounding worked that is of real interest here, it is more what occur when I tried to isolate
which of the connections was “the culprit”. The Dynon rep was putting his money on the transponder remote unit
since it was mounted to a non-metallic surface.
Once I had established that in fact the popping and music channel interruption had been eliminated I started disconnecting
the leads one at a time. An external speaker (that turned out to be critical in this test) allowed me to hear if the
music circuit was muted – tested that with a hand held. To my astonishment the music circuit did not revert to its
“problematic stage” even when I had all the leads again disconnected. Disconnecting the backbone also did nothing.
While puzzling over this I was distracted by a text message on my phone. Now the phone was connected via an
audio cable to the intercom – feeding it music. Whether this actually played any roll in the scenario is not known, but
probably unlikely and an empirical question I’m not sure I care about at this point. In any event while looking at the text
the speaker went haywire with popping and muting. In this case I was grateful to hear that.
I went back and touched the ground wire from my wire array to the intercom case on the dash unit and sure enough
muting stopped, no popping. HOWEVER, when I removed the wire everything remained good. Now, the only people
I can think of that would take that in stride have either never run a conditional experiment or have a whole lot more
intuitive knowledge than I do.
So I stood there and waited and it took 3-4 minutes, but eventually the circuit goes bad and the muting/popping starts
again. AND, it will not clear unless you ground it again. So, I’m sure Dave has a better understanding of what is
going on, but Dave (Dynon Dave) said something to the effect that the intercom unit is acting like a big capacitor and
once it gets full, the rest is history.
We also worked out why I was able to produce 35 degrees of error in the compass by moving the wires around. The
ADAHRS unit was the unit “with the best ground” - in this case it sits on an aluminum plate connected to the buss
tray. While there is not enough current running the ADAHRS unit itself to create an electrical field strong enough to
effect the magnetometer in the unit, when OTHER units try to find the best ground they added their current to the
path going past the ADAHRS unit. As a matter of fact it turns out this can be used as a nice little diagnostic tool.
With everything grounded as it is now there is no deflection in the compass (heading indicator) no matter where I place wires or which
wires go where – As it should be. So in the build process if one is able to deflect the compass (heading indicator on the Dynon display) – something is wrong.
I am sitting here looking at the SV-MAG-236 remote magnetometer I purchased to try and solve the compass issue.
It is probably unlikely I need it as I was able to recalibrate the compass yesterday but at $140 it’s going in the plane!
I am left with one decision – whether to go back to the monopole antenna. I have yet to fly into controlled AS since
the rebuild so I have no positive assurance I’m being seen. That is easy enough to test. But perhaps the bigger issue
is that while I had the transponder certified back in September I’d guess swapping the antenna is changing a major
component of that system. The one piece of luck is that I left a spare RG400 cable up in the gap seal area and
reinstalling the monopole is a matter of drilling one hole. Leaving the dipole and cable as is would be an easy way
to test if antenna location is indeed irrelevant."
That is the end of what I sent to my buddies.
One take-away here seems to be that if a dash panel (in my case there are four separate and non-connected pieces of metal cover with teak veneer) is separated from where the remote units are mounted the ground wires integrated in the harnesses may not be sufficient to insure proper operation.
I am grateful to Dave at Dynon for spending several hours examining photos of my set-up and discussing potential routes to proceed down. Fortunately, he "got it in one."