SV-261 Mode C altitude readout missing, reported by tower. Installed SV-261 transponder and 2020 GPS puck. I also have ADSB-470 installed. Initially loaded firmware 14.0 and subsequently loaded 14.1 followed by 14.1.1. Got all the right configuration indications as shown in installation manuals. A check with tower initially showed good information and altitude readout until I reached 5 miles from airport at which time the altitude readout became intermittent and eventually was lost completely. Turning back to the airport the altitude readout was again visible at about 5 miles and was observed all the way to touchdown. All other information, code, GS, etc seemed to be OK. If all info was lost, I would suspect antennae wiring but only altitude missing leads me to believe its a internal problem in the SV-261. Any ideas??
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Intermitent Mode C on SV-261 transponder
- Thread starter wolf1
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- Mar 23, 2005
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Wolf,
This sounds like maybe it's a new airplane? If so, have you done the 91.413 bi-annual transponder check yet? It is required on brand new airplanes before they first operate the transponder because it finds installation issues.
Your description sounds exactly like a bad antenna or coax leading to putting out minimal power. It takes a better signal to noise ratio to pick up the Mode C code than it does the Mode A or just a secondary reply. Ground Speed is calculated from only primary returns so that is not transmitted from the transponder. There's no way in the transponder for just the Mode C to fail, and with a SkyView the only way to lose ALT data is if the AHRS is offline. You'll see "NO ALT" in the transponder status area in the top bar too.
This sounds like maybe it's a new airplane? If so, have you done the 91.413 bi-annual transponder check yet? It is required on brand new airplanes before they first operate the transponder because it finds installation issues.
Your description sounds exactly like a bad antenna or coax leading to putting out minimal power. It takes a better signal to noise ratio to pick up the Mode C code than it does the Mode A or just a secondary reply. Ground Speed is calculated from only primary returns so that is not transmitted from the transponder. There's no way in the transponder for just the Mode C to fail, and with a SkyView the only way to lose ALT data is if the AHRS is offline. You'll see "NO ALT" in the transponder status area in the top bar too.
hstaton
New Member
When I had this issue on a previous airplane, the problem ultimately found was an unknown faulty connector in the cable that was hidden behind a panel. This would tend to support a cabling/high SWR issue as the problem.
- Joined
- Mar 23, 2005
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Yes, the transponder can handle any SWR and is not damaged by having a bad connection.
In many cases, we can actually report this error as well. Check the transponder setup page and see if you have any errors. This gets reset every power cycle so you need to do it after the transponder in running a bit (and not in SBY or GND).
In many cases, we can actually report this error as well. Check the transponder setup page and see if you have any errors. This gets reset every power cycle so you need to do it after the transponder in running a bit (and not in SBY or GND).