As some folks know, Louise and I put an entirely new glass-panel cockpit into her RV-6 last winter, the goal being to provide a solid platform for her to finish up her Instrument ticket. She has been signed off a couple of times to take the check ride, but in both cases, her old steam-gauge panel had enough squawks that it just didn't work out, so we wanted to make sure everything was ready before she goes for a finish up in the near future. The morning weather was good going into the long holiday weekend, so we took off for some hood work, and an opportunity to introduce her to flying with an HSI - but after take-off, it became apparent that we had a problem with the VOR NAV dropping in and out. We tried several nearby stations, and it was hit and miss - the signal would be strong for a few minutes, then lose lock. Since we have about 5 VOR's within 25 miles, it clearly was not a problem with stations - there was something wrong in the aircraft!
What we have in "Mikey" is an SL-30 sending serial data to the Dynon D180 and D10A (connected via the Dynon DSAB bus) and also to a Mid-Continent VOR/LOC/GS dedicated head (so much cleaner than an old-fashioned analog interface!). The behavior we saw looked a lot like a weak signal to me, but I didn't want to jump to conclusions, so I looked at things methodically, while Louise flew.
1) I noticed that the Dynon's and the VOR head lost lock at the same time. This pretty much told me that they were "seeing" the same thing on the serial bus, and that they were working. They indicated identically, showing either good data or flags. I declared the displays GOOD.
2) Next, I looked at the digital data on the SL-30 (it shows radial from the station, as well as a "Flagged" indication.) I figured that if it went in and out with the EFIS and head, that the serial data stream itself was good, and that the radio output was being accurately reflected. This turned out to be the case - data on the SL-30 display flagged at the same time as on the remote displays. From this I decided that the problem was upstream in the radio receiver, resolver, processor, or upstream in the antenna system.
2a) Curious about the fact that the VOR head OBS knob actually sends resolver data to the radio, I tested it to make sure that it also changed the OBS numbers and indications on the Dynon - this was the case, so again, I figured that the serial data and digital stuff was working.
3) So…down to a problem in the SL-30 or the antenna system. Now it was time for an ASSUMPTION - that being that the SL-30 is a very reliable piece of digital electronics that either works or it doesn't - and no smoke was emanating from it. This was just a guess to enable me to go look at the antennas, so we took the empennage fairing off to look at the cat-whisker antenna mounted under the horizontal tail. I had noticed SOME (possible) correlation between station position and heading - it seemed like if the main landing gear was between the tail and the station, we got a drop out - on either side. Hmmm…weak antenna? What can go wrong with an antenna like that? Well, not much, but it was worth a look. Visual and tactile (shaking it) inspections were good, so we chased the antenna wire up the chain to make and break a couple of BNC connectors. We found everything tight, and no signs of corrosion.
4) We pulled the SL-30 out of the rack and reseated it a couple of times, just in case there was corrosion on the antenna pin, and did a visual inspection down inside the rack. All OK.
5) Finally, it was time to "assume the position" underneath the panel, so I got out the pillows and cushions to pad the rudder pedals, and crawled up behind the radio to check the connection at the back of the unit. The avionics shop that delivered the SL-30 had pre-wired the rack with a one-foot "pigtail" for both of the antennas (Nav and Com). These connect to the rack with right-angle BNC connectors provided by Garmin that have a screw collar to anchor the Coax shield braid. Checking the BNC on the end of the pigtail showed nothing abnormal, but when I got my hand on the rack connector, the nut fastening the Coax braid to the body was loose - by about a turn! I tightened this up, and figured we might as well go flying - this was the third test flight (the second being after the digital/serial part of the T/S).
6) Success! We pulled in stations as far away as 75 miles from 1,000' AGL as soon as we took off. So the original "hunch" was correct - it was a signal strength (type) problem due to a bad antenna system - it just took time to figure out where the problem lay. In this case, a loose connection in the ground. I was quite amazed at the affect such a simple problem had on the radio's performance - I'm sure that the shield was connected in some way, it just wasn't as good as it should be. Oh…and I was glad it wasn't one of my own connections (this time….)
Patience and step-by-step troubleshooting one again proved to be the way to a solution. When we make the assumption that the new hardware is good, we can then concentrate on connections and wiring, which usually turns out to be the case.
Paul
What we have in "Mikey" is an SL-30 sending serial data to the Dynon D180 and D10A (connected via the Dynon DSAB bus) and also to a Mid-Continent VOR/LOC/GS dedicated head (so much cleaner than an old-fashioned analog interface!). The behavior we saw looked a lot like a weak signal to me, but I didn't want to jump to conclusions, so I looked at things methodically, while Louise flew.
1) I noticed that the Dynon's and the VOR head lost lock at the same time. This pretty much told me that they were "seeing" the same thing on the serial bus, and that they were working. They indicated identically, showing either good data or flags. I declared the displays GOOD.
2) Next, I looked at the digital data on the SL-30 (it shows radial from the station, as well as a "Flagged" indication.) I figured that if it went in and out with the EFIS and head, that the serial data stream itself was good, and that the radio output was being accurately reflected. This turned out to be the case - data on the SL-30 display flagged at the same time as on the remote displays. From this I decided that the problem was upstream in the radio receiver, resolver, processor, or upstream in the antenna system.
2a) Curious about the fact that the VOR head OBS knob actually sends resolver data to the radio, I tested it to make sure that it also changed the OBS numbers and indications on the Dynon - this was the case, so again, I figured that the serial data and digital stuff was working.
3) So…down to a problem in the SL-30 or the antenna system. Now it was time for an ASSUMPTION - that being that the SL-30 is a very reliable piece of digital electronics that either works or it doesn't - and no smoke was emanating from it. This was just a guess to enable me to go look at the antennas, so we took the empennage fairing off to look at the cat-whisker antenna mounted under the horizontal tail. I had noticed SOME (possible) correlation between station position and heading - it seemed like if the main landing gear was between the tail and the station, we got a drop out - on either side. Hmmm…weak antenna? What can go wrong with an antenna like that? Well, not much, but it was worth a look. Visual and tactile (shaking it) inspections were good, so we chased the antenna wire up the chain to make and break a couple of BNC connectors. We found everything tight, and no signs of corrosion.
4) We pulled the SL-30 out of the rack and reseated it a couple of times, just in case there was corrosion on the antenna pin, and did a visual inspection down inside the rack. All OK.
5) Finally, it was time to "assume the position" underneath the panel, so I got out the pillows and cushions to pad the rudder pedals, and crawled up behind the radio to check the connection at the back of the unit. The avionics shop that delivered the SL-30 had pre-wired the rack with a one-foot "pigtail" for both of the antennas (Nav and Com). These connect to the rack with right-angle BNC connectors provided by Garmin that have a screw collar to anchor the Coax shield braid. Checking the BNC on the end of the pigtail showed nothing abnormal, but when I got my hand on the rack connector, the nut fastening the Coax braid to the body was loose - by about a turn! I tightened this up, and figured we might as well go flying - this was the third test flight (the second being after the digital/serial part of the T/S).
6) Success! We pulled in stations as far away as 75 miles from 1,000' AGL as soon as we took off. So the original "hunch" was correct - it was a signal strength (type) problem due to a bad antenna system - it just took time to figure out where the problem lay. In this case, a loose connection in the ground. I was quite amazed at the affect such a simple problem had on the radio's performance - I'm sure that the shield was connected in some way, it just wasn't as good as it should be. Oh…and I was glad it wasn't one of my own connections (this time….)
Patience and step-by-step troubleshooting one again proved to be the way to a solution. When we make the assumption that the new hardware is good, we can then concentrate on connections and wiring, which usually turns out to be the case.
Paul
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